For any gearbox parts and needs, contact Matt Monson at Guard Transmission, here is his email address GTgears@Yahoo.com - Tel: (303) 530-1094 and website http://www.guardtransmission.com/index.html - Let Matt know Armando sent you.
The information below was found here http://www.rennsportsystems.com/2d.html
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.
After reading all the posts on short gears, curiosity is getting the best of me. I there a plausable method for calculating what gears are in my car without opening up the box?
I know that I have some odd short gears in the box, but not sure which ones. TIA"
Tom's link is one way to tell. You can also look at the gearbox number stamped on the underside of the gearbox housing, e.g. 901/02, 902/0, 915/xx, etc. If it hasn't been modified, look up the factory specs for the gearbox you have.
If there are no markings or the gearsets have been changed, use this method:
The overall gear reduction in each gear is the product of the gear ratio x the differential axle ratio.
If:
A = gear ratio
B = differential gear ratio,
then:
AxB = overall gear reduction
For example:
A = .96:1 (e.g. 5th gear)
B = 4.43:1 (typical 7:31 ring and pinion)
then .96 x 4.43 = 4.25:1 overall gear reduction
Thus, in gear position A, the engine rotates 4.25 revolutions for every 1 revolution of the drive wheel.
Working backwards, you can roll the car forward in each gear and count the number of turns the crank pulley rotates for every one revolution of tire rotation. Do this for every gear.
For example, chalk one of the rear tires at 6 o'clock, then roll the car forward in 1st one revolution (remove the spark plugs to make this task a little easier). If one revolution of the tire results in approx. 13.7 turns of the crank pulley. The overall gear reduction in first is then 13.7:1. Repeat and record the results for each forward gear. Create a Word table to keep track of your results.
To determine what part of that gear reduction is due to the gearbox ratio versus the differential ratio, divide each of the above by one of two commonly-supplied rear axle ratios.
Most 901 and early 915 gearboxes were equipped with a standard ring and pinion ratio of 7:31 or 4.42:1 (driven/drive teeth = gear ratio). The later 915 gearboxes have a 8:31 or 3.875:1. However, the factory provided some other ratios as well, but they're kind of rare. For example, I once had a 901-5-speed with a 6:29 differential (4.83:1), and some early 901-4-speeds had a 3.85:1 differential ratio.
Thus, either:
13.7 / 4.42 = 3.09 gear ratio or
13.7 / 3.875 = 3.53 gear ratio
In the 901 gear ratio chart I'm looking at, there isn't a 1st gear ratio that's as low as 3.53:1. So chances are the gearbox doesn't have a 3.875 differential. However, 3.09:1 equates to the gear designated as "A" (11:34 ratio). If it goes as smooth as this, assume the 4.42:1 gear ratio in all your calcs when repeating this for each of the remaining forward gears.
