[Venom 427]

Keith,

Oddly enough that was also the very first question that was asked on the cobra forum after my reply was posted there. So if I can quote my reply there again :

"The rear end we use is from the E36 3 series BMW through years 1992 to 1999. We do not modify any of the BMW rear end components including the control arms. The ratio we use mostly in the rear is a 3.15. The following ratios are an option depending on availability:
3.46:1 ~ 4.44:1 ~ 3.45:1 ~ 3.91:1 ~ 2.93:1 & 3.64:1

With respect to the horsepower and torque rating on the differential, after much research no definitive limits were discovered. What we can draw on is the experience of others with the differentials. A BMW suspension based Cobra runs on the 1/4 mile in Johannesburg SA with a 350 Chev engine, a 150 shot of Nitrous and on slicks. He states he has 500bhp, 600 ft/lbs of torque, does a 10.5 sec 1/4 mile and has never broken the BMW differential but has broken 2 T-5 transmissions. The question of how much torque the BMW rear end can take is very technical.
When will it break (not necessarily how much torque) depends on many factors including, tire diameter, tire width, diff ratio, coefficient of friction (traction), weight transfer to mention a few. Of course it would be interesting to know what is the weakest link as well - the c.v. joints, gear set or side shafts!

Having said that, the set up of the Venom 427 will handle the majority of applications required by customers. Our car is focused on the majority market satisfying drivability and a low entry price point, as opposed to the small fringe market of high torque applications such as drag racers. Over time we will develop a database of applications and the performance of the BMW rear end, and keep the Forum posted. I think the knowledge about Ford 9 inch rear ends and their limits was gathered in a similar way, through years of use in racing and street performance applications and the size of the US market for these products.

The article below is from a transmission website and does confirm a few factors involved
in drive-train design. I hope it helps.


Taken from http://www.gearzone.net (without permission)
When the concept of putting a 5 speed in an E-TYPE Jaguar was put in front of me I thought of many base platforms. I picked the T5 base simply because it could handle the power and torque of 6 and 12 cylinder Jaguar engines and yield a good life factor. It also was one of the finest shifting transmissions ever built. Many people advertise gearboxes that can handle 600 horsepower and 500 FT lbs of torque. The T5 is rated at 330 FT LBS and the Tremec is now rated at over 400 FT LBS. What does this mean? Even Tremec's test on the T5 and 3550 are not done under the same time / load frame. Gear boxes are rated on what the manufacturer's idea of how much torque a transmission should hold over a specific period of time. If no two tests are done exactly the same it makes it difficult to really say what a rating means when comparing gearboxes. You can apply more torque to the transmission above it's suggested rating but the life factor of the gearbox will decrease. Generally torque ratings are based on what torque the transmission may see in its life time. Most manufacturers will also fiqure on 150,000 miles as a life time. Holeshots or clutch dumps decrease that life factor. It is not uncommon to see a 400 plus horsepower Mustang GT run a T5 at the drags but break it weeks later. The box was never designed for this nor should it be put in that class. Put the same box in a 450 hp road car with little shock loads and to your surprise the T5 doesn't break. If your engine produces 400 FT LBS of torque at 3000 RPM that torque load is split between the transmission and rear. If you have a 1 to 1 rear axle ratio, most of it is absorbed in the gearbox. If you have a 4.56 rear axle ratio the torque load shifts more towards the rear axle. That is why most gearboxes break in cars with "dead rear end gears" such as 2.76 or 3.08 ratios. (Stock Mustang GT'S) I did not see a need to compromise quality shifting for a highly rated torque load that would never be exceeded in almost all of the conversion applications.

Regards,

Ross Sinclair
Performance Cars USA
www.performcars.com"