Ignitions, Cylinder Heads, Pistons & Cylinders, and the Bottom End 

In this second installment, we will look at Porsche ignition systems; factory and aftermarket, the various cylinder heads used on 911’s, piston and cylinder combinations for more power and the stuff inside the engine cases that keeps everything together.


Porsche has used almost all of the ignition technology that’s available, since the introduction of the 911. The ’65 through ’68 911’s used the reliable Kettering ignition consisting of a battery and coil system with breaker points to interrupt the primary current flow and induce a spark from the coil. This proved somewhat unsatisfactory with the rich-running Weber Carburetors and their crude idle circuits that caused some plug fouling problems when operated in traffic. In 1969, Porsche adopted a Capacitive Discharge ignition system that used the points for switching only. This was a major upgrade and made these engines run much better and less finicky to drive. Plus, the plugs and points lasted much longer.

Porsche used this CDI system until 1978 when the 3.0 litre engine received a breakerless ignition system that used a magnetic trigger instead of breaker points. From the 3.2 and 3.6 litre Motronic-equipped engines, the ignition was no longer triggered from inside the distributor. Instead Porsche used the flywheel as a crank-triggered ignition and the distributor merely sent sparks to the respective plugs.

Bosch has been the supplier of choice for Porsche until 2012. The factory used either Bosch and Marelli distributors, as well as Bosch and Permatune CD boxes when one vendor or the other couldn’t supply enough components to meet the demand. Since 1978, Bosch has provided all of the street and racing ignitions used by Porsche until the late water-cooled race cars.

The first twin-plug, point-type distributors used in the early 2.0 litre racing engines were made by Marelli. These, very well made but finicky units were used in the 906, 911R, and 911S 2.5 racers. When Porsche released the first 2.8 RSR’s, these were equipped with a Marelli until the 3.0 RSR engine which used a Bosch breakerless distributor along with some special CD boxes that were better able to withstand the engine vibration levels experienced in racing. All of the 934 and 935-series Turbocharged models used a similar Bosch breakerless ignition without an advance mechanism. Subsequent race cars like the 911SC RS used a Bosch system like the 3.0 SC engines.

Closer to the end of air-cooled production, Porsche applied twin-plug technology and benefits to the 3.6 litre engine beginning in 1989. Besides some initial problems with distributor drive belts, this has still been an excellent application on these engines. This has also made it more economically possible to retrofit the 3.2 Carrera engine with this system. By adding the Factory vent kit and replacing the distributor drive belt as part of the maintenance schedule, these systems are very reliable and relatively inexpensive to maintain.

The water-cooled engined use a coil-on-plug, direct-fire ignition with each spark plug lead having its own separate coil. The Motronic/Siemens DME fires each coil as programmed in the software.


Lets talk about ignition performance modifications to 911’s. These upgrades fall into three general categories:

  • Breaker-point upgrade with Magnetic, Hall-effect, or Optical triggers
  • CD box upgrade with hotter, better quality CD box and Coil
  • Twin-ignition upgrade (all air-cooled cars)

The first priority would be to replace the factory breaker points with a steadier trigger system that maintains timing much closer at high RPM. This also saves having to replace and maintain the points over time. In some cases, this solves a myriad of troubles with erratic tachometer readings when the points become dirty.  

The early Bosch distributors, ’65-’69 must use the Crane XR700 ignition system to replace the breaker points. These seem to be a little tougher to install and adjust, but their are no other options for these series cars, at the present. The later Bosch units, ’72-’77 can use a Pertronix Ignitor module to replace the breaker points. These have proven to be reliable and cost-effective.

The next level of modifications involves replacing the stock CD amplifier and coil with a High-Voltage, High-Current unit such as the MSD-6AL and a respective matching coils such as the High-Vibration Blaster or HVC E-core ones. These units provide a FAR hotter spark that is better able to fire the rich idle mixtures required by Weber/PMO carburetors and MFI. These systems permit the use of a wide plug gap has been proven necessary for best power and idle quality. These ignition boxes can make a race-cammed Weber’ed high-compression engine idle for hours without any signs of distress when installed properly. These multi-spark high current ignitions really help reduce the cantankerous nature of some carbureted and MFI-equipped engines that have mixture troubles caused by big venturies and high-overlap cams. These ignitions also feature some nice adjustable soft-touch rev limiting.

Twin-ignition is one of the ultimate upgrades for any  air-cooled 911 that isn’t equipped with this system. Although not particularly inexpensive to do, it does pay big dividends in the drivability, throttle response, octane requirements, and power. The degree of power improvement is related to the static compression ratio. Engines with less than 10:1 can expect a smaller power increase than ones that are above 10:1. In the case of big-bore engines, 98mm and up, twin-ignition is required due to their propensity for detonation at compression ratios above 10:1 on pump gas. The other extreme is the 2.0 litre engines with their small steep combustion chambers that require very high domed pistons for the compression ratios that provide best power. These heads are quite prone to detonation at or above 10:1. Twin-ignition is a necessity for these engines to live on the lousy pump fuels that we put up with.

Another benefit of twin-ignition is that heat gain from using a higher compression ratio is somewhat offset by not requiring as much ignition advance for complete combustion. Typically the twin-plug engine requires no more than 26-27 degrees total for best power compared to a single plug engine requiring 32-34 degrees.

Read the “Let’s Talk” column: Gasoline, Detonation, Timing, and Twin-Ignition for additional and more detailed information on this subject.

Installing a Twin-plug system on the various 911 engines isn’t inexpensive. The best time to do this is when you do an upper-end overhaul and replace the valve guides since the lower plug holes  can be drilled and tapped in the heads and the lower valve covers machined at the same time.

For the 2.0 through 2.2 litre engines, the early-type Marelli Twin-plug distributor is almost impossible to find so we use a new billet twin-plug distributor that has a magnetic trigger. These can be used to trigger Bosch or MSD CDI boxes.

The 2.4 through 3.0 litre engines can use either a new billet twin-plug distributor or one of the new reproductions of the Bosch RSR breakerless distributors and any desired CD boxes and coils.

3.2 Motronic engines can be adapted to Twin-ignition with either a new billet twin-plug distributor or the 3.6 dual-distributor and a special splitter unit for this purpose. The triggering comes from the OEM Motronic trigger at the flywheel and the 3.6 dual-distributor simply sends HV top each plug . All you need is the 2nd coil and a plug wiring harness. Non-Motronic 3.2 engines can use a new billet unit as above.

Cylinder Heads

Porsche has used several variations of cylinder heads from 1965 to the present. The small valves and ports of the first 911’s were soon enlarged for the 911S. These 2.0 litre heads were not particularly well designed with their small deep combustion chambers and wide valve angles. When used with high-domed pistons for compression ratios above 10:1, the tall domes shroud gas flow in the chamber and spark plug. This prevents the flame front started at the plug from progressing smoothly across the piston crown. The unburned fuel pockets that result from this cause detonation and pre-ignition.

The 2.2 litre engines used valve angles that were closer together to create a shallower combustion chamber which offered improved airflow over the 2.0 litre versions and much more resistant to detonation. These heads work well on all of the 2.0 engines unless the racing rules prevent this upgrade. Valve sizes remained very consistent from 1970 to 1977. The 3.0 engines were upgraded with larger ports & valves than their predecessors. In 1980, Porsche reduced the port sizes on the 3.0 litre engines; enlarging them again with the introduction of the 3.2 Carrera series. Valve sizes remained uniform throughout the 911 SC and Carrera series.

The 3.6 litre engine saw a further increase in port & valve sizes between the 964, 993 and 993 RS engines.

More recently, excellent aftermarket heads that outflow factory heads are available and used when power levels above 360 BHP are desired. Typically, these are installed on 3.8 to 4.0 engines and allow power levels beyond 400 BHP.

EngineValve SizesPort Sizes 
65-68 2.0 911In 39mm Ex 35mmIn 32mm Ex 32mm
67-68 2.0 911SIn 42mm Ex 38mmIn 36mm Ex 35mm
69 2.0 911TEIn 42mm Ex 38mmIn 32mm Ex 32mm
69 2.0 911SIn 45mm Ex 39mmIn 36mm Ex 33mm
70-77 2.2-2.7 911TEIn 46mm Ex 40mmIn 32mm Ex 32mm
70-73 2.2-2.4 911SIn 46mm Ex 40mmIn 35mm Ex 35mm
73 2.7 RSIn 46mm Ex 40mmIn 36mm Ex 35mm
73 2.8 RSRIn 49mm Ex 41.5mmIn 43mm Ex 43mm
74 2.7 911In 46mm Ex 40mmIn 32mm Ex 33mm
74 2.7 911SIn 46mm Ex 40mmIn 35mm Ex 35mm
74 2.7 Carrera(US)In 46mm Ex 40mmIn 35mm Ex 35mm
74 3.0 RSRIn 49mm Ex 41.5mmIn 43mm Ex 43mm
75-77 2.7 911S (all)In 46mm Ex 40mmIn 35mm Ex 35mm
76-77 3.0 CarreraIn 46mm Ex 41.5mmIn 39mm Ex 35mm
78-79 3.0 911SCIn 49mm Ex 41.5mmIn 39mm Ex 35mm
80-83 3.0 911SCIn 49mm Ex 41.5In 34mm Ex 35mm
83 3.0 911 SC RSIn 49mm Ex 41.5mmIn 43mm Ex 43mm
84-89 3.2 CarreraIn 49mm Ex 41.5mmIn 40mm Ex 38mm
89-94 3.6 C2/C4/RSIn 49mm Ex 42.5mmIn 41.5mm Ex 38mm
95-98 3.6 993In 50mm Ex 42.5mmIn 43mm Ex 39mm
96-98 3.8 Carrera RSIn 51.5mm Ex 43.5mmN/A



Well,……you are probably thinking: “This all very nice information but I want to know how to make more power and go faster!”

Power and durability must be considered together. For example, the larger, heavier valves used in the 3.0, 3.2, and 3.6 litre engines are more prone to float at high RPM than the smaller valves used in the 2.7 and earlier engines. This means that in an accidental over-rev such as what happens when you miss a shift, will likely result in valve-to-piston contact. Instant bent valves!

There are several ways to raise the RPM threshold at which this occurs. You can lighten the valve train components so that the rev limit is much higher by installing better valve springs such as the Aase springs, use Titanium valve spring retainers, and lighter Stainless Steel or Titanium valves (race only).

It is better to not miss shifts of course, but with Porsche transmissions, this can happen with worn shift linkage. Everyone misses a shift at one time or another “in the heat of battle” due to driver error or linkage wear. The trick is to make this event,……….uneventful.

Porting and flow testing the heads to increase airflow is one of the most important parts of increasing engine performance. Since any engine is simply an air pump, getting more air in and out, at all valve openings, is paramount to power increases. Done correctly, you will see a torque increase throughout the RPM range that translates into more usable power, not just at peak RPM. It’s too easy to just hog the ports out and lose almost all low and mid-range torque. It takes a lot of experience and skill to create major airflow increases from low RPM to the camshafts’ power peak and beyond.

Look at some CNC-ported heads and you can see the changes.

Porsche heads are largely interchangeable within the confines of cylinder head stud spacing. The 2.0 litre, ’66-’69, engines used the least desirable cylinder head design of all. This is due to the wider valve angles and deep combustion chamber that prevents the higher compression ratios necessary for higher power outputs. The tall piston domes necessary with these heads shrouds the valves quite badly and creates a detonation-prone atmosphere without twin-ignition. This is why these engines do not benefit very much from compression ratios over 11.5:1. Cylinder filling is quite compromised above this level. You can also use any 2.4 or 2.7 head on the 2.0 litre engines with some machining on the head sealing surface to match the cylinders being used.

