Transmissions & Clutches

Submitted by on January 11, 2009 – 9:34 pmNo Comment

Transmissions & Clutches

Transaxles and Clutches

Performance gearing has been a staple of Porsche performance since 1948. Selecting the proper transmission ratios for your gearbox is an integral part of improving Porsche 911 automobiles. It has been said that installing a set of shorter spaced gear ratios is like adding 50 HP to your car!

Transmission and Clutch Overview: 901-911

Let’s talk about the various transmissions installed in 911′s and 930′s since 1965. The first transmission designed for the 911 was called the 901 transmission. This unit was used in different variations in the 911, 912, 914 and 914/6. The early versions used sand-cast aluminum cases and the later ones used pressure-cast magnesium. These were 4 and 5 speed transmissions with a torque input rating of 138 lb/ft, later uprated to 148 lb/ft in 1969. The fact these transmissions have been used with engines producing more than 230 lb/ft of torque, albeit with shorter lifespans (measured in broken gear teeth), speaks volumes about the basic design. In 1970, Porsche changed the 901′s clutch actuation and called it a 911 type gearbox. These were still 901-type transmissions now coupled with a larger, pull-type clutch actuating mechanism. Clutches used in these cars were the 215mm unit used from 65-69 and the 225mm unit used from 70 to 71.

Transmission and Clutch Overview: 915

For the 1972 model year, Porsche used a larger, stronger transmission called the 915. Using a different shift pattern from the 901, this gearbox offered a torque input rating of 181 ft/lbs. Racing versions with the pump and cooler allowed this rating to be increased to 275 ft/lbs. Some people have used this transmission with the 930 Turbo engines with some success as well. 915′s were used in several variations until 1987 when Porsche introduced the G50 gearbox. Early 915 transmissions employed magnesium cases until 1977; 915 versions made in 1978 and later used aluminum cases that were stronger and had strengthened differentials.

The 915 came with two different final drive ratios, 72-74 units were equipped with 7:31 ring & pinions and the 75 and later transmissions had 8:31 final drive ratios. The 8:31 ring and pinions are significantly stronger than the 7:31 units and are the most desirable one to use for use with higher horsepower engines where service life is probably doubled. Starting in 1984, the 915 transmission used in the 3.2 911′s were equipped with an oil pump and integral cooler since the torque of the 3.2 engine could have shortened the life of the gearbox.

915 transmissions were equipped with several variations of Fichtel & Sachs 225mm clutches. Porsche used aluminum and cast iron pressure plates and spring centered discs. The ill-fated rubber centered disc was an attempt to reduce the low-speed gear rattle that all 915 gearboxes have. By now, all of the rubber centered discs should have been replaced with the noisier, but much more reliable spring center type disc. Replacing the iron pressure plate with a aluminum one is a good upgrade when it becomes necessary to replace the clutch assembly. This will help the engine rev quicker and shift better due to lower inertial moments.

Transmission and Clutch Overview: 930

The 930 or Turbo transmission was first used in Porsche street cars in 1975 with the introduction of the Turbo Carrera. This was a 4-speed gearbox that was much larger and heavier than the 915 which has proved troublesome when used with the 3.0 Litre Turbocharged engines. The 930 transmission is an aluminum cased unit capable of handling 326 ft/lbs of torque continuously. Racing versions with oil pumps and coolers were used in the 935 race cars that made over 750 HP albeit with shorter life spans. Turbos were equipped with a 240mm clutch to handle the extra torque that was moderately successful. There are some excellent upgrades to reduce the rotating weight and increase holding pressure in high-powered applications.

Transmission and Clutch Overview: G50

Porsche realized the shortcomings of the 915 transmission’s synchronization system as well as its torque limitations when the 3.2 engine was introduced. In 1987, the G50 gearbox was installed into the Carrera 3.2 911. This transmission used Borg-Warner syncromesh instead of the Porsche-design balk-ring system to improve the shift quality and lessen the effort required to change gears, especially from a stop. These gearboxes were rated at 221 ft/lbs of torque. The G50′s have been utilized with several variations of gearing and speeds. The 993 series was the first 911 offered with a 6-speed version of the G50 design. These later units also introduced significantly improved clutch cooling.

