Quote:
Originally Posted by DrM
Nyulak, I think that you misunderstood what WilyB was saying. He is correct.
If you experience a 13% drive train loss and you know the crank horsepower of your car, you figure out the wheel horsepower rating like this:
400hp (at the crank) * (1-0.13) = 348 wheel horsepower.
so:
Crank_hp * (1 - loss%) = Wheel_hp
Since we use chassis dynos, we know the Wheel_hp instead of the Crank_hp. So, if you wanted to estimate the Crank_hp you would do this:
Crank_hp = Wheel_hp / (1 - loss%)
which is the same as
Crank_hp = Wheel_hp * 1 / (1 - loss%)
Given that BMW estimates a 13% drive train loss on the manual cars, this would be 1/(1-0.13) = 1.149425287
So, if you knew the Wheel_hp you could simply multiply the Wheel_hp by 1.149425287 to get the Crank_hp.
In the example above 348 whp * 1.15 = 400.2 ~= 400 crank_hp
WilyB simply rounded this to 1.15. It is simply coincidence that it happens to be the value that BMW estimates as the drive train percentage loss for the automatic.
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Those formulas work OK initially for a rough estiamte, but I hate when they are applied to a modified car. The driveline loss stays constant unless you change something in the driveline. It just boggles my mind when tuners claim 800+hp kits for a car putting 6xx hp to the wheels because they are applying that same percentage loss.