Category: Let’s Talk Porsche

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.

Ignitions

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.

Modifications

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.

Engine	Valve Sizes	Port Sizes
65-68 2.0 911	In 39mm Ex 35mm	In 32mm Ex 32mm
67-68 2.0 911S		In 42mm Ex 38mm	In 36mm Ex 35mm
69 2.0 911T	E	In 42mm Ex 38mm	In 32mm Ex 32mm
69 2.0 911S	In 45mm Ex 39mm	In 36mm Ex 33mm
70-77 2.2-2.7 911T	E	In 46mm Ex 40mm	In 32mm Ex 32mm
70-73 2.2-2.4 911S		In 46mm Ex 40mm	In 35mm Ex 35mm
73 2.7 RS		In 46mm Ex 40mm	In 36mm Ex 35mm
73 2.8 RSR	In 49mm Ex 41.5mm	In 43mm Ex 43mm
74 2.7 911		In 46mm Ex 40mm	In 32mm Ex 33mm
74 2.7 911S	In 46mm Ex 40mm	In 35mm Ex 35mm
74 2.7 Carrera(US)	In 46mm Ex 40mm	In 35mm Ex 35mm
74 3.0 RSR	In 49mm Ex 41.5mm		In 43mm Ex 43mm
75-77 2.7 911S (all)		In 46mm Ex 40mm	In 35mm Ex 35mm
76-77 3.0 Carrera	In 46mm Ex 41.5mm		In 39mm Ex 35mm
78-79 3.0 911SC	In 49mm Ex 41.5mm		In 39mm Ex 35mm
80-83 3.0 911SC	In 49mm Ex 41.5		In 34mm Ex 35mm
83 3.0 911 SC RS	In 49mm Ex 41.5mm		In 43mm Ex 43mm
84-89 3.2 Carrera	In 49mm Ex 41.5mm		In 40mm Ex 38mm
89-94 3.6 C2/C4/RS	In 49mm Ex 42.5mm	In 41.5mm Ex 38mm
95-98 3.6 993		In 50mm Ex 42.5mm	In 43mm Ex 39mm
96-98 3.8 Carrera RS	In 51.5mm Ex 43.5mm	N/A

 

 Modifications

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!  

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.

Engine/Cams	Carbs	Venturiis
2.0 to 2.2 "S"	40mm Webers	32mm
2.4 to 2.7 "S"	40mm Webers	34mm or 36mm
2.8 "S"	40mm Webers	36mm
2.8 RSR	46mm Webers	38mm or 42mm
3.0 "S"	40mm Webers	36mm
3.0 RSR	46mm Webers	42mm
3.2 "S"	46mm Webers	38mm
3.5 "S"	46mm Webers	42mm
3.5 RSR	46mm Webers	42mm(Not really enough)
3.5 GE60 or 120/104	46mm Webers	42mm
3.8 GE80 50mm	PMO's	46mm 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

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.

Camshaft	Intake Lobe Center	Exhaust Lobe Center
911 T	216 deg. @ .387_	207 deg. @ .345_ 105 deg.
911 E	230 deg. @ .408_	222 deg. @ .393_ N/A
911 S	264 deg. @ .450_	236 deg. @ .400_ 101 deg.
Solex	244 deg. @ .439_	234 deg. @ .406_ 97 deg.
911 SC	229 deg. @ .455_	220 deg. @ .402_ 113 deg.
964 3.6	240 deg. @ .464_	230 deg. @ .425_ N/A
906	282 deg. @ .465_	252 deg. @ .406_ 95 deg.
RSR (sprint)	282 deg. @ .465_	266 deg. @ .450_ 99 deg.
3.8 RSR	272 deg. @ .485_	256 deg. @ .485_ 109 deg.
993 RS 3.8	240 deg. @ .490_	226 deg. @ .446_ 110 deg.
GE-40	256 deg. @ .470_	238 deg. @ .440_ 102 deg.
GE-60	266 deg. @ .490_	248 deg. @ .455_ 102 deg.
GE-80	274 deg. @ .500_	256 deg. @ .470_ 100 deg.
GE-100	284 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.

Engine	Header
2.0 to 2.4	1 ½" or 1 5/8"
2.5 to 2.8	1 5/8"
3	1 5/8" or 1 ¾"
3.2	1 5/8" or 1 ¾"
3.4 to 3.5	1 5/8" or 1¾"
3.6 to 3.8	1 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 Braces, Bump 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)

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.