Other versions of the basic design, called the G50/52 series, were used in the 3.3 C2 Turbo and 3.6 C2 Turbo cars. These transmissions also have stronger differentials and cases and a type of Limited Slip differential that locks 20% under power and near 100 % on the overrun to minimize trailing-throttle oversteer. All of the G50 cars use the 240mm clutch size introduced on the earlier Turbo cars and they are now hydraulically actuated. In 1990, the infamous dual-mass flywheel was introduced on the C2/C4 series. These flywheels were intended to help reduce low-speed gear noise however they have proven to be problematic on these cars. A popular conversion invloved installing the single mass, lightweight flywheel from the Euro Carrera RS into these cars for a performance increase and much improved durability. The well-known stalling issues can now be resolved.

Ring & Pinions and Limited Slip Differentials

In order to fit a compact transmission with the requisite features in the 911, Porsche uses very steep hypoid angles on the ring & pinion gears. This make these parts, the most highly stressed part of Porsche transmissions. This requires a GL-5 rated lubricant to protect these components against premature failure. Ring and pinions are problematic areas of these transmissions and require careful setup for any 901, 915, 930, or G50 used for competition or high-horsepower applications. The majority of 901 and 915 transmissions came with 7:31 or 8:31 final drive ratios.

Porsche also offered several other ring and pinions for racing including 7:37 and 7:33 ratios. These are not always available due to limited manufacturing qualities. The G50-series used a final drive ratio of 9:31 and the Turbo versions, G50/52 used larger, stronger ring and pinion gears.

If you do decide to change the ring and pinion in your gearbox, make sure that you get the appropriate one for either the mechanical or electronic speedometer.

Porsche has offered the ZF clutch-pack type limited slip differential in almost all 911′s and 930′s since 1966. These units are quite strong and can be set for locking factors of 40% to 80%. The clutches need replacement under racing conditions to maintain proper lockup but these are quite trouble free. An 80% locking factor is difficult to drive on the street, especially in wet conditions, but for open track racing these are quite popular. Street or Autocross usage requires the LSD to be set at 40%; simple to do with the proper parts.

There are now other LSD options for 901, 915 and G50 transmissions called Torque-Sensing differentials. These are gear-type units as opposed to the ZF clutch-type ones, and do not require any parts to maintain optimum performance. This unique, patented design allows full differentiation between the wheels as well as providing power to both sides and they produce less understeer than the Factory LSDs. Guard Transmission makes an excellent one as well as the unit from Quaife.

For Autocross and normal street driving ,the Torque-Sensing units are more benign in their operation but they do not improve the handing like the Factory clutch-pack units do. The ZF or Guard Transmission LSD’s really stabilize the car under braking and help reduce trailing throttle oversteer.

Transmission Modifications for Improved Performance

One of the first things a Porsche owner wishing to improve the acceleration of their car should consider is changing gear ratios. The wide gear ratios used by the factory are a result of designing for street drivability, fuel economy and emissions, not best performance!!

As we said earlier, a close-spaced set of gears in any 911 adds an “effective” 50 HP to the acceleration potential. Reducing the RPM drop each time you upshift keeps the engine in the most productive parts of the torque curve. One must experience this to fully appreciate this effect. Take a ride on a PCA Driver Education day in someone’s car that has a shorter gear stack and you will be convinced.

There are several options for shorter gear ratios depending upon usage, tire size and ring & pinion ratio. There are even computer programs that will help you select the proper gear ratios.

901 Modifications

These transmissions utilize 2nd gear as integral to the mainshaft. This means that to change this gear, you must either purchase a new mainshaft as part of a new 2nd gearset or use an often-scarce 904 mainshaft that allows you to change any gear with changing shafts each time, a much less expensive proposition. The gearsets used in the early 67 911S was A-F-M-S-X. This is a very nice set of street ratios that works quite well in almost any 901-equipped car. There are other combinations as well so be sure you factor your tire sizes!

The 901 magnesium-cased gearboxes will truly benefit from the installation of a forged, billet 6061 T-6 intermediate plate that carries the main bearings. This prevents the shaft deflection present on the stock gearbox that contributes to failure under severe conditions or when used with larger engines. The OEM magnesium plates are prone to cracking as well as being quite flexible and cannot support the transmission shafts as well as the aluminum one can. We also like to install a billet side cover to the differential housing that helps prevent the ring gear from “walking” away from the pinion gear.

We would also recommend the installation of a cooler and pump for maximum transmission longevity when used with 2.7 litre or larger engines. You can also install nozzles to squirt oil at critical points such as the ring & pinion and each gearset. This really helps carry the heat away from these highly stressed parts.