The heads used on the 2.2-2.4-2.7 litre engines can be modified to work quite well. Stock “S” heads have well designed, large ports that are suitable for street engines. Even the small-port CIS-type heads can be opened up and flowed to make good power on these engines up to 2.8 litres.

Porsche did manufacture special cylinder heads for the 2.8 RSR and 3.0 RSR engines with different valve angles and larger valves than the 2.7’s. These had a shallower combustion chamber to match the special 92mm and 95mm Pistons and Cylinders used for these engines. These RSR heads have very large ports and are most suitable for racing engines only. The 2.8 and 3.0 RSR engines only differ from each other from having different stud spacing patterns. Due their rarity, they are now quite expensive.

The first 3.0 SC engines, ’78-’79, employed larger ports and intake manifolds than the later, ‘80-‘83 versions. These early heads are more suitable for modifications and have a good deal more airflow than the later ones. For applications requiring major camshaft and compression upgrades, both types of SC heads should be opened up and flowed for maximum airflow at all valve lifts.

The 3.2 Carrera heads used on the ’83-’89 engines represented a major increase in airflow. These are excellent heads for street engine as well as mild race engines, just as they come, from Porsche. Although these will support up to 325 BHP in stock form, airflow improvements can improve these even more for racing applications.

We have been able to make these 3.2 heads flow as well as the Factory RSR/935 heads, two benchmarks of performance. For racing and/or high RPM use, we recommend using better valve springs to help control the large, heavy valves and help prevent valve float which can occur as low as 6800 RPM with older, stock valve springs.

The 3.6 heads used a new casting that had a 3-bolt intake flange for the first time and cast-in ceramic exhaust port liners for cylinder head heat reduction. Porsche also increased the number and size of the cooling fins for better heat dissipation considering the power potential. The 993-versions of these 3.6 heads received slightly larger valves and ports for more power.

The intake flow on these heads is quite good however, the exhaust flow can be improved. Due to the ceramic liner, this is not easy to do and requires special tooling and equipment.

These were the first heads that featured twin-ignition from the factory due to its high-compression and 100mm bore. Water-cooled engines use 4-valve heads that flow far more air than the air-cooled predecessors.  

Pistons & Cylinders

For more performance, changing pistons and cylinders should be at the top of the list. The adage, “There is no substitute for cubic inches” is a truism. All things being equal, a larger engine will produce more torque and power than a smaller one. For best acceleration, the additional torque afforded by a displacement increase is money well spent. Changing pistons and cylinders is also an opportunity to raise the compression ratio. This increases power throughout the whole RPM range. The only limitations here are fuel quality issues for street cars.

Compression ratios on Porsche engines above 9.8:1 require twin-ignition to enjoy the most benefit from the compression increase and maintain compatibility with pump gasolines. Bore sizes 98mm and larger should also have twin-ignition due to the burn time required for complete combustion in these big cylinders.

Porsche used a range of different types of pistons and cylinders in 911’s. Cast iron, aluminum with iron bores, and alloy cylinders with Nikasil® or Alusil® plated bores been used in the various engines. The most desirable as well as the most durable cylinders are the Nikasil ones made by Mahle. These are aluminum cylinders that have had a Nickel-silicon-carbide plating applied to the liner surfaces. This hard durable coating eliminates the steel sleeves and allows much closer piston-to-cylinder clearances than used previously in the cast-iron/aluminum finned Biral cylinders and less friction. You must also have the oil squirters in the engine cases to use these very close clearances since these lower piston crown temperatures by 120 degrees F. These can be retrofitted into any early Magnesium or Aluminum case by a competent machine shop. Most of the US 2.7 and 3.0 litre SC engines came with the Alusil cylinders that Porsche used to reduce costs. These were an aluminum cylinder with a high-content silicon surface that had good wear resistance. These used iron-plated pistons to reduce the piston scuffing that would occurred with the aluminum pistons running in aluminum bores.

Porsche’s premium pistons are made by Mahle of Germany. Their metallurgy was and still is, amongst the best in the world. They were the only foundry  capable of forging a high-silicon, low expansion piston for over 30 years. All other forged pistons made in the USA and Great Britain used alloys that required more clearance to prevent piston seizure when hot. Naturally, when cold, these pistons make a more noise until the engine heats up. Further, the tight clearances allowed by the Mahle pistons also allowed a better ring seal for better compression and improved leakdown figures. Currently, American piston manufacturers such as CP/Carrillo or JE make similar pistons which can be configured for reasonable clearances for competition or special purpose engines.

As an example, if you wanted a high-compression piston for your 2.7 RS-spec engine, you had to purchase the racing set from Mahle, cylinders and all, for a very high price. Now you can simply order a 90mm piston set with whatever compression ratio you desire.

Porsche has used cast pistons in all of their low and medium RPM engines since 1966. Only the high-RPM 911S has been equipped with forged pistons. The 911S models and 2.7RS as well as other high-performance and racing Porsches have all been equipped with forged pistons for their strength and durability at continuous RPM at or above 7000. Cast pistons may be used reliably up to 6700. Extended engine operation above this range would be much safer using forged pistons from either Mahle or JE.

Piston upgrades for 911’s consist of either installing a high-compression version of the existing components, or using larger pistons and cylinders.

2.0 Engines:

These can use the larger bore P & C set from the 2.2 in “E” or “S” versions. These engines can also use the 2.8 RSR piston & cylinder set to make 2.6 litres. 90mm RS sets result in a very low compression ratio unless major machine work or custom pistons are used. Nikasil cylinders will require that piston squirters be installed.

2.2 Engines:

Typically, we use the high compression set from the “S” or JE pistons in whatever compression ratio you want. You can also use the 2.8 RSR piston & cylinder set here for 2.6 litres, as well. This results in a usable compression ratio and makes a very strong street/track engine with Solex or “S” cams. Nikasil cylinders will require that piston squirters be installed.

2.4 Engines: 

These can use the 2.2 “S” piston sets and gain a nice jump in compression from 8.5:1 to almost 9.6:1. This is quite worthwhile doing!

2.7 Engines:

These usually use the 2.7RS set with the valve cutouts for use with higher lift cams. These are 8.5:1 and some machine work can increase this to around 9:1. Higher compression rations require the Mahle 10.3:1, 90mm racing set or JE pistons. The 92mm piston & cylinder set used on the 2.8 litre RSR is recommended only for track use due to the resultant high compression ratio from using the small chamber non-RSR heads. We would also strongly recommend installing twin-ignition to take full advantage of these pistons & cylinders.

3.0 Engines:

These engines have a wide selection of piston & cylinder sets available for many different configurations. You can use a high compression version of the existing 95mm pistons, or enlarge the displacement by using 98mm (3.2), or 100mm (3.5) piston and cylinder sets. The 95mm and 98mm sets are available in piston shapes compatible with CIS injection or Carburetors and Mechanical Fuel Injection. The 100mm sets will work OK with CIS although the dome shapes were made for high-lift cams and different induction systems. We recommend twin-ignition with any of the higher compression versions on these sets.

3.2 Engines:

The Carrera power plants also have a good selection of piston & cylinder sets to choose from. One of the most popular conversions is the 98mm “Max Moritz” wedge-dome set. This is a 3.4 litre upgrade with a 9.8:1 compression ratio that has a special dome shape compatible with CIS and Motronic engines. You can also use the 100mm set to make 3.5 litres. The 102mm 3.6 is also an option. The 100mm cylinders will require additional machine work on the case to open the spigots up. These large-bore engines should use twin-ignition with any compression increase for best power and durability.

3.6 Engines:

102mm piston & cylinder sets are available to make 3.8 litres and we have made some custom nikasil cylinders with 102.3mm bores to make an exact 3.8 litre engine. 4.0 conversions are also possible with a custom crank, rods and pistons.

No matter which piston and cylinder set you may decide to use, make sure that you have calculated the compression ratio, checked the deck height, and ensured adequate piston clearance at TDC as well as BDC.

The Bottom End

This section refers to the cases, crankshaft, connecting rods, and the oil pump.

Porsche used Aluminum and Magnesium for engine case material from 1965 to the present. The first 2.0 litre engines used a sand-cast Aluminum case from ‘65 to ‘68. In 1969, the factory used a pressure-cast magnesium case to save approximately 22 lbs over the equivalent Aluminum one. Porsche even won an international award for this technology of high-pressure casting of metal. This had only been done before with plastics. Various versions of magnesium cases were used from ‘69 until ‘77 until the higher operating temperatures as a result of emissions compliance made it unfeasible to use this material.

The two most common versions of the magnesium crankcases were the 4R/5R case and the final 7R version which was the strongest one. This had the most stiffening ribs throughout the entire casting and is the most desirable for building a powerful 2.7 or 2.8 engine. The internal physical differences between the 4R and 7R are quite apparent when compared side-by-side. Modifications to these mag cases are strongly recommended for reliability and include installing Case savers in the head stud holes, decking, and line-boring. Shuffle-pinning is recommended for race engines. We do NOT recommend “boattailing” the webs in these since they have enough natural flexibility that removing metal merely exacerbates the lack of stiffness in these cases.

Porsche switched to aluminum for the Turbo and subsequent 3.0-3.2 and 3.6 engine cases which excellent foundation for a high-horsepower engine. These cases can be “boattailed” quite successfully. This makes a marked difference in power when engine RPM’s exceed 7000 due to reduced windage losses.

The 3.0 and 3.2 cases are basically interchangeable. The 3.6 engine is quite different due to cast-in oiling passages, cylinder base sealing, and the use of hydraulic lifters in later versions.

Porsche crankshafts and connecting rods, with few exceptions, have been quite reliable and bulletproof in high-performance and racing applications. The 2.0-2.7 and 3.0 rods are quite strong with the 911S nitrided rods being preferred for those smaller engines. The 3.2 and 3.6 rods uses a smaller 9mm bolt and these should be replaced with ARP ones for durability and reliability when higher RPM use is expected. The stock rod bolts used in these engines are not reliable beyond 7000 RPM. Installing a performance chip into a 3.2 Carrera can raise the rev limiter to levels that are unsafe with the OEM bolts. If sustained RPM over 7000 is anticipated, we strongly recommend using Carrillo or Pauter Engineering 4340 steel rods. 993 rods are of a different design than earlier versions and we strongly recommend replacement with aftermarket rods if the engine is to operated beyond 6800 RPM!

Long-rod engines have been used successfully in certain competition applications. Changing the rod ratio by using longer, custom rods worked quite well in Porsche engines that use the longer-throw crankshafts like the 74.4mm and 76.4mm cranks. This modification improves the rod ratio and changes the torque curve somewhat as well as reduces the piston skirt side thrust loading for less frictional losses.

Aside from the non-counterweighted 2.2 crankshafts, the rest of the OEM cranks are suitable for racing provided they are prepared properly. A thorough cleaning, micropolishing of the journals and a precision balance is all that’s needed for a reliable street crankshaft. We recommend cross-drilling the crank (with the associated case and bearing modifications) for engines that will see continued operation above 7K. Porsche did make some special crankshafts for the 2.8 RSR and 3.0 RSR engines with larger crank fillets however, these only fit specific engine cases and require the special RSR bearings. Due to their rarity, these are now very expensive.

Oil Pumps

Porsche has used several different configurations for the dry-sump pump inside the cases. The oil pumps used in the 930 Turbo engine and the GT-3 RSR are still the largest of the production pumps with the most pressure and scavenge capacity, with the 3.6 pump running in second place. The next best pump to use is the 911SC/Carrera oil pump. This pump can be used inside any Porsche engine case by making the oil bypass modification, using the late oil pressure relief spring and valve, and the correct intermediate shaft. Porsche used aluminum and cast iron for the various pumps, finally using an magnesium cased pump for the 3.6 litre engines.

