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

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

Porsche used this 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. The Factory has used 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.

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, 911S 2.5 racers. When Porsche released the first 2.8 RSR’s, these were equipped with Marelli 2-point distributors until the 3.0 RSR engine was offered. These engines 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 any advance mechanism. Subsequent race cars like the 911SC RS used a system like the 3.0 RSR engines.

Now,..…..to the present. Porsche has 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 excellent 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 996 and Boxsters, use a direct-fire ignition with each spark plug lead having its own separate coil. The Motronic computer fires each coil in the correct firing order according to the software.

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

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 the Perlux IgnitorÒ module to replace the breaker points. These have proven to be quite 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 or Crane Hi-6 and their respective matching coils. These units provide a FAR hotter spark that is better able to fire the rich idle mixtures required by Weber carburetors and MFI as well as firing the wide plug gaps that have been proven necessary for best power. These ignition boxes can make a race-cammed Webered’ high-compression engine idle for hours without any signs of distress when installed properly. The multi-spark high current ignitions really help reduce the cantankerous nature of some carbureted and injected 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 the ultimate upgrade for any 911 or 914/6 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. 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 fuel 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 34 degrees.

Read the December 97 "Let’s Talk" column: Gasoline, Detonation, Timing, and Twin-Ignition for additional and relevant information on this subject.

Installing a Twin-plug system on the various 911 engines isn’t inexpensive. Each powerplant family varies in complexity and cost. The best time to do this is when you do an upper-end overhaul and replace the valve guides. This is the most cost effective time to have the lower plug holes drilled and tapped in the heads and the lower valve cover holes machined for the plug connectors.

For the 2.0 through 2.2 litre engines, the early-type Marelli Twin-plug distributor is almost impossible to find. Modified single-plug Bosch units are available that accept the Bosch RSR Twin-plug cap and rotor for this conversion on the early case engines. These can be used to trigger Bosch, Permatune, MSD, or Crane CD boxes.

The 2.4 through 3.0 litre engines can use either the later Marelli or Bosch RSR breakerless distributors and any desired CD boxes and coils. The Marelli caps and rotors are almost all gone and the special point sets are "unobtanium", however the Marelli distributor is a very well made, ball- bearing supported unit that can be adapted to the Perlux IgnitorsÒ for triggering. Bosch Twin-plug caps are rotors are still available but cost almost $1300.00 as of this writing.

The 3.2 Motronic engines can be adapted to Twin-ignition with the 3.6 dual-distributor and a special splitter unit made by Andial for this purpose. The triggering comes from the OEM Motronic trigger at the flywheel and the 3.6 dual-distributor merely sends the sparks flying. All you need is the 2^nd coil and a plug wiring harness. Non-Motronic 3.2 engines will use either the Bosch RSR breakerless unit, a Carrera distributor modified to accept the Bosch Twin-plug cap & rotor, or an Electromotive HPV-1 Crank-triggered Ignition System.

The Electromotive Distributorless Crank-triggered ignition systems are another option for adding twin-ignition to any 911 engine. These use no distributor at all; instead using internal electronics to set timing, dwell and fire the individual coils. The main advantage is that you have no expensive caps & rotors to buy and maintain. Two of these units would be required to install a twin-plug setup on any of the 911 engines. These come with a trigger wheel and sensor splitter. You must also buy two sets of plug wires and connectors since they use a special end on the GM HEI-derived coils. We highly recommend using the excellent MagnecorÒ plug wires and connectors when using these ignition systems. The high-tension wires create a large amount of RFI/EMI that causes stray triggers and misfires in the Electromotive units. These coil packs need a very high resistance wire to suppress RFI and help the coils saturate better. The 2.2 K/ft of these wires makes a noticeable difference in how the Electromotive units perform. These ignition units also need additional grounding due to their unusual susceptibility to EMI and RFI.

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 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. This was a much improved head over the 2.0 litre versions and was 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, introduced in 1989, saw a further increase in port sizes and the valves remain unchanged except for the 3.8 litre RSR racing engines. The 1996 993-series saw a slight increase in valve sizes.

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 far more prone to float at high RPM, than the small 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.

