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 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. 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. The larger plastic intake stacks as used on the "S" and Carrera RS engines are still available from Porsche as this is written.

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 are being re-manufactured by Andial for racers as well as restorers. 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.

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.

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 VarioramÒ -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 different camshafts. 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.

                        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 EFI, and Electromotive.

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 lpackaged with all the necessary fuel system hardware and 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, 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, sluggish throttle response and limited airflow capacity. For applications that require good fuel economy and drivability, this may be the best alternative.

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 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 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 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.

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

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

Here are some guidelines for 911 engines equipped with "S" cams or larger. Using RSR, Schrick or GE-80 camshaft profiles can allow the use of larger headers. You should consult your engine builder for advice.