Intake Systems – Camshafts – Exhaust Systems

Submitted by on March 1, 2018 – 4:03 pmNo Comment

Intake Systems – Camshafts – Exhaust Systems

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

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

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

Induction Systems

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

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

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

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

Solex Triple-throat

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

Zenith Triple-throat

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

Weber & PMO Triple-throat

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

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

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

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

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

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

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

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

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

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

Bosch Mechanical Fuel Injection

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

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

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

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

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

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

Bosch K-Jetronic (CIS)

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

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

Bosch Motronic

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

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

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

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

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

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

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

Individual Throttle Butterfly Intake systems

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

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

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

OK, But What do I use?

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

  • Budget
  • Tuning Adjustability
  • Ease of Maintenance
  • Reliability

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

Exhaust Systems

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

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

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

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

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

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

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

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

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

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

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

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