To help you understand the history and types of Porsche Transmissions:
Overview and History of Porsche Transmissions
Explanation by Transmission Types:
Type 901 Transmissions 1965-71
Type 915 Transmissions 1972-86
Type 930 Transmissions 1975-88
Type G-50 Transmissions 1987-98
Porsche Transmission Overview and History
When Porsche first started building their own brand of vehicles, they relied on many components which had already been engineered and put into production for Volkswagen. In fact, Porsche’s original concept was to build sports cars using Volkswagen components. As Porsche grew as a brand, it began to engineer and produce many of its own proprietary components for use in the first 356 models. Leopold F. Schmidt was a transmission designer for Porsche and was tasked with designing and producing a unique style of synchronization specifically for their new transmissions. This new design concept was to replace the Borg Warner style synchronizer which utilized a cone shaped dog gear. The new Porsche system was innovative but also a bit more complex than the cone-style synchronizer, and therein lie some of its more problematic idiosyncrasies. As with many of its designs like their transmission synchronizer system, Porsche had a tendency to stay with a concept and develop it as opposed to throwing it out altogether and starting fresh. The Porsche-designed synchronization was found in its first 911s, 912s and 914s models, as well as in several of its race cars including the 904 and 906. This early version transmission, from 1965 through 1976 had the internal factory designation “901.”
To be clear, the Porsche “transmission” should be referred to as a “transaxle” as it incorporates the final drive assembly; ring gear, pinion gear and differential, in the same main housing as the transmission itself. As the 911 became more powerful, the 901 transaxle needed to be beefed up to handle this additional power and the 915 transaxle was produced. The most notable difference to the driver was that the shift arrangement of the five speed 915 had reverse gear in the same gate as fifth gear, where reverse gear in the 901 transaxle shared its gate with first gear. Other internal differences included larger gears and shafts, as well as a stronger case. The 914 gearbox was simply a 901 transmission that had been adapted to working in a mid-engine application by flipping the ring gear and re-working the linkage to accommodate the shifter.
Overhaul of either the 901 or 915 transmission requires a certain number of special tools, some of which are imperative to have, others can be worked around. Many of these tools are available in the aftermarket at relatively low cost, while others can be purchased only through Porsche at significant expense. The tooling to set up the relationship of the ring and pinion gears (pinion depth, side bearing pre-load and ring gear backlash), for example, are not available through aftermarket suppliers and must be purchased directly from Porsche or sourced used from a technician selling off tools. Some or all of the adjustments to the ring and pinion gears may not be necessary if certain components affecting the relationship of the gears are not replaced or serviced. The components devoted to servicing the transmission portion of the transaxle do not require a daunting investment and are the areas which are more prone to require repair and rebuilding. Do not, however, attempt to overhaul a Porsche transaxle without a workshop manual or a reasonable facsimile sourced through one of the several repair information providers to our industry.
The Porsche/Schmidt style of synchronizer incorporates five main components; a brake band, an anchor, a stop, a friction ring and “dog teeth.” An operating sleeve, which is fixed to its respective shaft, first contacts the friction ring which has a surface that has a rough spatter coating. As the operating sleeve grabs the synchronizer, it turns until it hits the stop. The stop, in turn, then energizes the brake band, which then applies more tension on the synchronizer ring. This action is all to either speed up, (in the case of downshifting), or slow down, (in the case of upshifting) the gear that is going to be engaged. As the gear speed matches the spinning shaft, pointed “dog teeth” allow the operating sleeve to lock the selected gear to the shaft. All of this is happening in split seconds and the integrity of the individual components is key if a clash-free gear shift is to occur. The system is effective only if the key components are working properly and not worn. If the operator misses a shift, or doesn’t fully de-clutch when moving the shift lever, stress is put on one or more of these components.
The rough coating (like a cat's tongue) on the synchronizers eventually can be worn smooth with normal use, which then can cause the dog teeth and operating sleeve to clash and wear down the points which are there to assure clean engagement. The effect of improper use, or a missed shift, will begin a “snowballing” effect as wear on one component compromises others. This is why it is key during an overhaul to carefully assess each component for wear or damage due to past use. If the transmission is relatively high in miles, all synchronizer rings should be replaced as a matter of course, as these are the front line of the entire shifting process. Always pay close attention to the dog teeth as well as the teeth on the operating sleeve. Slight wear or evidence of minor contact to the teeth are okay, but the points of the engagement dog teeth should be distinct and relatively sound. The operational caution here is: if in doubt, replace it.
