(303) 530-1094
gtgears@yahoo.com
How do I choose a differential for my vehicle?
The most typical installation choices for our differentials are as follows:
(a) Street / autocross - Torque-biasing suitable for most applications, especially autocross. 40/60 (or 40%) LSD for all Turbo and high-performance street cars.
(b) Street / track - Torque-biasing for light track use, with stock suspension. 40/60 (or 40%) LSD if car has stiffer suspension or power upgrades.
40/60 (or 40%) LSD for any car used primarily for track.
(c) Track only - 50/80 LSD Special order “Zero-Preload” LSD, with 4, 6, or 8 friction discs
Limited-slip differentials provide lock-up on both acceleration and deceleration. The amount of lock-up on accel and decel can be adjusted by selection of the internal plate sequence and/or ramp selection. Lock-up on deceleration allows late braking and aggressive entry into high speed turns, reasons why all professional Porsche road- racing teams utilize LSDs, rather than TBDs.
Torque-biasing differentials provide lock-up on acceleration only. The amount of lock- up increases as the amount of torque increases. On deceleration, lock-up is negligible, making the TBD ideal for the slower-speed turns of Autocross. (In the same slow speed turn, an 80% LSD would most certainly cause unwanted understeer.)
A torque-biasing differential performs like an open diff whenever one of the drive wheels approaches lifting off the ground. Lock-up and traction are lost until both drive wheels are again planted firmly on the pavement. Distinct limitations exist when a TBD is incorrectly used in a rear-engined or high speed track race car.
A great deal of confusion is caused by optimistic sales information distributed by companies that only offer torque-biasing differentials, or when a TBD is described by a vendor as a “limited-slip differential”. As Guard offers both differential types, we try to present as much information as possible, so that an informed choice can be made based on suspension set-up and intended use of the vehicle. Too often, we have seen the full potential of a car severely limited by use of the incorrect differential type, or by a low quality Chinese knock-off “LSD” that fails to provide long-term performance.
Thank you for your interest ---
Paul Guard
What is the service life of the low ratio gears?
The 2000 & 2001 GT3Rs have air-to-oil transmission oil coolers. In these early models, transmission oil temperatures could be expected to rise as high as 275- 300 degrees Fahrenheit in any race longer than 1 hour in duration.
In 2002, Porsche introduced the water-to-oil oil coolers. In these later GT3Rs, transmission oil temperatures could be expected to run at around 240 degrees Fahrenheit, even in races of 12 or 24-hour duration.
3rd gear idlers 1.550 (20:31) and lower (numerically higher) have a notoriously shorter life span. This issue becomes more pronounced when higher oil temperatures are experienced, and especially pronounced with ultra-low ratios.
Comparable life expectancy of low-ratio gears in the above two examples are:
| Oil Temperature | Life Expectancy |
|---|---|
| 275-300º F | 15 Hours |
| 240º F | 30 Hours |
As can be seen above, it is a risky affair to use ultra-low ratios for extended periods of time when oil temperatures are excessively high. However, efforts to reduce these high temperatures can yield substantial benefits.
“Life expectancy” is defined as “period of running time before obvious signs of pitting and/or galling, especially at the root of the individual gear teeth”. While a gear exhibiting these conditions would not likely experience catastrophic failure immediately, every effort should be taken to replace the gear shortly after these conditions become apparent.