Intake Systems – Camshafts – Exhaust Systems

(click here for Part 2, “Ignitions, Cylinder Heads, Pistons & Cylinders, and the Bottom End”)

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 as the air-cooled 911 engine. From the dry-sump oiling system to the 8 main bearing bottom end, these powerplants have been a 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 legendary 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 not optimized for best performance. Some of these such as Bosch K-Jetronic Fuel Injection, were intended to meet emissions and fuel economy laws, not best performance.

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 Continuous Injection System)
• Bosch Motronic (various versions/drive-by-wire after 2000)

Each of these systems has strengths and weaknesses for performance purposes. The best performance potential will be found using Weber/PMO Carburetors, Mechanical Fuel Injection, and some aftermarket user-programmable Engine Management Systems in concert with an individual throttle intake system. MoTec, Electromotive, and EFI are just a few of the EMS systems available. The features somewhat dictate what they cost so these systems do range in price quite a bit.

Vintage racing may dictate the use of the Solex overflow carburetors. The Factory could not cure the “flat spot” that they were known for, although are now some tuners that can make these run very well and these have the potential to run very well.

Zenith Triple-throat

The excellent Zenith 40 TIN carburetors are not a good choice for performance applications. These carbs were equipped with 27mm venturis that restrict power potential to well below 175 HP. Since there is a very limited range of jets and other tuning parts, engines equipped with these are best upgraded to Webers or PMO’s, due to the great range of venturi sizes and jet combinations.

Weber (and PMO) Triple-throat

The ubiquitous Weber IDA 3C-series of triple-throat carburetor and its much more modern derivative; the PMO carburetor, remains the mainstay 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 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 system 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. Venturis sizes are largely determined by engine displacement, camshaft profile and the operating 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.

Venturis and Manifolds

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 have less than satisfactory success using Weber or PMO’s, and problems can be traced to dirty or contaminated fuel, setup and maintenace. 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 and RSR versions. 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. Pacific Fuel Injection and Eurometrix are two of the most reputable re-builders of injection pumps and throttle bodies.

Injection pumps can be recalibrated to almost any engine size and camshaft. 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. These 50mm throttles work quite well for 3.0 litre and larger racing engines. Another upgrade to consider are the RSR throttle slides from the 74 3.0 RSR. These were being re-manufactured by Andial but availability is now limited. Other aftermarket throttle slides and fuel rails are available for the 3-bolt flanges used on 3.6-3.8 litre engines. You can even fit excellent quality aircleaners to these throttle assemblies. Typically, FI pumps used for racing are recalibrated to RSR-spec and the idle-cutoff solenoid is removed.

Here is a ’73 2.7 RS with MFI and custom aircleaners. RSR fuel lines were used for clearance as well. The sound this system makes at full throttle is incredible. [insert picture]

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 and the torturous path that the air must travel make this system inadequate in this regard. Another major problem is the inability to use any sort of performance camshaft profile with the CIS injection. Performance camshafts have more duration and overlap that creates severe sensor plate pulsations. CIS-type camshafts are low-duration / limited-lift parts.

Having said that, 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 3.6 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 1984-2001 include performance chips, mass-airflow sensors and enlarging the throttle body for more airflow. The intake manifolds used on the 3.2 Carrera engines suffers from unequal air distribution between cylinders. Airflow in the stock intake runners vary between 180 to 290 cfm. After modification, these runners will flow 300 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 of these timing and fuel curves to achieve different results. One thing to be careful of is, too-aggressive ignition timing figures that are more prone to detonation unless racing fuel is used.

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. Many East Coast chip tuners do not make allowances for the lower octane fuels used on the West Coast.

Mass-airflow sensors and enlarged throttle bodies are both quite expensive for what they do. These components are best used on engines equipped with substantial exhaust changes and there isn’t anything left to modify. The cost per HP for these two items is quite high.

The 3.6 litre engines have much better 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 of 7-9%. The Motronic systems used in the 96 and later 993, 996, Twin-Turbo’s, and Boxster, , 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. Porsche offers a re-programmed computer for the 993 Twin-Turbo that adds over 25 HP when coupled with the larger oil cooler from the Turbo “S”. 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. 996 and Boxsters really respond to proper chip tuning and typically see torque increases around 15-18 lb-ft. 996 Twin-Turbo’s with performance chips will see a 60-70 HP improvement from software, alone.

Racing Engine Management Systems

Porsches used mainly on racetracks as well as non-budget minded individuals should consider the installation of one of these high-tech Engine Management Systems. There are good offerings in a range of budgets from MoTec and DTA.

These are all laptop-programmable EMS systems that allow the user to tailor the fuel sequence, timing, quantity as well as the ignition events and mapping from the on-board microprocessor. These can be used to drive a distributorless ignition system that uses individual coils for each plug or a conventional breakerless twin-plug distributor. Some of these units offer a data acquisition system as well as transmitting real-time data via an RF link to the pit, where its legal to do so.