High-horsepower cars used in competition should use the GT-3 RSR (above) or Turbo pump for best lubrication and scavenging.

A related issue is the oil thermostat. The Factory-style oil thermostat is a very special device. This regulates oil pressure, as well as oil temperature, to and from, the front auxiliary oil cooler. The pressure relief function of the Factory thermostat ensures that the inlet side of the oil pump is never starved due to the pressure drop in the long lines to the front cooler. Aftermarket oil thermostats such as the Mocal unit do not have this feature and you risk starving the engine of oil pressure during peak pressure demands.

 As always, this is merely an overview on things to consider when modifying the 911 engine. This is by no means, a thorough discussion of all issues related to this subject. When deciding on a plan to improve engine performance within a given budget structure, consult with your engine-builder and use a systematic approach for a program of performance modifications. There isn’t a great deal of material to read on this subject since the most successful people in this area consider some information proprietary. Because of the sheer expenses involved here, do your homework!  

Intake Systems – Camshafts – Exhaust Systems

One of the main attractions of the Porsche 911 is that fabulous engine and the sound it makes! Besides the physical aesthetics, this one of the reasons why the 911 has enjoyed such loyal ownership since 1965.

No other engine design in recent times has been so over-engineered. From the dry-sump oiling system to the 8 main bearing bottom end, the air-cooled 911 engine is another Porsche benchmark of performance, reliability and durability. This is the main reason why the 911 has been such a successful race car all over the world. Let’s look at this powerplant and see how we can improve its performance even more without losing any reliability.

Since the internal combustion engine is nothing more than an air pump that harnesses the expansion of hot gasses to perform work, making changes to increase the airflow in and out of each cylinder is the prime directive. The more air and fuel that can be delivered to the cylinders, the more power will be generated. Other modifications that enhance the efficiency as well as durability will be discussed.

Induction Systems

Porsche 911’s have been equipped with a wide range of induction systems; some of which were optimized for emissions compliance, not performance. Once example is Bosch K-Jetronic Fuel Injection which was used from ‘73 to ‘83 and intended to meet stricter emissions and fuel economy regulations.

Here’s a lineup of Porsche 911/930 induction systems used since 1965:

  • Solex 40 PJ Triple-throat Carburetors
  • Weber 40 IDA – IDS 3C Carburetors
  • Weber 46 IDA 3C Carburetors (racing only)
  • Zenith 40 TIN Carburetors
  • Bosch Mechanical Fuel Injection (various sizes and configurations)
  • Bosch K-Jetronic Fuel Injection ( also called CIS)
  • Bosch Motronic (various versions-some drive-by-wire)

Each of these systems has strengths and weaknesses for performance purposes. The best performance potential will be found using Weber or PMO Carburetors, Mechanical Fuel Injection, and some aftermarket EFI intake systemsuser-programmable EFI Engine Management Systems such as MoTec and AEM.

Solex Triple-throat

Some vintage racing venues may dictate the use of the original Solex overflow carburetors. The Factory could not cure the “flat spot” that they were known for however, there are now some tuners that can make these run very well.

Zenith Triple-throat

The excellent Zenith 40 TIN carburetors are not the best choice for performance applications due to a lack of venturis and jet options. These carbs were equipped with 27mm venturis that restrict power potential to well below 175 HP. With the limited range of jets and other tuning parts, performance engines are bestgwith these are best upgraded to PMO or Weber carburetors.

Weber & PMO Triple-throat

The ubiquitous Weber IDA 3C-series of triple-throat carburetor and its much more modern derivative; the PMO carburetor, remains one of the mainstays of Porsche performance upgrades for street and racing. Available in 40mm, 46mm and 50mm (PMO only) throttle sizes, these carburetors offer excellent reliability, horsepower and adjustability. These features have endeared themselves to Porsche owners worldwide. The overall adaptability of these carburetors to be used on any size engine from 2.0 litres to 3.8 litres, and their owner friendliness makes these an excellent choice. Fuel economy is not their strong suit; simplicity and reliability is what Webers and PMO’s are all about.

These carburetors are not without their faults and idiosyncrasies. They need absolutely clean fuel and carefully regulated fuel pressure to be reliable and trouble free. For best throttle response, the float levels must be set very precisely with the proper Weber float level gauge and this process requires a lot of patience. PMO’s use sight glass windows on the float chambers for the same purpose. Time invested here will pay off with a very crisp and responsive running engine. The older generation 46mm Weber carburetors also need additional modifications to the idle and progression circuits for best throttle response.

Water and dirt are the enemies of any fuel delivery systems and this holds true especially for Webers. Using a Racor Model # 110 Fuel filter/Water separator and the Holley 1-5 lb fuel pressure regulator set at 3.5 psi, makes a very reliable and trouble free installation for a carbureted 911.

One mistake that some people make is using a too-large main venturi that will ruin the excellent throttle response that these carbs are known for. Venturi sizes are largely determined by engine displacement, camshaft profile and the operating RPM range of the engine.

Here are a few combinations that work. Variations are made for different operating conditions, engine configurations and gear ratios.

2.0 to 2.2 "S" 40mm Webers32mm
2.4 to 2.7 "S" 40mm Webers34mm or 36mm
2.8 "S" 40mm Webers36mm
2.8 RSR46mm Webers38mm or 42mm
3.0 "S" 40mm Webers36mm
3.0 RSR46mm Webers42mm
3.2 "S" 46mm Webers38mm
3.5 "S" 46mm Webers42mm
3.5 RSR46mm Webers42mm(Not really enough)
3.5 GE60 or 120/10446mm Webers42mm
3.8 GE80 50mmPMO's46mm venturis (Or larger)

Remember these are only guidelines. Other factors such as well-flowing heads, camshaft profile and very high compression ratios can change these numbers.

Other performance-enhancing options for Weber-equipped cars are the tall, auxiliary venturis developed for the 906 engine. These really enhance the metering signal “seen” by the main circuit to sharpen the throttle response when using slightly-too-large main venturis for maximum power. The tall manifolds used on the 906 and 911R were developed to help contain the reversion in the intake tract from using camshafts with lots of valve overlap. Reversion causes a ‘fuel cloud’ to form over the individual intake trumpets that plays havoc with fuel mixture at higher RPM’s. Spacing the carburetor farther away from the intake valve with taller intake manifolds, increases torque and throttle response, especially with racing-sized venturis.

Richard Parr at PMO has completely re-designed, improved and updated the Weber IDA 3C-series carbs with his own PMO’s. These are now available in 40mm-46mm-50mm sizes that address all the flaws and shortcomings of the mid-fifties Weber designs. PMO’s run much better at less than full-throttle than the equivalently configured Weber and their ball bearing supported shafts will outlast Webers, hands down.. The 50mm carburetors provide an alternative to racing FI systems for large, high-powered engines used on the track.

A personal note about carburetors; Some people have experienced issues using Weber or PMO’s and such  can be traced to dirty or contaminated fuel, setup and maintenance. If Weber’s or PMO’s are precisely installed and setup, are supplied with clean fuel without dirt or water, this induction systems is every bit as reliable and trouble-free as the CIS and Motronic FI systems. Most of the carbureted cars that we have setup in the past 25 years have required little, or no attention whatsoever. This is due to precise fuel pressure control, first-rate fuel filtering and careful float adjustment and tuning. Human factors and installation errors are FAR more responsible for carburetor problems, than their inherent design or layout.   

Bosch Mechanical Fuel Injection

Bosch Mechanical Fuel Injection is a splendid, if not inexpensive, performance induction system. It was used on the 911S and 911E from ‘69 to ‘73 as well as the ‘73 Carrera RS. Porsche used various types of MFI on factory racing cars from ‘67 to ‘84. This is a very complicated fuel delivery system that, if set up properly and in good condition, makes excellent power and throttle response.

The trouble is, few people anymore possess the necessary knowledge and experience to make these systems run well and Bosch has little documentation available for the owner. MFI Werks, Pacific Fuel Injection and Eurometrix are the most reputable re-builders of injection pumps and throttle bodies.

Injection pumps can be recalibrated to almost any engine size, however camshafts remain a limitation You cannot go deviate from factory cam profiles since the space cam inside the pump must match the fuel delivery requirements. 2.7 RS-spec engines equipped with these systems continue to be quite popular since the requisite space cam for the pump is still available. Eurometrix can enlarge the throttle bodies to 40mm for larger engines as well.

Tall butterfly throttle stacks from the ’73 2.8 RSR and 911 SC/RS are one of the good upgrades for engines used for racing purposes and the 50mm throttles work quite well for 3.2 litre and larger racing engines. Another upgrade to consider are the RSR throttle slides from the ‘74 3.0 RSR. These have been re-manufactured by various European tuners for racers as well as restorers.

Here is a ’73 2.7 RS with MFI and custom aircleaners. RSR fuel lines were used for clearance as well.

The early FI pumps are better suited for the RSR modification and the later pumps used on the 2.4 litre engines are better candidates for an RS-type upgrade.

Bosch K-Jetronic (CIS)

The Bosch K-Jetronic (CIS) FI systems are not well suited for those seeking more horsepower. There are several factors that restrict the airflow potential of this design. Intake manifold size and configuration, airbox and sensor plate restrictions do not permit big HP numbers. Another major problem is the inability to use any sort of performance camshaft profile with the CIS injection due to severe intake pulsations caused by long-duration camshafts. 964 or Webcam 20/21 profiles are the only choices.

Large displacement (3.5 litre) CIS-equipped engines can be great performers where emissions testing makes any induction substitution illegal.

Bosch Motronic

The Bosch Motronic system, also called Digital Motor Electronics, has been used on 911’s since 1984. The Turbo didn’t get a Motronic systems until the introduction of the 993 Twin Turbo in 1996 and the latest versions use Drive-by-Wire technology instead of a throttle cable. The rate at which the throttle is opened is determined in software, not by the driver’s foot.

Motronic FI is a fully computer-controlled engine management system with various sensors on the engine for intake air temperature cylinder head temperature, altitude, throttle position, airflow, fuel mixture, and crankshaft position.

Modifications that are feasible for Motronic-equipped 911’s made from 1984-2006 include performance chips and in a few cases mass-airflow sensors (Carrera & 964). The intake manifolds used on the 3.2 Carrera engines suffers from unequal air distribution between cylinders due to casting variations. Airflow in the stock intake runners vary between 180 to 290 cfm. Using the Extrude-Hone process, these runners can flow 300 cfm to 335 cfm.

Performance chips change the ignition timing and the fuel delivery curves to values that sharpen the throttle response and extend the RPM range. Power increases for the OEM chips vary from the 3.2 to 3.6 litre engines. Each chip manufacturer uses different mapping in these timing and fuel curves to achieve different results. Caution is advised where overly aggressive ignition timing mapping that may not accommodate the fuel octanes in your area.

The best advice we can offer is to purchase a chip from a reputable manufacturer that stands behind their products and will make you happy. Remember, many East Coast chip tuners do not make allowances for the 91 octane fuels used on the West Coast.

Mass-airflow sensors and enlarged throttle bodies are both quite expensive for what they do and are best used on engines equipped with substantial exhaust changes and displacement increases. The cost per HP for these two items is quite high.

The 3.6 litre engines have improved intake manifolds that do not need further enlargement for street use. You can install a chip in these ‘89-‘95 3.6’s for improvements in throttle response and power improvements ranging from 10-15 BHP depending on fuel. The Motronic systems used in the ‘96 and later 993, along with the water-cooled cars , use new software and hardware incorporating On-Board Diagnostics II, or OBDII. This fully adaptive system is more difficult to reprogram for performance increases. This must be done at the code level and few people in the world are capable of doing this without creating fault codes or shut downs. The Varioram-equipped 993’s make more power and have better throttle response than the earlier versions of this engine due to the unique high-velocity, three stage intake manifold and slightly larger ports and valves. All of the water-cooled cars really respond to proper chip tuning and typically see torque increases around 15-18 lb-ft. 996/997 Twin-Turbo cars with software upgrades will see a 60-70 HP improvement.