Porsche does not sell the dog teeth portion of its gears separately. As one can imagine, replacing a gear set, whether through Porsche or one of the several aftermarket suppliers, can be an expensive proposition. Fortunately, there are suppliers who make the dog teeth separately and they can be purchased for a fraction of what a gear set will cost. The replacement of the dog teeth requires the use of some very special tooling. While this equipment is not cheap, the tools will easily pay for themselves after a very few uses and they add tremendous value to the cost of a rebuild. It is vital to pay attention and follow the tool manufacturer’s procedures for a proper job. The replacement dog teeth gears are relatively thin and can be easily damaged during installation. It is also a good idea to utilize a high strength locking fluid to assure a reliable repair. This procedure, while not condoned by Porsche, has been done countless times by professional Porsche technicians and proves to be a perfectly reliable fix if performed professionally and conscientiously.
On both the 901 and 915 transaxles there are internal adjustments to the shift rails. The procedures for this must be followed very carefully as per the factory repair manuals. The shift rails and linkages are adjustable on two planes; fore and aft, as well as their relationship to one another, side to side. Failure to get this part of the repair done properly might necessitate a complete disassembly to access the pinch bolts on the shift rails that facilitate the adjustments. It is a great idea to engage gears while they are exposed prior to installing the housing and observe the movement of the shift rails to assure that they are properly adjusted.
The fluid that lubricates a Porsche transaxle is subjected to two major challenges. It must be highly shear stable to provide protection and wear of the heavy loads of the ring and pinion gears, and it also needs to be light enough to allow the synchronizers to function properly. Modern synthetics are far superior to what came before and there are several products to choose from. One stickler that requires consideration is the fact that most synthetic oils lubricate a little too well to allow the synchronizers to perform their duties. Lubricant manufacturers have become aware of this and have blended “synthetic” oils that actually allow the synchros to operate by not being too slippery while providing the shear stable characteristics of a heavier 75W-90 hypoid oil that the ring and pinion requires. A popular synthetic oil provider actually has an “NS” designation for “Non-Slippery” transmission oil that addresses this very issue.
The differential service components include ring and pinion gears, pinion carrier bearings and the differential itself. All are addressed as a separate unit even though they share the housing with the transmission. The setup of a ring gear is only necessary when a new ring and pinion or a new differential has been installed. The relationship between ring and pinion gears is critical to the longevity of the components as well as quiet running of the gears. While there is nothing new about what is measured in a Porsche ring and pinion gear setup, how they are measured is quite unique. Specifically pinion gear depth, one of the most critical dimensions to achieve, requires highly specialized tooling, and there is no way to make the proper adjustments without them. Each ring and pinion set is matched to one another and the pair should never be mixed with a different set. There is an “r” value assigned to every pinion gear after it has been finally machined and matched to its respective ring gear. This “r” value is a small deviation from the depth specification and must be factored into the adjustment.
The method for setting pinion depth is accomplished by putting shims behind the pinion shaft bearing, and this can be measured only with the factory designed equipment. This cannot be “fudged” or estimated or even set up with the original shim selection in the hope that it will work. The tapered roller bearings on each side of the differential housing can often be replaced without upsetting the ring gear adjustment, but checking the differential pre-load, ring gear backlash and gear patterns are all standard operating procedures after replacement and can be accomplished using more generic or even locally fabricated equipment.
In 1987 Porsche solved many of its gear shifting issues by switching to a Borg-Warner cone style synchronizer ring. This device, which is used widely in the industry now, has far fewer parts than its Porsche-engineered counterpart and is extremely effective and reliable. Porsche started using this with the 3.2 liter 911’s and has stayed with it through the 964, 993, and 996 models. The dog teeth on the cone-style synchronizer are shaped much like those on the Porsche style counterpart. The friction component, however, is on the inside of the synchronizer cone and it is pushed by an operating sleeve directly onto a corresponding cone on the gear that is about to be engaged. As the friction does its magic, the gear about to be engaged is either sped up or slowed in order to match the shaft speed before the dog teeth of the synchronizer locks onto the dog of the gear itself. Wear is much more easily measured on the cone rings, as there is a specification for the distance between the synchronizer ring and the face of the dog teeth on the gear.