Limited slip differentials such as Guard, OEM ZF, or some others will really help get the power down with higher-horsepower engines. These almost always require a change in driving style to accommodate the additional understeer created from the added traction. As mentioned previously, the Guard/ZF units can be adjusted by changing plate thickness for setting the locking factor as well as change ramp angles for assymetric operation (Guard only).

Clutch Modifications

For street driven 901-equipped cars, installing the 69 “S” clutch with the drilled flywheel and aluminum pressure plate assembly is a bullet-proof and relatively lightweight unit.

915-equipped Porsches should use the aluminum pressure plate instead of the cast iron one and a spring center disc. There are other racing discs and RSR pressure plates available for the high-horsepower cars that need the extra clamping force.

There are various custom setups for Turbo Porsches. 993-996-997  Twin-Turbos’  use a LWF/GT-2 package that have greater clamping power, are much lighter, and are more durable.

Many people with G-50 equipped Porsches install the European RS single-mass flywheel setup for quicker throttle response and much better durability. Due to the engine’s propensity to stall easier, modifications must be done to the idle stabilizer and in some cases, Motronic software to compensate for the 50% reduction in weight. The only disadvantage is some additional transmission noise in the car at idle.

Race cars use some the ultra-lightweight racing clutches available for the 901 equipped cars that make shifting much easier as well as allow the engine to rev MUCH quicker. This really reduces the wear and tear on the syncros, operating sleeves, and balk-rings. These are not suitable for street driven cars.

915 Modifications

The 915 transmission, unlike its cousin the 901, has 1st gear machined onto the mainshaft. This means that you must change the entire shaft when this gear is to be changed. Since this isn’t always cost effective to do, most folks simply change 2nd through 5th gears to achieve the desired spacing. There is a good selection of 915 gears now available on the aftermarket that allow you to build a custom gearbox for your exact application. Here is a real nice close-ratio street setup that has been very successful:

  • 1st Gear 11/35 (stock)
  • 2nd Gear 17/31
  • 3rd Gear 21/31
  • 4th Gear 24/27
  • 5th Gear 27/24 or 28/24

Consult your Porsche tuner or contact Rennsport Systems to find out what gears are available and what to use in your application.

The 915 doesn’t require additional stiffening besides the Wevo bearing retaining plate and differential side cover. Machined billet side covers for the magnesium and aluminum-cased 915′s help eliminate the side thrust in the ring and pinion gears that contribute to failure. Adding a pump and cooler is a very good idea when this transmission is used with engines producing over 250 HP. You can either use the Carrera RS oil pump and internal squirters or the 3.2 Carrera oil pump and integral cooler. Both of these modifications will require a new end cover on the gearbox.

Sustained power levels over 300 HP require the installation of oiling nozzles at the most critically loaded areas of the gearbox for maximum durability.

Limited slip differential options are the same as the 901. You may elect to use the Guard, OEM ZF or some other units based upon preferences and availability. The pinion depth and backlash should be checked anytime you install an LSD.

930 Transmission Modifications

This transmission is relatively easy to change for better performance. While the Turbocharged 911 truly benefits from installing a 5-speed, this is not inexpensive to do! The 930 gearbox can be made to work very well by changing  gear sets to reduce RPM drops at each shift. These transmissions need cooling when the power levels climb about the 550 HP level or for racing. Installing a cooler and pump to supply pressurized oil to the ring and pinion makes these gearboxes last much longer.

G50 Transmission Modifications

These transmissions as used in the Carrera, C2/C4, C2 Turbo and 993-series cars are more expensive to buy gears for, compared to the 915 and 901 units. The availability of gearsets is very good with these units. Porsche’s racing program and the availability of high-quality aftermarket gears make these transmissions very attractive for street & racing applications.

When used with very high torque engines and large rear tires, the ring and pinion assemblies are prone to premature failure. Differential side cover deflection and undersized ring and pinion gears for the power levels are responsible for transmission failures not generally seen in Porsches used for endurance racing to such a degree. Auxiliary cooling and pressurized lubrication systems are necessary in these applications with this transmission. The G50/50-series is probably the strongest of this generation of gearbox although these are 5-speed units.

