The Porsche Transmission:
The most misunderstood and overlooked component of any Porsche has to be the transmission. How many gearbox upgrades, and hot-rod equipment do we see ads for? Not many. Engine upgrades are a plenty. The fact of the matter is that the transmission transmits power to the ground and it has to function well for the car to work well. It is usually the last thing that people talk about or even consider when undergoing upgrades to their cars. The parts are contained within a case of some sort so all of the working parts are closed off and the gearbox itself is usually hidden from view. When considering engine enhancements, make sure that your gearbox is up to the task and has had the service it needs. Often the gear lube is never changed during the life of the vehicle. This would go a long way toward gearbox performance. Services abound and vary in cost depending on needs.
Another consideration is the shift linkage. 1986 911 and earlier cars through the 356 come into our shop continually with “needing transmissions” and shifting can be dramatically changed with service to the shift linkage. The nylon bushings in Porsche shift linkages are many times deteriorated from age and use. Check these details before you think you need a transmission rebuild.
Some background on Porsche transmissions follows.
901-series transmissions were used in the 911/912 from 1965 until 1971. The 1970 and 1971 models employed a slightly different case that permitted the new 225mm pull-type clutch. 901’s were also used in the 914 and 914/6, but differ by the ring and pinion swapped for the mid-engine configuration. 901 gearboxes are also four and five speeds. The casting type number will decipher which is which.
These gearboxes work well for engines in the 2.0L to 2.4L engine sizes without significant modifications. We do recommend installing the aluminum intermediate plate in a 901 that will see any competition. This adds a good deal of strength to help support the shafts. If more than 200hp is produced, an oil cooler and pump is needed for the transmission. Race cars should also get a pressurized lubrication system to help with reliability.
These transmissions can be re-geared quite successfully for racing and/or street applications. They share similar components that were used in the 904 and competition 901 cars, so plenty of parts are available to work in any application.
The 915 appeared in 1972 911s with a major upgrade in strength, compared to its 901 predecessor. The first ones didn’t shift as well as the later ones due to some improvements in the shifter and synchronizers. 915 gearboxes were made from magnesium cases from 1972 until 1977. Some of them had aluminum final-drive cases in 1977 before the entire gear case was made from aluminum in 1978 with the introduction on the 911SC. 915s used 7:31 ring & pinions until 1975 when they switched to the stronger 8:31. These later ring & pinions are the preferred item for racing or high-horsepower applications and the later 915’s had stronger differentials and main shafts. The 915 gearbox was used through the 1986 911 model year. The 930 four speeds are a variety of the 915, and were continued until 1988.
All Porsche transmissions use a very steep hypoid angle in the differential due to the necessity of close shaft spacing. Since these gearboxes all use the Porsche-design balk ring synchromesh, they depend upon some friction for this brake-band type synchronizer to work properly. Selection of gear lube will dictate how the gearbox will work. Off-the-shelf synthetics have a slip angle that is so high that it will not allow enough friction for the gearbox to work properly. Hypoid gear lube traditionally works better in most of the older gearboxes, but under high power and heat, they have a tendency to oxidize.
The best lube we have found is the Mobilube SHC, it works very well in all Porsche gearboxes. It is factory fill in all 996 GT3/GT3 R cars.
There are currently more gear ratio choices for the 915 transmission than any other Porsche gearbox. It is a favorite for installing in early 901 bodied hot-rod street cars and has been used in 930s and others due to its versatility.