The later, water-cooled Porsche models 996 were fitted with the G96 transmission. This too utilized the cone style synchronizers as did the G50 transaxle. The biggest difference internally is the use of larger gears and bearings to handle the additional power to be delivered through the gearbox. An unusual twist is the use of sealed bearings inside these newer style transmissions. Porsche utilized a sealed bearing internally on the intermediate shaft of the early Boxster engines with disastrous results. The use of sealed bearings seems puzzling but they seems to work reliably on these transmissions. Due in part to its larger gears and shafts, the G96 transmission has components requiring much greater press force to assemble and disassemble the gears and bearings from the shafts. Prior to tackling one of these newer transaxles a smart technician should be certain that a press capable of up to 25 tons of pressure is available for use.
From the onset of the G50 trans to contemporary transmissions, internal adjustments have been removed from the hand of the servicing technicians as now the accuracy of manufacturing has become precise enough that adjustment is no longer necessary. Instead of the shift forks clamping to the shift shaft via a pinch bolt, the forks are now secured in the proper position via a set of roll pins that do not permit any adjustment error. This, however, does not mean that things can’t go awry. Always check the internal shift rods for straightness and replace any if you find them bent. Pay attention to the position and movement of any detents or gear lockout pins as well, as these can either block a shift rail or allow two gears to be engaged simultaneously if not in their correct positions.
Both the pinion shaft and the mainshaft, also referred to as the input shaft, are supported by three bearings each. There are two roller bearings and one “four-point” bearing, which is essentially just a ball style bearing with the center race split into two components. The roller bearings support the shafts and control lateral play in the shaft and the four point bearings control fore and aft movement of the shaft. Special attention must be paid to all the bearings during inspection, but the four point bearings require especially close scrutiny. The ring and pinion gears carry a lot of load and forces are constantly trying to push them apart. It is the duty of the four point bearing to control the forces trying to push the pinion shaft away from the ring gear. Likewise, the differential carrier tapered roller bearings control the loads on the ring gear and they are subjected to extreme pressure and consequently higher wear than many of the other bearings in the transaxle assembly.
Inspection of these as well as the roller bearings on the pinion and mainshafts is a bit tricky as there is almost always what appears to be slight degradation of the surface of the bearing races. Some of this degradation occurs very quickly and is normal. Consider the expense and the role each bearing plays and consider how long the vehicle might be in service. A classic Porsche that is driven only a few thousand miles lightly each year might be given very different consideration than one that is driven year round or dedicated to track events only at high performance driver education events.
It is absolutely critical for the clutch to be fully disengaged (foot right down to the floorboard) before attempting to move the shift lever. Each time a gear is clashed or a synchronizer is disengaged with power still being applied, components suffer and become compromised. Inspection of the clutch itself is also imperative, as a dragging clutch, one not fully disengaging, will cause synchronizers to clash or be overworked. Installing a new clutch assembly along with any transmission work is considered standard operating procedure unless the clutch assembly has a service record of having been replaced or looks and measures to new specifications.
Transmission work can seem daunting at first to the uninitiated. Any job undertaken for the first time requires research, care and diligence. With thoughtful self-training, however, this area of repair can be conquered with minimal pain to a conscientious technician. Transmission repair work is an area of service that has become more common as the classic Porsche models age. The rewards for taking on this type of work and developing expertise will eventually pay off in the form of significant revenue enhancements.
901 Series Porsche 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.
The 901 series Porsche transmissions was a four or five speed longitudinal transaxle designed and manufactured by Porsche between 1964 and 1976. The transmission was used primarily in the 911 but variations were also used in the 904, 906, 912 and 914 production road and race cars. Transaxle is the correct definition instead of transmission due to the differential being part of the overall design. However for simplicity purposes this article will use the term transmission.
As part of the creation of the type 911 (901) car, Porsche created an all new transmission which became the 901 transmission. The initial version utilized a cast aluminum housing with later models having a cast magnesium housing. The basic design consists of a main segment that houses the gears, differential and bell-housing. However the two shafts and gear sets themselves are attached to an intermediate plate that is sandwiched between the larger casting and the end plate (sometimes referred to as the “nose cone” or “tail cone”.) The transmission was designed as a 5-speed however 4-speed versions were offered. The shift pattern is considered to be odd with reverse at the top left and first on the bottom left. This makes for a more difficult 1st to 2nd up-shift. The Porsche designed sychronizer system (aka “Balk Ring” design) was used on all 901 series transmissions. It is possible to convert from the Porsche synchronizer system to a dog gear racing style system.