Our Web Site has a picture of a G50 6-speed transmission that was extensively modified for the Daytona 24 hour race. This gearbox was used in a 993 3.8 RSR that finished 12th overall and 4th in GT-3. The pressurized lubrication system in the photo provides cooled oil to all gear sets and the differential. This unit also was equipped with a special side cover and custom-made 4th and 5th gears. Needless to say, this was not an inexpensive enterprise due to the number of hours required to accomplish this. Many G-50 6-speeds utilize a fixed 2nd gear so a new Motorsports mainshaft is required to regear these transmissions. Even the new GT-3 street cars come this way.

914/6 Notes

The unusual case of the 914/6 requires a few comments. Since the transmission in these cars resides behind the engine, they operate at much higher temperatures than in a 911. Regardless whether you use a 901 or a 915 gearbox in these cars, you should install a pump and cooler to maintain transmission life with engines 2.7 litres or larger. For racing applications, we like to install a pressurized lubrication system in the 914/6 for maximum component life. The clutch also runs hotter in this configuration so don’t neglect this part as well. It doesn’t hurt to add some ventilation to the bellhousing under racing conditions.

Miscellaneous 915 Application Notes

One issue that surfaces with the early 6-bolt flywheels used in racing applications is keeping the flywheel from coming loose. Everyone wants to use a lighter flywheel, and there are many benefits to having one. Better and faster shifting, extended syncromesh life, and faster speed out of corners as the engine picks up revs quicker makes his very desirable.

The commonly used RSR steel flywheel is quite notorious for vibrating loose when the engine is revved beyond 8000 RPM. This is caused by a combination of crankshaft vibrations and flexing of the steel flywheel when used with that pull-type clutch. Shifting at engine speed over 8000 RPM will shake the flywheel loose and cause serious damage to the crankshaft as well as contribute to a DNF.

For these applications, we recommend that the flywheel bolts be installed with Blue Loctite and slightly overtorqued. We also advise using the aftermarket aluminum flywheel that is more rigid than the RSR unit.

A racing clutch such as our Tilton 5 ½” unit doesn’t place so much stress on the crankshaft as the factory-style unit. Aluminum flywheels are now available which provide a less expensive and streetable alternative to a full racing clutch system.

The later 9-bolt crankshafts used in the 3.0 SC and Carrera engines do not seem to suffer as much from this phenomenon.

Other modifications are the use of aftermarket bearing retainer plates and billet side covers that stiffen and maintain precise shaft spacing to help protect the ring & pinion. These are especially important when using a 915 with a 3.6 or 3.8.

Another problem that has surfaced with time are elongated main bearing races in the aluminum 915′s (’78-’86). Unlike the magnesium-cased 915′s, the later versions didn’t have a steel insert that held the main bearing. Over time, the aluminum castings soften a bit and the bearing begins moving around which elongates the machined recess it sit in. This causes the bearing to spin in its bore and allows the pinion shaft to move, relative to the ring gear of the differential. Needless to say, this compromises the life of the ring & pinion and the carrier bearings. The fix is to weld the damaged hole and machine the aluminum differential housing to accept a steel insert to properly locate the  bearing (just like the mag-cased versions). This makes for a very robust transmission that better than how it left the factory.

Lubrication Issues

Oils are always a subject of opinion and controversy. Everyone’s opinion is based upon personal experience. The more experience one has in the area in question, the more weight we give to the opinion being offered. In our experience, the 901, 915 and 930 transmissions seem to shift and operate better with non-synthetic oils. The Porsche designed balk-ring syncromesh needs more friction to operate smoothly than synthetics provide. We use and recommend the Swepco line of gearlubes for these gearboxes. They simply shift better than any other oils that we have used. The G50-series trannys, on the other hand, really work well with a full-synthetic gearlube like Mobil 1 Delvac or Redline. The Borg-Warner type syncros work better with a slippery oil. We do not use or recommend any transmission additives besides the LSD improvers sold by GM dealers for the times that you need to eliminate LSD-clutch chatter.

Other gearlubes might be just fine too, but “caveat emptor” applies here.

As always, we have just touched on this subject. Consulting a gear chart for your transmission will show you what your gear splits are and what you can change. Not all of those gearsets are always available so factor this accordingly. Setting up a transmission for racing or even aggressive street use may be best left for professionals due to the component costs, special tools necessary, and the consequences of a small mistake. Ring and pinions require a special fixture to set the pinion depth and ring gear backlash properly.

Since changing gearsets can make a stock engine much more fun to drive, this should be near the top of your performance modification list. Installing a set of close-ratio gears is the equivalent of adding 50 HP to the performance of your car.

For further information or consultation, please contact us: 503 244 0990