All of these EMS Systems require a set of intake manifolds to work with. Rennsport Systems offers the tall butterfly EFI system pictured above packaged with all the necessary fuel system hardware and this one uses a DTA ECU.

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, money no object, are the Engine Management Systems from MoTec and EFI. These can be configured with knock-sensors and other sensors to be adaptive and are completely programmable.

K-Jetronic

This is a simple, reliable FI that meets its design criteria, but it isn’t optimal for performance applications due to its intolerance of any performance-type camshaft, sluggish throttle response and limited airflow capacity. For applications that require good fuel economy and drivability, this may be the best alternative.

Motronic FI

This 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 option.

In the case of street-driven 3.2, 3.6 and the 3.4 litre 996 engines, Motronic can be improved upon and made to work quite well in an emissions conscious environment. These systems tend to quite reliable with only some sensors giving much trouble. We would recommend anyone with this system carry an extra DME relay and cylinder head temp sensor. This system in its street-configuration will not flow as much air and fuel as the racing-type Motronic found on the 3.8 RSR and 996 GT-3R (RS).

Mechanical FI

This 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. This may be a drawback for some people since this service is not inexpensive. All things being equal,(and they rarely are) the Bosch MFI will make more top-end power than its equivalent carbureted setup. The introduction of the PMO carburetors has changed this. 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

These in their many sizes and configurations remain the overall favorite 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.

Now that EFI systems have become far more affordable and with their inherent flexibility and tunability, these are now the preferred choice for all 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 throttle stacks 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 its’ effective operating 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.

Porsche offers a plethora (I’ve always wanted to use this word in that context) of camshaft choices. Each has its own particular disposition and requirements to integrate properly with the induction and exhaust systems.

There are a few things to consider when deciding what cams to purchase and install in your engine:

• Application
• Engine size
• Compression ratio
• Gearing
• 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 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.

This just a small sampling of the camshaft profiles that are available. Consult your engine builder or Rennsport Systems for specific recommendations.

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 litre engine and you will get characteristics like good power from 4000 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, GE80/GE100-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”.

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!

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 exhaust manifolding like the pre-75 system. 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 Industries (heat exchangers)
• Monty Mufflers (mufflers)
• Dansk (heat exchangers & mufflers)

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, GE-80/GE-100 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 1/2" or 1 5/8"
2.5 to 2.8	1 5/8"
3.0	1 5/8" or 1 3/4"
3.2	1 5/8" or 1 3/4"
3.4 to 3.5	1 5/8" or 1 3/4"
3.6 to 3.8	1 5/8" or 1 3/4" or 1 7/8"

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.

We have had excellent results using European Racing Headers made by George Narbel. These are made in different sizes and are of equal-length construction and make better torque across the RPM range than the factory ones. Futher, we have some custom versions made expressly for small-displacement engines that work even better.

Next, we will discuss Ignitions, Cylinder Heads, Pistons & Cylinders, and the Bottom End. We will cover various configurations and making everything bulletproof.

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

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

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 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.

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 the Perlux Ignitor 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. Currently, there are now custom, billet twin-plug distributors made to fit these early cases that work VERY well. Modified Bosch units are also 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. There are alos custom, billet twin-plug distributors that work very well using Marelli Jaguar caps with a custom rotor. Porsche 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 Ignitors 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 2nd coil and a plug wiring harness. Non-Motronic 3.2 engines will use either a custom, billet twin-plug distributor, the Bosch RSR breakerless unit, a Carrera distributor modified to accept the Bosch Twin-plug cap & rotor, or an Electromotive 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 Magnecor 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 KΩ/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.

Here is an abbreviated rundown of Porsche Valve and Port sizes:

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 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  and 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 RSR/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 such as Extrude-hone.

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.

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 9.7:1 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, Nikasil and Alusil 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.

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 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

There are several options that exist for these large engines. 102mm, 103mm, 104mm & 105mm piston & cylinder sets are available to make 3.8,  3.9 and 4.0 litres. One other factor is high piston weights at these bore sizes that put quite a strain on the rods. Using  lighter CP,  JE, or Omega box-design pistons in these applications with some custom lightweight wristpins will dramatically lower the reciprocating weight and allow the engine to spin up quicker and significantly 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.

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 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 in mag-cased engine. 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. There is a suite of oiling modifications that should be done to ANY 911 that will see extended operation above 7K to protect the integrity of the rod bearings.

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 911′s used in competition should use the GT-3 RSR  or 930  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.

We strongly recommend only using the Porsche thermostat or the Troutman thermostat since they both have the same internal architecture and use the same Behr element.

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  integrated 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!