Individual Throttle Butterfly Intake systems

These intake systems, also called ITB’s, are single-throttle-per-cylinder intakes offered much better throttle response and airflow capacity for increased performance. Since these systems are not as sensitive as OEM single-throttle intake manifolds, one may use any camshaft profile that’s appropriate for the application. Engine Management such as Motec must be used with any of these systems.

Generally speaking, there are two different types of ITB systems: individual stacks such as what Porsche used on the old MFI (’73-’75) RSR and resonance-plenum/ITB systems where the throttles are located under the individual runners of the resonance plenum. GT3 resonance plenums are used in these applications with custom ITB’s and linkage.

There are variants of resonant-plenum/ITB systems which allows one to tailor the application based on engine displacement, camshaft profile and type of use.

OK, But What do I use?

As you can see, there are lots of choices when deciding how you are going to get the fuel & air into your engine! There are several factors to consider before making this decision.

  • Budget
  • Tuning Adjustability
  • Ease of Maintenance
  • Reliability

Some of these system prices lie beyond what most people can spend on this part of their project. The very best systems to use where maximum performance is the main objective and there are few budgetary restrictions would be an ITB/Engine Management Systems with MoTec.

K-Jetronic, while a simple, reliable FI that meets its design criteria, isn’t optimal for performance applications due to its intolerance of any performance-type camshaft, somewhat sluggish throttle response and limited airflow capacity. For applications that require emissions compliance, good fuel economy and drivability, this may be the best alternative. The only concern with CIS is the lack of new parts which will be problem as time elapses.

Motronic FI can be configured to work quite within its performance parameters. Intake manifold modifications, airflow sensors and re-programmed chips all combine to make this a good for cars that must remain smog-compliant.

In the case of street-driven 3.2, 3.6 and the water-cooled cars,  the factory Motronic software can be improved upon and performance improvements work quite well in an emissions conscious environment. These systems tend to quite reliable with only some sensors giving much trouble. We do recommend anyone with this system carry an extra DME relay and cylinder head temp sensor.

Mechanical FI is one of the best of the performance-oriented induction setups available to Porsche owners. Not the cheapest by far, these systems make great power on a range of engine sizes. This system does require specialized knowledge to setup and maintain properly. Adjustments are limited to some mixture adjustment and idle air for each stack. Other, major adjustments for fuel tracking must be done by a specialist. If possible, start with the “S”-spec throttles and stacks when deciding to use this induction systems in performance applications. Throttles can be bored to larger sizes and FI pumps recalibrated to RSR spec. The magnesium throttle stacks found on the 70-71 911S, flows a little more air than the later plastic versions. You can also fit aircleaners much easier to the early type throttle stacks.

Weber & PMO carburetors in their many sizes and configurations remain one of the overall favorites for performance upgrade and racing applications. Their tunability and parts repertoire make these very adjustable as well as suitable for street and track operation. By changing main venturis and jets, these carburetors can be made to work on a wide range of engines. In addition, these are very reliable units and almost anyone can maintain these with minimal hand tools and a copy of the excellent Weber manual from Haynes. As mentioned earlier, the PMO carburetor addresses all of the shortcomings of these systems and may prove to be the ultimate solution in many cases.

EFI systems have become far more affordable in recent times and with their inherent flexibility and tunability, these are now the preferred choice for most performance applications; street or race. High quality engine management systems from MoTec and DTA are amongst the best and easiest to tune although they all require some dyno time to optimize. Proper tall-butterfly throttles or resonance-plenum/ITB’s are designed to center up over each intake port (unlike carbureted and MFI systems) and offer much greater power potential and drivability.


Camshafts are said to be the “heart” of an engine. Nothing could be more true in the case of the Porsche 911 powerplant. Camshafts determine the “personality” of the engine and  effective operating RPM range. Since they also affect emissions, you should check the applicable laws in your location to see of this would be legal in cars that are driven on the street.

There is a wide range of camshaft choices to choose from and each has its own particular disposition and hardware requirements so you get the performance that you expect.

Several factors to consider when deciding what camshafts to purchase and install in your engine would be:

  • Application (street or track)
  • Engine size
  • Compression ratio (pump or race gas)
  • Gearing (close ratio is necessary for high-overlap cams)
  • Budget

Since street engines spend most of their time below the power peak, picking a camshaft profile that will idle well, make good usable torque and throttle response, and adequate peak power is very important. Fortunately, Porsche has offered some excellent cams as well as some great aftermarket ones. Racing engines require as much camshaft as one can stand without destroying the mid-range torque characteristics. It does no good to have a camshaft that produces its best power between 7000 RPM and 8500 RPM, when you have many slow corners to accelerate out of. This sort of power range will require a lot of shifting that may be hard to make up with a track full of slow speed turns and short straights.

Here are some camshaft timing numbers. Look at the differences in duration and lift to see the spread of camshaft characteristics. Another clue to camshaft “personality” is the lobe centers.

The closer that the lobe centers are spaced together, the longer both valves are open at the same time; so the smaller the lobe center value, the closer they are. This is what causes the fuel reversion above the intake stacks and makes the powerband “peakier”. Notice the “Solex” cam numbers, this cam has closer lobe centers that the “S” , but less duration and lift. This shows how close this cam is to the “S” in power characteristics with the “S” cam having a little more peak RPM.

CamshaftIntake Lobe CenterExhaust Lobe Center
911 T216 deg. @ .387_207 deg. @ .345_ 105 deg.
911 E230 deg. @ .408_222 deg. @ .393_ N/A
911 S264 deg. @ .450_236 deg. @ .400_ 101 deg.
Solex244 deg. @ .439_234 deg. @ .406_ 97 deg.
911 SC229 deg. @ .455_220 deg. @ .402_ 113 deg.
964 3.6240 deg. @ .464_230 deg. @ .425_ N/A
906282 deg. @ .465_252 deg. @ .406_ 95 deg.
RSR (sprint)282 deg. @ .465_266 deg. @ .450_ 99 deg.
3.8 RSR272 deg. @ .485_256 deg. @ .485_ 109 deg.
993 RS 3.8240 deg. @ .490_226 deg. @ .446_ 110 deg.
GE-40256 deg. @ .470_238 deg. @ .440_ 102 deg.
GE-60266 deg. @ .490_248 deg. @ .455_ 102 deg.
GE-80274 deg. @ .500_256 deg. @ .470_ 100 deg.
GE-100284 deg. @ .520_266 deg. @ .490_ 100 deg.

Compression ratio and gearing go hand-in-hand in making power and helping the car to accelerate well. Using long duration camshafts such as 906, RSR, Schrick and GE80’s leaves both valves open for a period of time that lowers effective cylinder pressures at low RPM. This affects low and mid-range torque until the engine comes up closer to its torque peak. Raising the compression ratio helps “fill-in” the lack of torque with racing camshafts at off-peak RPM’s. You can run more static compression with racing cams than a street-type camshaft without risking as much detonation problems. Requirements for Twin-ignition and racing fuel remain the same!This just a small sampling of the camshaft profiles that are available. Your engine builder should be the person who makes this critical decision.  Engine size determines the “personality” of a given camshaft. For example; an “S” cam in a 2.2 or 2.4 litre engine makes a sweet, revvy, high-RPM engine that really takes off from 5200 to 7300 RPM. Not much happening below 4000 RPM! Now,…..install this same camshaft into a 3.2 or larger engine and you will get characteristics like good power from 3000 to 7000 with no “big hit” at 5200. Just good smooth power and idle qualities much like CIS cams. These characteristics are even more evident in a 3.6 litre engine. To get that revvy sensation and a big power hit in those larger engines, you must use a 906, RSR, or GE80-type cams. Using a 906 cam profile in an engine 3.0 litres and below, makes a very peaky combination, much like a 67 “S”.

Close-ratio gears also keep the engine RPM into its power range, allowing a peakier camshaft to be used without as much penalty from waiting for the revs to come up.

Budget is also a very important factor here. Many camshafts such as the popular “S” cam, require additional piston-to-valve clearance to avoid catastrophe. You cannot install these cams in a 3.0 litre CIS or 3.2 litre Motronic engine without installing the proper pistons that have the reliefs for both valves machined into the piston crown. In some cases, you should replace the stock cast pistons with forged ones for strength at the higher RPM that the new camshafts will allow. In the case of the 3.2, 3.3 Turbo and 3.6 litre engines, the OEM rod bolts are prone to fail at RPM’s close to 7000 and the cost of replacing these with Raceware or ARP rod bolts must be factored.

These two items alone can make this an expensive proposition. Regarding the engine as a system will keep everything in perspective regarding the various upgrades. Reliability should be paramount in the execution of any performance program.

Exhaust Systems

Since the introduction of the 67 911S, Porsche has used some of the best exhaust systems on street-driven cars until the 1975 911. This excellent 3-into-1 header, wrapped by steel shrouding for heating, was installed on all 911’s from 1968 until 1974. From 1975 until 1994, 911’s sold in North America have been forced to comply with ever-increasing emissions standards and noise regulations. This has forced a re-design of the original exhaust system design that has cost a good deal of power. The introduction of the 993 brought an exhaust design reminiscent of the pre-‘75 systems. This makes a nice upgrade for the 89-94 C2/C4 cars that retains the durability and heat capability of the factory car. An aftermarket muffler may be added to these cars for further power increases.

Current Federal Law makes it illegal to tamper with any part of the emissions system and this includes everything from the engine to the catalytic convertor. Mufflers are free to change or modify.

The ‘75-‘89 911’s used for track events can benefit from using the pre-‘75 exhaust systems with the header-type, 3-into-1 heat exchangers from SSI and B&B with a dual-inlet muffler. Not all of these systems are trouble-free so do your research! The average 911SC & 3.2 Carrera will see a power increase of 15 to 22HP with this upgrade depending upon muffler choice.

Some of the aftermarket exhaust systems create a loud droning inside the car at cruise speeds that is quite tiring after a while. We suggest speaking to your shop as well as other users of different systems to see what they like or dislike about their system.

Aftermarket Stainless Steel exhaust components are made by several different manufacturers. Here is a partial list in no particular order:

  • SSI Heat Exchangers (now made by Dansk)
  • Monty Mufflers (expensive)
  • Dansk (much more reasonable)
  • Factory GT3 (used for 3.6 & larger engines)

Some of these systems use proprietary components that only allow compatibility within that manufacturer’s product line so be sure and do your homework. The factory-type heat exchangers from SSI allows any OEM-compatible muffler to be used.

Racing 911’s can install headers instead of heat exchangers for weight reduction and improved cooling from the exclusion of the heating shrouds. Power improvements from the use of headers vary with camshafts, cylinder head flow characteristics and muffler choices. A common mistake is using headers with primary pipes that are too large for the engine size and configuration. Nothing will kill the low and mid RPM power quicker than headers that are too large! The other critical dimension is the pipe length from the header collectors to the muffler. If this length is too short, the engine will not develop much torque. Cross-over pipes increase overall torque and help reduce the noise levels.

As racetracks around the country and the rest of the world become noise-conscious, the consequence of properly designed racing mufflers will be very important. A delicate balance must be struck between flow, volume, weight and noise levels. Some racetracks already require a noise level of no more than 90 db and this is difficult to attain without restricting power on 911’s with racing type camshafts

Here are some guidelines for 911 engines equipped with “S” cams or larger. Using RSR, Schrick or custom camshaft profiles can allow the use of larger headers. You should consult your engine builder for advice.