The Porsche 911 transmission took on a huge improvement in reliability, durability, and ease of operation in 1987 with design 950, commonly called the G50 gearbox. They are used from 1987 to present day in all 911, 964,and 993 bodied cars. These gearboxes, made by Getrag, use the Borg-Warner (Ford) style synchromesh and have an input torque rating of 221 lb-ft. A variant of this is still used in the 996 GT3, GT3R, and 996TT/GT2 cars. As with the earlier Porsche transmissions, Porsche has manufactured a “ton” of gear ratios, as well as ring and pinion sets to allow the gearboxes to be setup for virtually any kind of driving, as well as race track. Most of the street car gearing is selected with fuel economy in mind so there are many opportunities for performance improvements with changing gear sets. The first version of the gearbox was a 5 speed, but with introduction of the 993 brought on a 6 speed version of the G50 gearbox. The 993TT and four-wheel drive 993 and 993 4S use a similar gearbox with a driveshaft that extends through the nosecone to drive the front wheels. It has always proven to be a very reliable gearbox often working with twice the horsepower it is originally designed for. Change the gear lube every couple of years and it should last.
The G96 gearbox was introduced at the start of production 996/986 cars. A 986 (Boxster) gearbox has the G86 gearbox designation. The G86 is basically the same as a 996 gearbox with the ring and pinion swapped for the mid-engine configuration. The most noticeable difference in these gearboxes is the cable shifter. All 996/986 cars other than 1998 996 GT3 cars use a cable shifter.
The standard 996 gearbox (G96.00, G96.01, G96.30, and G96.31) is a development all its own which is similar to some current VAG (VW Audi Group) products, but shares almost nothing with other Porsche transmissions. There are many 996s that have suffered from ring and pinion failures, synchronizers, and other maladies at a variety of mileages. The only fix is to replace with a Porsche remanufactured unit, or a good used one. Parts for these gearboxes cannot be obtained by the manufacturer or Porsche. They unfortunately cannot be fixed.
The 996 GT3 gearbox is different (type G96.90, G96.93, G96.96) from the standard 996. The GT3 gearbox has evolved from the 993 GT2 which in turn evolved from the 993 TT, and before that the 1989 930 G50.50. The current GT3 box is also used in the 996TT/GT2 and 996 GT3 Cup/GT3 RS/RSR race cars. It has a dedicated oil pump and external oil/water intercooler, steel synchronizer rings on gears 3-5, and interchangeable gear ratios (main shaft assembled from individual ratios that are positioned, not pressed into place), and a 40%/60% asymmetrical clutch type limited slip differential.
The Tiptronic transmission is a development of the traditional automatic transmission, offering fully automatic shifts as well as the ability to manually shift up and down as with a manual gearbox, not needing to use a clutch pedal in-between shifts. The first Tiptronic transmission appeared in a Porsche production car in the 1989 964 (Carrera 2). Porsche had undergone many developments since the first concept in the early 20th century. In the 1920s, André Citroën’s collaborator, Adolph Kégresse, made important experiments in this area, and in 1939 ZF filed a patent for a dual-clutch gearbox. But it was not until the mid-1980s that a feasible solution for Porsche was established. This was named Porsche Doppler-Kupplungs (PDK, or double clutch device) used in 1985 by Porsche’s 956 Gruppe C car and Audi’s quattro S1 rally car. The PDK allowed the next gear change to be made while the clutch-disc and flywheel were still frictionally involved and transmitting power to the driven wheels. Thus, for the first time in history, there was no pause for a break in transmission of engine power to the driveline of the vehicle.
These transmissions have evolved now (in 986, 996, 997, Cayenne, etc.) to be computer controlled and part of the other systems in the car. The engine ECU and Tiptronic ECU work together to maximize efficiency of the transmission. The technology allows the transmissions to be more reliable, more economical, and smoother operation. Occasionally there can be computer component faults that will cause the 986, 996 and in cars to act up, and can be fixed with mostly external parts.
Tiptronic cars for the most part are fairly trouble free, but still need their proper maintenance of fluid changes and inspections to make sure they are working up to full efficiency. Leaks, overheating, and contaminated or improper fluid are usually the reason these transmissions fail. Follow your owner’s manual and you will most likely be fine over the life of the vehicle. Rebuilding Tiptronics is not cost effective, and parts are hard to come by for anyone. The options are either installing a good used unit, or a Porsche Remanufactured transmission.