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Calculating Horsepower

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I have had several discussions about crank horsepower vs Rear wheel horsepower. I wanna got more opinions form the Porsche board. Mostly from what I have read, most people assume a 15% loss from the crank to the tires.

For this calculation, you would mutiply the crank horsepower by .85

Example. A 300 horsepower 911 has (.85)*300 = 255 wheel horsepower.

Now, the person who dynoed my car says he multiplys 1.15 to the wheel horsepower. This is not the same.

Example. 255*(1.15) = 293.25 crank horsepower

I also looked at the TPC Supercharger website. They also use the 1.15 method.

Example. They claim the supercharger has 378 wheel horsepower, and 434 crank horsepower. 378*(1.15) = 434 crank horsepower.

My point is, this 1.15 method does NOT assume 15% loss from the crank to the wheels. It assumes 13.9% loss to the wheels.

Example. 434*(.87) = 378 wheel horspower.

So if TCP or the dyno shops use this method, and it truely is a 15% loss from the crank to the wheels, they are shorting their calculations at the crank.

Any thoughts?

Edited by IFLYASA
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Dynos really should not be used to compare different cars, but to compare changes made to a car. None of them are truly accurate as there is no NIST (gov't recognized) standard for hp (accuracy is the average deviation from a true value...standard). The more important characteristic is precision. The problem you have is the dyno gives wheel numbers which are then converted to crank hp numbers...so you have to start there. So you need to multiply the wheel numbers by some figure...might as well just use what you want as 1999 says

Edited by wross996TT
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15% is way too much, at least on a C2. on a C4S maybe, but on a C2, you have fewer mechanical parts being driven by the motor. On a C2, it is about 7 to 8%.

If that were the case using 8%, my car with the mods would only have about 285hp at the crank. That would be a decrease of 11 horsepower, rather than an increase of 14 horsepower. With headers, exhaust and intake, I don't believe 8% is correct.

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I was basing my satement on two facts. 1. My bone stock 2002 C2, factory rated @320HP produced 341HP at the crank per engine dyno. 2 My tuner, who did the dyno advised that he believed it to be no more than 8% loss. He is a certified Porsche factory trained tech, who recently left to go to the independent tuner. I had a 97 C5 Vette, factory rated at 345HP(LS1) motor, had a 15% loss, verified on a chasis dyno at 292RWHP bone stock. Everything on the C5 vette is heavier and there is simply more mechanical loss to the rear wheels and that number translated to 15%. If we do the math at 6 1/2% loss on the C2, we arrive at 320HP. I have also heard from sources that the Porsche number is a rear wheel rating, although I am less sure of that. It is defintely less than 15% on the C2. I believe 7 to 8 is just about right. Less mechanical motion to get to the wheels!!! I would like to hear further opinion. Loren, anyone?

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