2.0 to 2.41 ½" or 1 5/8"
2.5 to 2.81 5/8"
31 5/8" or 1 ¾"
3.21 5/8" or 1 ¾"
3.4 to 3.51 5/8" or 1¾"
3.6 to 3.81 5/8" or 1¾" or 1 7/8"

We have had excellent results using European Racing Headers since they are inexpensive and perform well. These are made in different sizes and are of equal-length construction and make better torque across the RPM range than the factory ones. Other stainless-steel merge-collector headers are used for 3.8 and larger engines.These sizes are based upon street and racing applications. In the case of 911’s driven on the street as well as track events, use the next smaller size if in doubt. SSI Heat Exchangers are only available in 1 1/2″ so they have limited applications on engines larger than 3.2 litres.


The best way to make your Porsche faster is to learn to drive it at its fullest potential! As delivered, Porsche automobiles are capable of performing far in excess of most owners’ ability. Investing in Driver Education and formal racing instruction is more cost effective than any piece of hardware that you can install on your Porsche. Driver Education and track time will make you a better driver than initially upgrading springs, shocks, or swaybars.

That said, the main goal in improving your Porsche’s handling is actually making it easier to drive and more controllable when driven near or at the limit of your ability. The factory suspension settings are a compromise between handling and ride quality, therefore any upgrades change the package to one that is specifically tailored to the way you use and drive your car. Some of these components have a negligible effect upon ride quality while others can transform your car into a race car with the attendant degradation in ride quality. We always indicate which items have the smallest impact on maintaining a streetable ride.

911’s, 930’s, 964’s, 993’s, 996’s and Boxster’s all have different components and suspension design differences. We will discuss the various parts that may be upgraded with specific recommendations for each model for street and occasional track use. There are four main suspension components that can be changed for higher performance:

1) Torsion Bars & Springs

2) Dampers (shock absorbers)

3) Anti-Sway Bars

4) Suspension Bushings or Bearings

Other items that are usually considered are Front Shock Tower BracesBump Steer kits, Spring Plate kits, and complete suspension systems for racers. This last item usually consists of replacing torsion bars with coil-over suspension pieces. There are some specialty parts such as ERP Products that replace the entire front and rear suspension with custom made pieces that are derived from the Porsche 935. These are very high quality components that save weight and make suspension adjustments much easier and more precise for competition.

Torsion Bars & Springs

These two items perform the same function respectively, for the 911/930 as well as the 964/993 and water-cooled Porsche automobiles. Porsche used torsion bars of varying sizes on the 911 from introduction in 1964 to 1989. The 930 also used torsion bars from 1975 to 1989. In 1989 the Carrera 4 (964) was equipped with coil springs and the rest of the 964 and 993-based cars followed suit. All of the water-cooled cars use coil springs.

Torsion Bar sizes can be upgraded to larger bars in a wide range of sizes. The modern hollow ones help to alleviate the added weight penalty of using larger, stiffer torsion bars. Some of the aftermarket bars have not been of particularly high quality and have exhibited sagging as well as not being the same spring rate as the factory offerings so we are quite particular about which ones we use.

Installing larger torsion bars has a small effect on ride quality up to a certain point. You can install bars that are 25% stiffer without a serious degradation, unless you live where the roads are really terrible. Removal and installation require just basic tools.

Coil springs can also be upgraded with shorter, stiffer versions that reduce lateral weight transfer and lean angles of the 964/993 and water-cooled cars as well as closing the excess gap between the fender and the tire. Porsche Cup suspension system used by the factory for various racing series are too stiff for street use unless you have masochistic tendencies. This stuff is really stiff!!

964, 993 and water-cooled cars gain the greatest handling improvements with the installation of a good aftermarket coil-over system of matched shocks and springs such as Bilstein PSS-10, KW Variant 3’s and Motion Control shocks. This permits ride height adjustments from 0 to-2″, precise corner-weighting, and the fitment of some larger, adjustable swaybars.

Dampers (Shock Absorbers)

Shrader valve on cannister

Shock absorbers (correctly called dampers) perform a multitude of tasks that are critical to optimum handling especially on bumpy streets and race tracks that have many changes in direction.

Dampers MUST be matched to the springs for the suspension to function properly. This is accomplished by selecting components from a manufacturer with a great deal of experience. The manufacturer must offer several choices that are very close to optimal, or adjustable dampers that will allow a range of adjustments. These may not have very wide range of adjustment. You may need to choose the type of damper, specifically for street, sport or racing, to get the range of adjustment that you need.

Dampers that are properly selected and valved manage the rates of lateral, longitudinal and diagonal weight transfer during cornering and braking. They also dampen spring oscillations and keep the tires in contact with the pavement. Dampers control both the amount of weight transfer and the rate at which this happens.

Basically, this means that all weight transfer transients, such as when entering and leaving a corner or changing direction, are controlled by the damper settings. The rate of weight transfer caused by braking and acceleration is also affected by damper valving. Dampers come in different valving combinations depending upon whether they are for street use or track. The damping rates are expressed in two numbers, rebound over compression. These number are expressed in kilograms or pounds at a certain velocity, depending upon the maker. For example, an RSR shock for the 73-89 911 has 180/170; rebound/compression damping rates. The rear Turbo(930) dampers are 136/65 for comparison.

Some dampers are adjustable (Motion Control, JRZ, KW V3) and some are not (Boge/Monroe). Either way, the object is to have the dampers adjusted to match the street/track surface, spring rates, and sprung/unsprung weight of the car.

A common mistake in setting up a street 911 is to have too much compression damping in the front end. Many people install sport-type dampers on the front needlessly, and suffer the consequences of an insensitive and skittery front end. Another common mistake is to have the ride height too low in the rear. When the rear dampers bottom in a corner due to surface changes or weight transfer, the effective spring rate leaps to infinity and the car can spin quite quickly without much warning and this can be quite exciting! Using dampers that are shorter than the stock ones will really help maintain travel when the car is lowered.

There are three ways to set the dampers correctly. First, install double adjustable types, such as Motion Control, JRZ, KW V3, and experiment with settings until the car feels better and the lap times confirm this. Second, use off-the-shelf components that have been optimized by the manufacturer. Third, use those same parts that have been re-valved with a custom setting optimized for your car by someone with experience and knowledge in this area.

All three are quite acceptable methods and vary only in the time required to set the car up.

The MacPherson struts used on the front suspension present a different set of challenges. Porsche has used Bilstein, Boge, Koni, Fichtel & Sachs, and Woodhead struts on the front of these cars since 1965.

Shock manufacturers have conveniently painted their products to make identification much easier. Bilsteins are usually green or yellow, Konis are red or yellow, Boges and Monroes are black.

All but the Bilstein struts share the same basic configuration and design. The High-pressure DeCarbon design used by Bilstein allows the damper to be operated in any position. To lower unsprung weight, these attach to the body at the top of the strut so that the unit ‘s heavy end is bolted to the car instead of the suspension arm. Bilsteins basically operate in the “upside down” position, compared to the other brands of dampers and some H&R struts employ the same technology.

Bilstein’s design has another advantage for the 911/930 owner, the ability to place the spindle in a more optimum location on the strut tube for better geometry and handling. Since the damper cartridge is “upside down”, the tube body is a uniform diameter with no hydraulics behind the support tube. This allows one to relocate the spindle upward, to restore lost suspension travel after lowering the car. This will raise the front roll center back up where it belongs and restore lost suspension travel. The spindle height limit is determined by the choice of wheel diameters. 15″ wheels allow no more than 18mm change in spindle height due to ball joint interference with the inside rim. Using larger wheels will allow higher spindle heights.

When the spindle is raised, one must use a proper bump steer kit such as ERP or Elephant to allow one to adjust and fine-tune the bump-steer curve for best handling.

Anti-Sway Bars

Anti-roll bars, commonly called “sway bars”, are actually transverse torsion bars that attach to each side of the suspension arms. These function by offering varying degrees of resistance to body lean and lateral weight transfer. Changing the position of the attachment point or the length of the moment arm that the torsion bar acts on, allows you make adjustments in the effective stiffness of a given bar diameter.

Some bars offer slider adjusters, spaced mounting holes, or rotating blades to allow you to fine tune the stiffness. There are even some bars that feature cockpit adjustable units that allow you to adjust the chassis balance for fuel load and track conditions. These are for racing only since the adjustment tower is placed where the passenger’s feet are. Installing bars of larger diameter will also increase the lateral stiffness.

Using the swaybars as part of the overall suspension system is necessary for chassis balance. There are several opinions on this, but we feel that the vehicles’ springs should carry the majority of the desired roll stiffness, not the swaybars. These should be used as tuning tools, not primary roll stiffness components.

To that end there are several factors to consider when selecting and using swaybars to make your Porsche handle better. One is swaybar mounting methods, the other is chassis stiffness.

Mounting methods vary most with the 911/930 series. The Porsche factory offered two types of front mounts, the through-the-body style used from ‘65 to ‘73 (and ’75-‘76 Turbo), and the bottom style, used from 1974 to 1989. The rear bars also varied slightly where the droplink attached to the suspension. The early style is superior, and in fact necessary if you wish to install an adjustable front bar. The Factory bottom-mount bars have no provision for relieving preload which is important during corner-weighting.

The earlier, or through-the-body swaybar must be mounted correctly to prevent damage to the body. Also, the rear swaybar mounts need reinforcement if large sticky tires are utilized, otherwise they are liable to be torn off due to stress.

Several manufacturers provide different swaybars to the Porsche aftermarket, including Porsche Motorsports for the late 964-993 and water-cooled cars.

Depending upon one’s budget, you can use the swaybars made by Tarett or Smart Racing. These are similar in price, quality, and durability. Like anything else, you get what you pay for.

Adjustable bars should be installed in pairs so that you can adjust the roll stiffness and roll couple of the car at each end. These are available in sizes from 20mm to 31mm, Custom sizes are also available at extra cost from the various vendors.

Adjustable droplinks are extremely important for removing swaybar preload. Pre-load is caused by the car not sitting perfectly level on its suspension and pre-stressing the swaybars so that the bar is loaded more on one side of the car. This can be adjusted away by using spherical-bearing adjustable length links between the swaybar end and the suspension part that it attaches to. Adjustable droplinks are available for the front and rear and allow the swaybars to be adjusted to function uniformly in right or left turns. This also removes any cross-weight that has occurred as a result of pre-loading.

The performance of any aftermarket swaybar is dependent upon chassis stiffness. Targas and cabriolets may not respond the same way as a coupe when using large swaybars due to chassis flex. These cars will require additional chassis reinforcement to take full advantage of suspension upgrades. The water-cooled cabriolets and Targas are far stiffer than their predecessors and do respond very nicely to suspension upgrades.

Suspension Bushings

Porsche installs rubber bushings of varying durometers (hardness) in the suspension pivots and swaybar mounting points and these were an excellent compromise for a car that is used mostly for street purposes.

For maximum performance however, the natural flexibility of the rubber introduces a certain degree of imprecision when driving close to the limit. Replacing the OEM bushings with either harder rubber ones or metallic bushings can make the car handle and turn more crisply, with an increase in noise and harshness. Replacing the rubber swaybar bushings will make the bar function quicker due to less squirm in the bushing. This delays the load into the bar until the bushing has compressed.

Most bushings in use today are made from either rubber or urethane that has been impregnated with a lubricant to help keep it from binding or squeaking., however they still can squeak and make noise unless they are installed correctly. Using the proper greases, zerk fittings with urethane ones, and careful fitting will go a long way toward making these quiet as possible. Using these on the swaybars only, will not affect the ride or noise levels appreciably.

One good choice for dual-purpose cars are the Poly-Bronze ones from Elephant Racing. These are a steel-bronze bushing assembly with a polyurethane outer layer for noise reduction and employ zerk fittings for external lubrication. Suspension friction is far less than rubber or any other plastic bushings for improved ride comfort and control.