The transmission was used frequently in Porsche racing cars partially due to the wide selection of gears that were available. Variations include the type 901 used in early 911 cars, type 902 used in the 912 car, type 911 used in later 911 cars and the type 914 used in the 914 car. While not technically part of the 901 series, the type 905 is a Sportomatic version that uses many of the design specifications (such as distance between the input and output shaft, gears, etc.) of the 901 series. Specific models are given an official Porsche “type” designation and this typically designates a specific configuration (gearing, intended use in a specific car, etc.) An example would be the “Type 914/12” which was used in the 1973-76 Porsche 914 car. A feature of the mainshaft on a 901 series transmission was that 2nd gear is part of the shaft. A special main shaft that was used on the 904 car (commonly known as the 904 mainshaft) does not include 2nd gear on the mainshaft. This allows for easier ratios changes for 2nd gear. Real 904 mainshafts are relatively rare and reproductions are available.
Typically these transmissions were provided with a standard “open” differential. However some examples were provided from the factory with a ZF clutch type Limited Slip Differential (LSD). The 901 series transmission was eventually replaced by the 915 series transmissions.
901 Transmission 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. We also offer 2nd gear ratios that are welded in place that have almost no failure rate in racing applications but the gear ratios can no longer be changed. 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.
Limited slip differentials from ZF, Quaife, or OS Giken 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 and OS Giken units can be adjusted by changing plate thickness for setting the locking factor.
The basic design changed somewhat over the production life. Main transmission case material changed from aluminum to magnesium. The intermediate plate that caries the main gear stack changed from a thinner to a thicker design. The differential experienced a number of evolutionary improvements. A number of other various internal items changed during production. As you would expect with the transmission being used in various cars there are design features specific to those cars. Those used for mid-engine placement have the differential “flipped”. There are casting differences based upon shift linkage configuration, transmission mounting methods, clutch activation methods and other reasons. The number of differences are to many to list here. While there are a number of differences there remains a number of things that changed very little. The basic gear design was unchanged during the life of production (you will find small manufacturing variations however). This makes it easy to re-gear these transmissions due to the wealth of different gears that have been manufactured over time.
When used within it’s design parameters the 901 series transmissions are well built and tough. However those that are pushed beyond these design limits, have high mileage and/or are poorly maintained can expose the weak spots in the design.
Magnesium Corrosion. Even the newest example of a 901 series transmission is now 35 years old. The thin chromate conversion applied to the bare magnesium and Cosmoline like Tectyl waxy protection wears off within a few years. Over time the magnesium case can experience internal and external corrosion.
Use with high torque engines. It is believed that Porsche set 138 lb/ft (sand cast aluminum) and 148.5 lb/ft (die cast aluminum) maximum torque values for the 901. However this value most likely represents a torque level that maximized longevity and that while the 901 can handle higher torque values, the use with engines that produce higher amounts of torque will reduce the overall longevity of the transmission.
Weak first gear. The location on the input shaft that holds first gear is the weak point on the input shaft. Combining high torque, grip levels from modern tires and hard launches, you can twist first gear off the input shaft.
"Simplied" Differential. The design for the differential used roughly between 1968 and 1970 can lead to catastrophic failure of the transmission when the “teats” on the end of the output flange stretch bolts wear off.
Ovaling of intermediate place bearing carriers. Over time the holes machined into the intermediate plate that hold the input and output shaft bearings can experience an oval wear pattern. This allows the bears to loose support and eventually cause failure of the bearings.
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:
1. 1st Gear 11/35 (stock)
2. 2nd Gear 17/31
3. 3rd Gear 21/31
4. 4th Gear 24/27
5. 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, Quaife, OS Giken, or Wavetrac units based upon preferences and availability. The pinion depth and backlash should be checked anytime you install an LSD.
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.
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.
Turbo Porsches can use a custom 993 Twin-Turbo base setup or a Sachs 935 clutch or a 930 Sport clutch assembly that has greater clamping power, much lighter, and are more durable. We also offer a Kennedy Competition Clutch that has proved to be very durable.
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.
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.