Originally Posted by critter

Good thread. The dyno issue was glossed over, though. The question at hand is the comparison of flywheel horsepower and rear wheel horsepower, or engine dyno vs. chassis dyno. A Dynojet does not measure horsepower. It does not even measure torque. I measures acceleration (well, measures speed and differentiates to get acceleration) of a drum of known weight. Given that, the acceleration of all rotating components (wheels, axles, posi, driveshaft, trans shafts, cankshaft, flywheel, clutch, etc) will consume HP and reduce the acceleration of the drum, and thus will not be measured. The dyno software then corrects its computed number to try to compensate, but as was pointed out earlier, the horsepower of the vehicle under test will skew this as more power will be "lost" in the faster acceleration of the non-drum rotating components. (And in fact, Dynojets seem to over correct, giving numbers too large) Given all this, there is simply no way you can write a simple formula that will correlate fw and rw hp. For a graphic demonstration of this, you might try dynoing in 2nd and 4th, or with 3.23 gears and then 4.56 gears and compare the readings. However, if you use a Mustang dyno (which does measure torque) and don't use the acceleration mode, you will get real RW HP and stand a much better chance of deriving a conversion formula.

Originally Posted by Chris ARE 360

It has been verifed multiple times that the drivetrain loss through an M6 with stock rear is right around 50hp.

This is a constant value and doesn't fluctuate as a percentage.

ie: if my car dynos 450RWHP and I am running stock T56/rear/clutch/flywheel/DS then my engine horsepower is right around 500.

Cheers,
Chris

Originally Posted by trackbird

Go here, scroll down. There is a dyno sheet in 3rd and 4th for my car and a 3,4,5 dyno pull up to the speed limiter. It made more HP in 4th gear then either 5th (reduction of torque from overdrive) or 3rd ("RPM effect").

http://home.columbus.rr.com/trackbirdsz28/my_z28.htm