Replacing the bushings with spherical bearings, also called monoballs or uniballs, can virtually eliminate all friction and binding that always occurs with rubber or plastic bushings. This is not for a street-driven vehicle, however the handling difference has to be experienced to be believed!

This is not an inexpensive modification, but overall is very worthwhile and cost effective for track-only cars due to the overall improvement in handling and response. Having little or no friction in the suspension also permits very accurate corner-weighting.

Other Suspension Modifications

Three of the most important things, beside what has been discussed above, that make a really good handling 911 are fine tuning the bump steer, corner weight, and alignment.

Tuning the bump steer inherent in every 911 can be as simple as installing a rack spacer kit to installing an adjustable bump steer kit using spherical rod ends, instead of tie rod ends, to precisely position the steering arms. This is only necessary when the car has been lowered from the stock ride height. Rack spacer kits  are comprised of two alloy spacers and longer bolts to raise the rack that levels the tie-rods. The spherical rod-ends and hardware which replace the OEM tie rod ends offer a much greater range of adjustment, albeit at higher cost.

Corner weights must be set precisely so that the car handles the same in left and right hand turns as well as settling correctly over sharp rises that unload the wheels. Accurate corner weights also affect braking since unequal wheels weights create erratic braking characteristics.

Anytime the car is lowered from the stock height, you must place the car on 4 equal scales and adjust the springs or torsion bars so that the diagonal weight distributions are correct. Using computerized racing scales makes this much easier to optimize since the scales’ computer will display the desired wheel percentages and diagonal weights so necessary for a well-behaved 911. The front end of the ’65-’89 cars is easy to adjust and is done with an 11mm socket on the factory screws. However, the rear suspension is quite a challenge since the spring plates must either be re-clocked on torsion bar splines or use adjustable spring plates.

Adjustable spring plates makes this task much easier to dial-in. One can use either the Carrera two-piece plates or the Elephant Racing ones that allow adjustment with a ¼” Allen wrench. 964, 993 and the water-cooled cars require threaded-body struts and shocks to allow changing corner weights.

Alignment settings are critical, as are tire pressures, in the final chassis tuning to optimize your car for the street or each track you drive it on. Tire choices and sizing also determine the optimal alignment due to variations in tire construction techniques.  

Camber, Toe, and Caster are all adjusted for your specific use by a qualified alignment technician. Be absolutely certain that the person doing your alignment knows Porsches and understands the way you intend to use the car. 993’s also have a kinematic toe adjustment that is quite critical for proper handling. Your alignment shop and personnel must be equipped and experienced to align these cars correctly.

Chassis Strut Braces are utilized to tie the front strut towers together to eliminate flex. These bars attach to the top of the upper strut mounting plates and maintain the installed camber setting in a corner when the chassis is under load. These are available in steel, aluminum, and carbon fiber. All Porsches except the 914 and 996 cars truly benefit from the usage of these bars.

Once again, we have just barely touched on these subjects. We would recommend reading,“How to Make your Car Handle” by Fred Puhn and “Prepare to Win” by Carroll Smith as a good beginning to understanding chassis dynamics and how you car behaves. Bruce Anderson’s “Porsche Performance Handbook” also has some good information.

As always, you may direct any further questions to us at Rennsport Systems. We will be happy to help people as time permits.


Gasoline, Detonation, Timing, and Twin-Ignition

This column is about Fuel, Detonation, Timing and Twin-Ignition for air-cooled cars. These subjects are all inter-related in the optimization of performance.


Fuel quality problems in the USA have been present since the EPA mandated the removal of lead in automobile gasoline and the major oil companies have struggled to maintain minimum octane requirements as well as comply with oxygenated and reformulated fuel regulations. The challenge to maintain seasonal and regional drivability and maintain resistance to detonation has resulted in motor fuels that have some unpleasant characteristics. In addition, the compression ratios in Porsche cars have gradually crept upward since 1982 in the effort to improve performance, fuel economy and overall efficiency. The air-cooled Porsche racing engines and 3.6 litre versions of street engines equipped with twin-ignition haven’t suffered quite as much from the latest gasoline formulations and the 4-valve, water-cooled heads used in the 996/997/991, Boxster/Cayman cars are much more tolerant of differing fuels.

Performance issues that affect all cars including Porsche naturally aspirated as well as turbocharged models are the constant variations in the chemical makeup of motor fuels and the seasonal changes made by the oil companies to comply with Federal Clean Air Act regulations. Not all areas of the USA mandate the use of oxygenated or reformulated gasolines, however there are changes in the fuel composition due to seasonal temperature changes. The net effect is the change in power and driveability that most Porsche owners regard as normal or perhaps the need for a tune-up. Throttle response, acceleration, knock resistance, and overall power are affected by these changes in gasoline. Racing gasolines have been widely accepted now as the main solution for maintaining peak performance in Porsches used for DE events and competition. Besides the obvious octane issues, the main reason why most racers use racing gasoline is to eliminate the variables in specific gravity, vapor pressure and octane present in today’s pump fuel. Racing fuel formulations and offerings do vary among the different manufacturers as well for different applications so one should try several brands & variants to optimize power.

Each gasoline formula performs differently in Porsche cars and one should try several brands within a similar octane group to determine which one offers the best throttle response and power. There is a big difference! Pump gasoline octane rating numbers posted at the gas pumps do not tell you very much about what to expect using that fuel. That yellow sticker is the average of the Research Octane Number and the Motor Octane Number of that particular fuel. The RON of a fuel affects low to medium speed knock and engine run-on. This number is typically 8-10 numbers higher than the MON. The Motor Octane Number is determined under higher RPM and high loads. This affects the high speed and part throttle knock characteristics and this is of greater importance to owners of high-performance cars. If the MON is too low, you could experience detonation during passing maneuvers and when climbing hills under load. If the RON value is too low, then you might experience part-throttle detonation.It is VERY important to realize that some engines respond better to a higher RON and others will perform better with a higher MON depending upon combustion chamber shape,compression ratio and spark plug position. Since RON and MON values ratios are proprietary to each manufacturer of gasoline, you must try different fuel to see which one performs best in your car. Load dyno testing will also help determine what fuel to use. These variations in RON and MON will not change the posted average octane rating at the pump.

Although some unleaded racing gasolines are illegal to use in cars that are driven on public highways, blending unleaded racing gasoline with pump fuel can help eliminate these differences in the fuel used in your Porsche and raise the overall octane rating of the blended fuel.  Now you can find EPA-approved racing gasolines which are oxygenated for compliance. This is always a good idea when participating in Driver Education or any other Track events. Be absolutely certain that the racing gasoline that you may use is UNLEADED. Do not use leaded racing gasoline in any car with a catalytic converter, even for track use.

Other factors that affect driveability in Fuel injected and Weber-equipped 911’s are the volatility and inclusion of ingredients that attack the rubber components in the fuel system. Gaskets, O-rings, fuel lines, and float needles are all affected by the compositions of gasoline. Some brands of fuel have been known to dissolve the interior of rubber fuel hoses so these items will need constant observation and regular replacement to prevent engine problems.  This is why its foolish to simply add some Toluene, Aniline, and Zylene to the fuel tank, even though those compounds are proven ingredients in racing fuels. For example, a leaking rubber-tipped float needle in a Weber-Carbureted 911 can fill a cylinder with fuel causing hydro-lock and result in engine damage as well as a major fire hazard. Unleaded racing gasolines generally do not cause these problems since they are formulated not to attack rubber and plastic such as the foam inside racing fuel cells. This can very among fuel manufacturers. We strongly recommend using ethanol-resistant fuel hose and Viton seals/O-rings in the fuel system for durability.


Now is the time to mention one of the most common octane-enhancing agents present in in pump gasoline, Ethanol. With an octane rating of 112, Ethanol does a commendable job of increasing octane when used in correct quantities, however it has some drawbacks that you need to know. In most cases, the Ethanol content is posted on the pump so that you know how much has been blended in the product. These blends can change during the fall and winter seasons. Ethanol is an alcohol that has very clean-burning characteristics, but its also hygroscopic, miscible, and will scour out any dirt or impurities present in the tank and deposit them downstream into the fuel filter plugging it up very quickly. The use of Ethanol will require the frequent replacement of fuel filters. In addition you will  experience a drop in fuel mileage due to Ethanol’s lower energy content. The mixture changes caused by blending Ethanol is somewhat self-compensated by most fuel injection systems, however carbureted cars may require some adjustments or jetting changes to maintain performance. Today fuel formulations requires the constant use of fuel system cleaners to maintain throttle response and power due to carbon buildups. This is a slow process and sometimes the performance loss isn’t apparent until hesitations, misfires, hard starting and rough idling manifest themselves. Regular use of Chevron’ Techron or LubroMoly’s Jectron together with Ventil Sauber products will prevent these problems. In addition, some Porsche dealers and independent repair facilities that employ the MotorVac fuel system cleaning process, can restore full power and driveability.

Here is some quick data about 3 major brands of Unleaded Racing Gasoline that illustrates some differences that you should consider when choosing which fuel to try. Leaded racing fuels are illegal for road use and will contaminate a catalytic converter. Unless you have a static compression ratio over 11.5:1, these unleaded racing fuels should work just fine, especially with twin-ignition.

 UNOCAL 76Trick UnleadedSunoco GT+

As you can see, changing the RON and MON combinations may not make a change in overall octane, but their burning characteristics and anti-knock qualities can vary enough to make a significant difference in how your Porsche runs! The Reid Vapor Pressure, or RVP, is an indication of volatility which affects the fuel’s resistance to vapor lock, its ability to start easily when cold, and its throttle response. The fuel’s specific gravity specifications, also affect the float levels in carbureted cars and mixture settings. If the fuels’ RVP is too low, 5.5 or less, this can cause poor throttle response and stumbling when the throttle is suddenly opened. RVP’s that are too high, 8.0 or more, can create vapor lock and fuel percolation in float bowls.

Here is a mixing chart for blending 91 Octane Unleaded Premium with 100 Octane Unleaded Racing Gasoline.

Gallons100 oct race gas1234567891011
91 oct pump gas

Porsche has used twin-ignition in their air-cooled racing engines since the 4-cam Carrera engines. The high-domed pistons necessary for high compression ratios requires another spark plug to start another flame front on the other side of the piston dome. The best place for a spark plug is the middle of the combustion chamber to ensure even flame propagation.  A centrally positioned plug allows the flame front to travel the least distance for complete ignition. This reduces the need for ignition advance to start and finish the combustion process when the piston reaches Top Dead Center. Since the spark event is starting closer to TDC in the compression cycle, there is less pressure from the beginning of ignition that is pushing ‘back’ down on the piston crown as the combustion event progresses. This lessens the ‘negative’ work done by the expanding gasses and allows all of the pressure building in the cylinder to push the piston in the correct direction, making the engine more efficient.

Single Plug combustion pattern

The offset-plug position in the air-cooled 911 delays the combustion process since it takes longer for the flame front to progress across the piston to the opposite side. By installing two spark plugs per cylinder, the combustion process is accelerated and can reduce the required advance by as much as 10 degrees, thus lowering cylinder head temperatures. In terms of power, twin-ignition will add some 3-5% or more depending upon compression ratio over a single ignition system. RPM can increase as much as 700 RPM at top speeds. If high compression ratios are to be used, twin-ignition allows all of the power benefit to be gained from the increase. Another benefit is twin-plug equipped 911 (and 930’s) run much crisper and cleaner with lower cylinder head temperatures and improved throttle response. Plus, a twin-plug 911 is much less prone to plug fouling with today’s fuel.