It is better to not miss shifts of course, but with Porsche transmissions, this isn’t too likely. 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 heads on our Website at: /1-c.html

You will see a stock port passage as well as one that has been extensively flowed.

Not all stock Porsche heads are created equal. As George Orwell said: "Some are more equal than others". Porsche heads are 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 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 models. These early heads are a little more suitable for modifications and have 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. Additional 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 935 heads, a benchmark 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 on the C2/C4’s from '89 to '94 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 over 25%. Due to the ceramic liner, this is not easy to do and requires special tooling and equipment.

These were the first production heads that employed twin-ignition due to the 100mm bore the necessity for starting two flame fronts for best combustion and help prevent detonation. Since '89, all 3.6 litre engines have used twin-ignition. The new 996 4-valve water-cooled engines have centrally located spark plugs in the most efficient position at the top-center of the combustion chamber. These new heads, used on the Boxster and 996, no longer need twin-plugs for good combustion and are quite detonation resistant.

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. 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 cylinders with aluminum finning, NikasilÒ and AlusilÒ cylinders have 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-carbide coating 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 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 much more clearance to prevent piston seizure when hot. Naturally, when cold, these pistons make a lot of noise until the engine heats up. The tight clearances allowed by the Mahle pistons, also allowed a better ring seal for better compression and leakdown. Currently, the American piston manufacturer, JE makes a similar piston that is much lighter than Mahles, can be used at reasonable clearances, available in custom sizes and dome shapes, and most important, just as strong. These have been in use for the past 4-5 years with success. Since JE has made these pistons available, many previously unusable cylinders can be employed in various 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 enough dome material to have almost any 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; 2.0-2.2-2.4 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.

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 powerplants also have a good selection of piston & cylinder sets to choose from. One of the most popular conversions is the 98mm "Max Moritz" 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: Fewer options exist for these, already large, 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. One other factor is high piston weights at these bore sizes that put quite a strain on the rods. Using the much lighter JE’s in these applications with some custom lightweight wristpins will dramatically lower the reciprocating weight and allow the engine to spin up quicker and reduce rod loading at high RPM.

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.

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 started 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 to address some case cracking problems.

The two most common versions of the Magnesium crankcase were the 4R/5R case and the 7R case. The latest and strongest case was the 7R one. This had the most stiffening ribs cast throughout the entire casting and is the most desirable, by far, for building a powerful 2.7 or 2.8 engine. The physical differences between the 4R and 7R are quite apparent when compared side-by-side. Modifications to Magnesium cases are merely for reliability. Installing timeserts on the head stud holes and shuffle-pinning the main bearing webs to minimize the fretting that occurs at the case half parting lines. We do NOT recommend "boattailing" the webs. These cases have enough natural flexibility that removing metal merely exacerbates the lack of stiffness in these cases.

The aluminum cases used on the 3.0-3.2-3.6 engines are an excellent foundation for a high-horsepower engine and are able to handle a great deal of power without stress. 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 case is quite different yet due to the hydraulic valve lifters and other cast-in oiling passages and uses a lot of new castings.

Porsche crankshafts and connecting rods, with few exceptions, have been quite reliable and bulletproof in high-performance and racing applications. The 2.0-2.2-2.4-2.7 and 3.0 rods are quite strong with the 911S nitrided rods being preferred for those smaller engines. The 3.2 rods and 3.6 rods must have the OEM bolts replaced with ARP or Raceware rod bolts for durability and reliability. The stock rod bolts used in these engines are not reliable beyond 6700 RPM. Installing a performance chip into a Motronic equipped 3.2 Carrera can raise the rev limiter to levels that are unsafe with the OEM rods bolts. If sustained RPM over 7000 is anticipated, we strongly recommend using Carrillo or Pauter Engineering steel rods. The 3.6 rods are of a different design than earlier versions and we strongly recommend replacement with aftermarket rods if the engine is to operated beyond 6700 RPM! With these larger engines the stroke length combined with heavier piston assemblies, places a premium on rod integrity that the OEM rods no longer have at higher RPM levels.

Long-rod engines have been used successfully in some applications. Changing the rod ratio by using longer, custom rods has 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 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 crankshaft. 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 very expensive.

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 911’s 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.