The 901 Transmission
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.
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 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 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 from ZF or Quaife 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 ZF units can be adjusted by changing plate thickness for setting the locking factor.
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.
Turbo Porsches can use a custom 993 Twin-Turbo base setup or a 935 clutch assembly that have greater clamping power, 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. Back to top...
The 915 Transmission
With the power and torque increases, the 2.4 L cars also got a newer, stronger transmission, identified by its Porsche type number 915. Derived from the transmission in the Porsche 908 race car, the 915 did away with the 901/911 transmission's "dog-leg" style first gear arrangement, opting for a traditional H pattern with first gear up to the left, second gear underneath first, etc. Some say this was because the dog-leg shift to second gear was inconvenient for in town driving, other say it was due to Porsche’s desire to put 5th gear outside the main transmission housing where it could easily be changed for different races. The Sportomatic transmission was still available, but only as a special order.
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.
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
The 915 doesn’t require additional stiffening besides the 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 ZF or Quaife units based upon preferences and availability. The pinion depth and backlash should be checked anytime you install an LSD. Back to top...
The 930 Transmission
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. Back to top...
The G50 Transmission
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.
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. Back to top...