Twin plug combustion pattern

Besides the damage to one’s bank account, there isn’t one drawback to installing this system and enjoying the benefits in throttle response, power and great drivability that twin-ignition adds to any 911 or 930. Quite a difference! In some cases, it is a necessity to realize the maximum gains from a complete set of engine modifications. The water-cooled engines (Mezger/M96/9A-series) have a much more optimal spark plug location at the top and center of the combustion chamber that negates the need for two spark plugs per cylinder.

Twin-ignition Hardware

Now comes the spendy part,…how to implement this in 911 and Turbo engines.

There are 4 basic methods to installing and using dual plugs in these engines:

1) Stock Distributor converted to RSR configuration.

This is done by taking the Bosch OEM unit and mounting a billet adapter ring to accept the Bosch 12-nipple RSR cap and machining the distributor shaft to accept the RSR rotor. The cap and ring must be phased to the rotor alignment using a distributor machine and the advance curve should be modified to suit. can be converted This setup can trigger either the OEM Bosch CD boxes or a pair of MSD 6AL’s with MSD matching coils for best performance. This setup is ideal for all 2.0-3.5 litre engines with carburetors, EFI or MFI systems.  For 930’s, this is the hardware of choice since this retains the all-important boost retard feature that is critical to engine life and proper throttle response.

2) 964 Dual-distributor converted for 2.4-2.7-3.0-3.2 litre Engines.

This one is done by installing a trigger from a donor SC distributor and using the appropriate crank gear, depending upon the engine being used. This unit will not fit the Turbos’ due to interference with the boost plumbing. Again, these can be triggered by OEM Bosch CD boxes, MSD’s or a proprietary splitter unit for Motronic-equipped engines. This setup is ideal for any Motronic motor and works very well in carbureted or MFI-equipped engines. The 3.6 crank gear must be used for these conversions.

3) Crank-triggered Distributorless Ignitions.

The ubiquitous Electromotive coil-pack and TEC-III Engine Management systems fall into this category as does the superb but expensive MoTec ones. These can be challenging to install and require additional RFI and EMI shielding to prevent stray ignition signals that create random misfires. We use additional grounding straps and high-quality plug wires to eliminate this possibility and maintain the integrity of the ignition signals. Some of these coil-pack ignition systems operate at system bus voltage and do not produce the amount of ignition current needed to keep the plugs clean in a rich mixture environment. Carbureted and MFI-equipped engines do operate with rich idle mixtures needed for best throttle response and will foul plugs MUCH easier with these ignitions.

The other big drawback with some of these DIS is that you cannot use plug gaps larger than .032 without misfires. Its well proven and documented that plug gaps in the .040-.045 range make more overall power and result in better idle qualities due to the larger flame kernel and the resulting more thorough combustion process.

4) Custom Twin-plug distributors.

There are several options for brand new twin-plug distributors that employ magnetic triggering, adjustable advance mechanisms, and ball-bearings. While not cheap, these are one of the best solutions for installing twin-ignition on any engine.


There is a wealth of information available on these subjects and we have only scratched the surface here. If you would like more information on these issues or other performance-related Porsche questions, please contact:



Transmissions & Clutches

Transaxles and Clutches

Performance gearing has been a staple of Porsche performance since 1948. Selecting the proper transmission ratios for your gearbox is an integral part of improving Porsche 911 automobiles. It has been said that installing a set of shorter spaced gear ratios is like adding 50 HP to your car!

Transmission and Clutch Overview: 901-911

Let’s talk about the various transmissions installed in 911′s and 930′s since 1965. The first transmission designed for the 911 was called the 901 transmission. This unit was used in different variations in the 911, 912, 914 and 914/6. The early versions used sand-cast aluminum cases and the later ones used pressure-cast magnesium. These were 4 and 5 speed transmissions with a torque input rating of 138 lb/ft, later uprated to 148 lb/ft in 1969. The fact these transmissions have been used with engines producing more than 230 lb/ft of torque, albeit with shorter lifespans (measured in broken gear teeth), speaks volumes about the basic design. In 1970, Porsche changed the 901′s clutch actuation and called it a 911 type gearbox. These were still 901-type transmissions now coupled with a larger, pull-type clutch actuating mechanism. Clutches used in these cars were the 215mm unit used from 65-69 and the 225mm unit used from 70 to 71.

Transmission and Clutch Overview: 915

For the 1972 model year, Porsche used a larger, stronger transmission called the 915. Using a different shift pattern from the 901, this gearbox offered a torque input rating of 181 ft/lbs. Racing versions with the pump and cooler allowed this rating to be increased to 275 ft/lbs. Some people have used this transmission with the 930 Turbo engines with some success as well. 915′s were used in several variations until 1987 when Porsche introduced the G50 gearbox. Early 915 transmissions employed magnesium cases until 1977; 915 versions made in 1978 and later used aluminum cases that were stronger and had strengthened differentials.

The 915 came with two different final drive ratios, 72-74 units were equipped with 7:31 ring & pinions and the 75 and later transmissions had 8:31 final drive ratios. The 8:31 ring and pinions are significantly stronger than the 7:31 units and are the most desirable one to use for use with higher horsepower engines where service life is probably doubled. Starting in 1984, the 915 transmission used in the 3.2 911′s were equipped with an oil pump and integral cooler since the torque of the 3.2 engine could have shortened the life of the gearbox.

915 transmissions were equipped with several variations of Fichtel & Sachs 225mm clutches. Porsche used aluminum and cast iron pressure plates and spring centered discs. The ill-fated rubber centered disc was an attempt to reduce the low-speed gear rattle that all 915 gearboxes have. By now, all of the rubber centered discs should have been replaced with the noisier, but much more reliable spring center type disc. Replacing the iron pressure plate with a aluminum one is a good upgrade when it becomes necessary to replace the clutch assembly. This will help the engine rev quicker and shift better due to lower inertial moments.

Transmission and Clutch Overview: 930

The 930 or Turbo transmission was first used in Porsche street cars in 1975 with the introduction of the Turbo Carrera. This was a 4-speed gearbox that was much larger and heavier than the 915 which has proved troublesome when used with the 3.0 Litre Turbocharged engines. The 930 transmission is an aluminum cased unit capable of handling 326 ft/lbs of torque continuously. Racing versions with oil pumps and coolers were used in the 935 race cars that made over 750 HP albeit with shorter life spans. Turbos were equipped with a 240mm clutch to handle the extra torque that was moderately successful. There are some excellent upgrades to reduce the rotating weight and increase holding pressure in high-powered applications.

Transmission and Clutch Overview: G50

Porsche realized the shortcomings of the 915 transmission’s synchronization system as well as its torque limitations when the 3.2 engine was introduced. In 1987, the G50 gearbox was installed into the Carrera 3.2 911. This transmission used Borg-Warner syncromesh instead of the Porsche-design balk-ring system to improve the shift quality and lessen the effort required to change gears, especially from a stop. These gearboxes were rated at 221 ft/lbs of torque. The G50′s have been utilized with several variations of gearing and speeds. The 993 series was the first 911 offered with a 6-speed version of the G50 design. These later units also introduced significantly improved clutch cooling.

Other versions of the basic design, called the G50/52 series, were used in the 3.3 C2 Turbo and 3.6 C2 Turbo cars. These transmissions also have stronger differentials and cases and a type of Limited Slip differential that locks 20% under power and near 100 % on the overrun to minimize trailing-throttle oversteer. All of the G50 cars use the 240mm clutch size introduced on the earlier Turbo cars and they are now hydraulically actuated. In 1990, the infamous dual-mass flywheel was introduced on the C2/C4 series. These flywheels were intended to help reduce low-speed gear noise however they have proven to be problematic on these cars. A popular conversion invloved installing the single mass, lightweight flywheel from the Euro Carrera RS into these cars for a performance increase and much improved durability. The well-known stalling issues can now be resolved.

Ring & Pinions and Limited Slip Differentials

In order to fit a compact transmission with the requisite features in the 911, Porsche uses very steep hypoid angles on the ring & pinion gears. This make these parts, the most highly stressed part of Porsche transmissions. This requires a GL-5 rated lubricant to protect these components against premature failure. Ring and pinions are problematic areas of these transmissions and require careful setup for any 901, 915, 930, or G50 used for competition or high-horsepower applications. The majority of 901 and 915 transmissions came with 7:31 or 8:31 final drive ratios.

Porsche also offered several other ring and pinions for racing including 7:37 and 7:33 ratios. These are not always available due to limited manufacturing qualities. The G50-series used a final drive ratio of 9:31 and the Turbo versions, G50/52 used larger, stronger ring and pinion gears.

If you do decide to change the ring and pinion in your gearbox, make sure that you get the appropriate one for either the mechanical or electronic speedometer.

Porsche has offered the ZF clutch-pack type limited slip differential in almost all 911′s and 930′s since 1966. These units are quite strong and can be set for locking factors of 40% to 80%. The clutches need replacement under racing conditions to maintain proper lockup but these are quite trouble free. An 80% locking factor is difficult to drive on the street, especially in wet conditions, but for open track racing these are quite popular. Street or Autocross usage requires the LSD to be set at 40%; simple to do with the proper parts.

There are now other LSD options for 901, 915 and G50 transmissions called Torque-Sensing differentials. These are gear-type units as opposed to the ZF clutch-type ones, and do not require any parts to maintain optimum performance. This unique, patented design allows full differentiation between the wheels as well as providing power to both sides and they produce less understeer than the Factory LSDs. Guard Transmission makes an excellent one as well as the unit from Quaife.

For Autocross and normal street driving ,the Torque-Sensing units are more benign in their operation but they do not improve the handing like the Factory clutch-pack units do. The ZF or Guard Transmission LSD’s really stabilize the car under braking and help reduce trailing throttle oversteer.

Transmission Modifications for Improved Performance

One of the first things a Porsche owner wishing to improve the acceleration of their car should consider is changing gear ratios. The wide gear ratios used by the factory are a result of designing for street drivability, fuel economy and emissions, not best performance!!

As we said earlier, a close-spaced set of gears in any 911 adds an “effective” 50 HP to the acceleration potential. Reducing the RPM drop each time you upshift keeps the engine in the most productive parts of the torque curve. One must experience this to fully appreciate this effect. Take a ride on a PCA Driver Education day in someone’s car that has a shorter gear stack and you will be convinced.

There are several options for shorter gear ratios depending upon usage, tire size and ring & pinion ratio. There are even computer programs that will help you select the proper gear ratios.

901 Modifications

These transmissions utilize 2nd gear as integral to the mainshaft. This means that to change this gear, you must either purchase a new mainshaft as part of a new 2nd gearset or use an often-scarce 904 mainshaft that allows you to change any gear with changing shafts each time, a much less expensive proposition. The gearsets used in the early 67 911S was A-F-M-S-X. This is a very nice set of street ratios that works quite well in almost any 901-equipped car. There are other combinations as well so be sure you factor your tire sizes!

The 901 magnesium-cased gearboxes will truly benefit from the installation of a forged, billet 6061 T-6 intermediate plate that carries the main bearings. This prevents the shaft deflection present on the stock gearbox that contributes to failure under severe conditions or when used with larger engines. The OEM magnesium plates are prone to cracking as well as being quite flexible and cannot support the transmission shafts as well as the aluminum one can. We also like to install a billet side cover to the differential housing that helps prevent the ring gear from “walking” away from the pinion gear.

We would also recommend the installation of a cooler and pump for maximum transmission longevity when used with 2.7 litre or larger engines. You can also install nozzles to squirt oil at critical points such as the ring & pinion and each gearset. This really helps carry the heat away from these highly stressed parts.

Limited slip differentials such as Guard, OEM ZF, or some others will really help get the power down with higher-horsepower engines. These almost always require a change in driving style to accommodate the additional understeer created from the added traction. As mentioned previously, the Guard/ZF units can be adjusted by changing plate thickness for setting the locking factor as well as change ramp angles for assymetric operation (Guard only).

