(Contributed by Karlo)
Porsche G50 Transmission Information
Authored by CJ www.ultima-canam.info/index.htm
This is for Informational purposes. The info has not been double Checked.
There a two variants of a G50; "long" & "short". The "long" box as the name suggests is longer. The "short" box has a transverse mounting at the rear and as the name suggests is slightly shorter.
Porsche changed the design of the release fork in 1988. Pre. mid 1988 the release fork had needle roller bearings inside it ran on a 15mm dia bar. The release bearing guide tube was 34mm dia. After mid 1988, The release fork (now PN 950 116 712 3R) was revised to run on an 18mm dia bar with the bearings in the box at each end of that bar. It is possible to have the box modified who have the facility to machine out the 15mm hole to 18mm to accept the new bearings . Also the release bearing guide tube is now 32mm dia. If you do buy a clutch separately, it is a simple matter of modifying the box. TIP: When fitting the box to the engine, tie the release fork to the release bearing using an elastic band. This will hold it in place and allow you to get the pivot bar into the box and will be lost when you start the engine
Always buy a box complete with:
rear mount beam ("short only")
These parts can be expensive to replace.
1987 911 G50s have an 8 digit code stamped in the bottom, for example 'G50/0073H'_'12/00050'
G50/00 = G50 trans
7 = trans for 6 cylinder
3 = 5-speed Rest of World; 4= 5-speed USA/Japan/M298)
H = 1987 model
00050 = serial number
A 1988-89 trans has a 12 digit code, such as 'G5000 1 J 00903'
G5000 = trans type (see below)
1 = normal differential (0= no diff, 2= ZF ltd. slip diff)
J = 1988 model (K= 1989)
00903 = serial number
G50/00 = 911 Carrera, R.o.W.
G50/01 = 911 Carrera, USA/CND/J/AUS
G50/02 = 911 Switzerland (very short gear ratios)
G50/50 = 911 Turbo world wide, 1989 only
G50/52 = later Porsche turbo boxes ZF ltd. slip
Transmission and Clutch Overview: G50
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 G50s 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.
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 G50 used for competition or high-horsepower applications.
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 911s and 930s 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 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 an builder wishing to improve the acceleration of their car should consider is changing gear ratios. The wide gear ratios used by Porsche are a result of designing for street drivability, fuel economy and emissions, not best performance!!
A close-spaced set of gears in any car 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.
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.
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. Porsches 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.
The Rennsport website 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 it.
Oils are always a subject of opinion and controversy. Everyones opinion is based upon personal experience. G50-series trannys really work well with a full-synthetic gearlube like Mobil 1 or Redline. The Borg-Warner type syncros work better with a real slippery oil. Generally speaking, transmission additives are not recommended 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.
I 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 depth and 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 the car.