Your logic is no more sound than the original estimated drivetrain loss. Your "ex ante" assumption is that the factory rating of 240hp at the crank is accurate. From that assumption, you're calculating 9% driveline loss.

"No, that doesn't work."

Sir, your calculations are no more valid since you take BMW's 240hp rating at face value.

The positive value of the original post is that the HP and torque at wheels was dyno measured, not calculated. VenD at least acknowledged that he was estimating "typical" driveline loss.

Do you have any other suggestions for accurately measuring driveline loss? I would think that the only accurate way would be to have hp/torque values directly measured at the crank and the wheels.

Dean.

The estimate of drivetrain loss is an ex ante guess prior to getting your car dyno'ed. Once you have your dyno results, you can pretty accurately figure out your estimate drivetrain loss. Once you know your horsepower at the rear wheels, you can't just assume 15% and then use that to figure out actual horsepower. No, that doesn't work.

If he is getting 217horsepower at the rear wheels, and the M coupe engine is rated at 240 horsepower, then his loss is 1-(217/240) or 9.58%, not 15%. 15% could be a number you would assume before you dyno'ed your car.

In this case, you would make an ex ante assumption that your drivetrain loss would be 15%. You would expect your car to show 240-.15(240) or 204 horsepower on the dyno. Your result of 217 horsepower would then negate this assumption.

-Ben FR