Clutch Modifications

For street driven 901-equipped cars, installing the 69 “S” clutch with the drilled flywheel and aluminum pressure plate assembly is a bullet-proof and relatively lightweight unit.

915-equipped Porsches should use the aluminum pressure plate instead of the cast iron one and a spring center disc. There are other racing discs and RSR pressure plates available for the high-horsepower cars that need the extra clamping force.

There are various custom setups for Turbo Porsches. 993-996-997  Twin-Turbos’  use a LWF/GT-2 package that have greater clamping power, are much lighter, and are more durable.

Many people with G-50 equipped Porsches install the European RS single-mass flywheel setup for quicker throttle response and much better durability. Due to the engine’s propensity to stall easier, modifications must be done to the idle stabilizer and in some cases, Motronic software to compensate for the 50% reduction in weight. The only disadvantage is some additional transmission noise in the car at idle.

Race cars use some the ultra-lightweight racing clutches available for the 901 equipped cars that make shifting much easier as well as allow the engine to rev MUCH quicker. This really reduces the wear and tear on the syncros, operating sleeves, and balk-rings. These are not suitable for street driven cars.

915 Modifications

The 915 transmission, unlike its cousin the 901, has 1st gear machined onto the mainshaft. This means that you must change the entire shaft when this gear is to be changed. Since this isn’t always cost effective to do, most folks simply change 2nd through 5th gears to achieve the desired spacing. There is a good selection of 915 gears now available on the aftermarket that allow you to build a custom gearbox for your exact application. Here is a real nice close-ratio street setup that has been very successful:

  • 1st Gear 11/35 (stock)
  • 2nd Gear 17/31
  • 3rd Gear 21/31
  • 4th Gear 24/27
  • 5th Gear 27/24 or 28/24

Consult your Porsche tuner or contact Rennsport Systems to find out what gears are available and what to use in your application.

The 915 doesn’t require additional stiffening besides the Wevo bearing retaining plate and differential side cover. Machined billet side covers for the magnesium and aluminum-cased 915′s help eliminate the side thrust in the ring and pinion gears that contribute to failure. Adding a pump and cooler is a very good idea when this transmission is used with engines producing over 250 HP. You can either use the Carrera RS oil pump and internal squirters or the 3.2 Carrera oil pump and integral cooler. Both of these modifications will require a new end cover on the gearbox.

Sustained power levels over 300 HP require the installation of oiling nozzles at the most critically loaded areas of the gearbox for maximum durability.

Limited slip differential options are the same as the 901. You may elect to use the Guard, OEM ZF or some other units based upon preferences and availability. The pinion depth and backlash should be checked anytime you install an LSD.

930 Transmission Modifications

This transmission is relatively easy to change for better performance. While the Turbocharged 911 truly benefits from installing a 5-speed, this is not inexpensive to do! The 930 gearbox can be made to work very well by changing  gear sets to reduce RPM drops at each shift. These transmissions need cooling when the power levels climb about the 550 HP level or for racing. Installing a cooler and pump to supply pressurized oil to the ring and pinion makes these gearboxes last much longer.

G50 Transmission Modifications

These transmissions as used in the Carrera, C2/C4, C2 Turbo and 993-series cars are more expensive to buy gears for, compared to the 915 and 901 units. The availability of gearsets is very good with these units. Porsche’s racing program and the availability of high-quality aftermarket gears make these transmissions very attractive for street & racing applications.

When used with very high torque engines and large rear tires, the ring and pinion assemblies are prone to premature failure. Differential side cover deflection and undersized ring and pinion gears for the power levels are responsible for transmission failures not generally seen in Porsches used for endurance racing to such a degree. Auxiliary cooling and pressurized lubrication systems are necessary in these applications with this transmission. The G50/50-series is probably the strongest of this generation of gearbox although these are 5-speed units.

Our Web Site has a picture of a G50 6-speed transmission that was extensively modified for the Daytona 24 hour race. This gearbox was used in a 993 3.8 RSR that finished 12th overall and 4th in GT-3. The pressurized lubrication system in the photo provides cooled oil to all gear sets and the differential. This unit also was equipped with a special side cover and custom-made 4th and 5th gears. Needless to say, this was not an inexpensive enterprise due to the number of hours required to accomplish this. Many G-50 6-speeds utilize a fixed 2nd gear so a new Motorsports mainshaft is required to regear these transmissions. Even the new GT-3 street cars come this way.

914/6 Notes

The unusual case of the 914/6 requires a few comments. Since the transmission in these cars resides behind the engine, they operate at much higher temperatures than in a 911. Regardless whether you use a 901 or a 915 gearbox in these cars, you should install a pump and cooler to maintain transmission life with engines 2.7 litres or larger. For racing applications, we like to install a pressurized lubrication system in the 914/6 for maximum component life. The clutch also runs hotter in this configuration so don’t neglect this part as well. It doesn’t hurt to add some ventilation to the bellhousing under racing conditions.

Miscellaneous 915 Application Notes

One issue that surfaces with the early 6-bolt flywheels used in racing applications is keeping the flywheel from coming loose. Everyone wants to use a lighter flywheel, and there are many benefits to having one. Better and faster shifting, extended syncromesh life, and faster speed out of corners as the engine picks up revs quicker makes his very desirable.

The commonly used RSR steel flywheel is quite notorious for vibrating loose when the engine is revved beyond 8000 RPM. This is caused by a combination of crankshaft vibrations and flexing of the steel flywheel when used with that pull-type clutch. Shifting at engine speed over 8000 RPM will shake the flywheel loose and cause serious damage to the crankshaft as well as contribute to a DNF.

For these applications, we recommend that the flywheel bolts be installed with Blue Loctite and slightly overtorqued. We also advise using the aftermarket aluminum flywheel that is more rigid than the RSR unit.

A racing clutch such as our Tilton 5 ½” unit doesn’t place so much stress on the crankshaft as the factory-style unit. Aluminum flywheels are now available which provide a less expensive and streetable alternative to a full racing clutch system.

The later 9-bolt crankshafts used in the 3.0 SC and Carrera engines do not seem to suffer as much from this phenomenon.

Other modifications are the use of aftermarket bearing retainer plates and billet side covers that stiffen and maintain precise shaft spacing to help protect the ring & pinion. These are especially important when using a 915 with a 3.6 or 3.8.

Another problem that has surfaced with time are elongated main bearing races in the aluminum 915′s (’78-’86). Unlike the magnesium-cased 915′s, the later versions didn’t have a steel insert that held the main bearing. Over time, the aluminum castings soften a bit and the bearing begins moving around which elongates the machined recess it sit in. This causes the bearing to spin in its bore and allows the pinion shaft to move, relative to the ring gear of the differential. Needless to say, this compromises the life of the ring & pinion and the carrier bearings. The fix is to weld the damaged hole and machine the aluminum differential housing to accept a steel insert to properly locate the  bearing (just like the mag-cased versions). This makes for a very robust transmission that better than how it left the factory.

Lubrication Issues

Oils are always a subject of opinion and controversy. Everyone’s opinion is based upon personal experience. The more experience one has in the area in question, the more weight we give to the opinion being offered. In our experience, the 901, 915 and 930 transmissions seem to shift and operate better with non-synthetic oils. The Porsche designed balk-ring syncromesh needs more friction to operate smoothly than synthetics provide. We use and recommend the Swepco line of gearlubes for these gearboxes. They simply shift better than any other oils that we have used. The G50-series trannys, on the other hand, really work well with a full-synthetic gearlube like Mobil 1 Delvac or Redline. The Borg-Warner type syncros work better with a slippery oil. We do not use or recommend any transmission additives besides the LSD improvers sold by GM dealers for the times that you need to eliminate LSD-clutch chatter.

Other gearlubes might be just fine too, but “caveat emptor” applies here.

As always, we have just touched on this subject. Consulting a gear chart for your transmission will show you what your gear splits are and what you can change. Not all of those gearsets are always available so factor this accordingly. Setting up a transmission for racing or even aggressive street use may be best left for professionals due to the component costs, special tools necessary, and the consequences of a small mistake. Ring and pinions require a special fixture to set the pinion depth and ring gear backlash properly.

Since changing gearsets can make a stock engine much more fun to drive, this should be near the top of your performance modification list. Installing a set of close-ratio gears is the equivalent of adding 50 HP to the performance of your car.

For further information or consultation, please contact us: 503 244 0990

Brake Upgrades


6-piston Brembo M caliper

One of the most important improvements to any Porsche is improved braking performance. This can be accomplished by installing better Brake Pads, Racing Brake Fluid, Cooling Kits, Larger Calipers & Rotors and even Stainless Steel Brake lines. We offer various upgrades for all models from simple cooling and pad upgrades, to rotor/caliper replacements to complete racing brake systems for high-horsepower vehicles. Brakes MUST be configured and installed with compatible components so that the car’s braking balance isn’t disturbed. Front-to-rear brake bias can be adjusted by sizing the proper calipers and master cylinders as well as manual brake bias control. The biggest and best brakes will not work properly without adequate rotor cooling so we offer several brake cooling options.

Porsche High Performance Brake Systems

Porsche Brakes have set the standard by which all other manufacturers have been compared since 1964 when they started installing ATE disc brakes on the 356C. Today, the 993 TT & 997 GT-3 brakes are the benchmarks of brake performance for the rest of the world. To that end, we have several options for increasing the braking power of your 911/930, 964, 993, 996, 997 and Boxster models. Brake improvements consist of upgrading 5 major items: Calipers, Rotors, Pads, Fluid, and Cooling. Rennsport Systems offers several levels of brake upgrades from simple caliper replacements to full 4-wheel systems incorporating larger rotors and calipers. We also have high-performance brake pads from Pagid, Brembo, & Mintex. A major part of any brake upgrade is using the proper brake fluid and additional cooling for the rotors and calipers. We use and recommend and sell Motul Racing Brake Fluid, ATE Super Blue, and Castrol SRF . The wet boiling temperature is very important for climates that see a lot of moisture, or for Porsche owners that change brake fluid infrequently. The higher the wet boiling temperature, the less moisture will be absorbed by the fluid. Moisture causes corrosion inside the caliper and contributes to sudden brake failure by lowering the temperature at which the fluid will boil. You can read more technical information about Porsche Brakes on the “Lets Talk” page: http://www.rennsportsystems.com/brakes.html

Brake Cooling for 911’s Here is the superb Brake Cooling kit made by AJ/USA for 911’s 69-89. Together with the inner air block-off plates, this makes a significant difference in the performance of these brakes. This is a very effective solution for cooling the brakes found on the 69-89 911’s, and 930 through Carrera braking systems.




Brake Upgrades

These kits use various Brembo Calipers and Rotors. Used by many Formula 1 teams, ALMS teams, and most Porsche racers, Brembo Racing Brake components are among the finest brake systems available anywhere in the world. Some of these rotors are drilled however, so please remember that all cross-drilled rotors can develop cracks, sooner or later. Gas-slotted rotors are much more durable with a small increase in weight over its cross-drilled equivalent. This list is just a sample of the Brake Upgrades that we have available.

Here are just a few of our Brake kits:

930 brake kit for 911

930 Turbo Brakes for all 911’s 1969 – 1989 930 Turbo: Front: Factory 930 calipers (new, or perfect used ones) with custom front 306mm x 32mm rotor/hat assemblies and 309mm x 28mm OEM rear rotors, all mounting bolts, factory front spacers & lock washers, custom rear spcers & lock washers, short front and rear brake lines, OEM street pads, Ate Type #200 gold brake fluid, 23mm master cylinder. Some cars may need 3mm or 6mm wheel spacers for caliper clearance if the outer fins are left intact and we do carry those.