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!