How to calculate RWHP vs Crank HP - Ford Mustang Forum
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Old 11-02-2008 Thread Starter
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How to calculate RWHP vs Crank HP

Alright, I am still confused about the difference between these. I mean I know crank horsepower is just about the power the engine is creating at the crank and rwhp is the amount of hp actually making it to the wheels but is there any formula to figure how much rwhp you have? Like a stock 05+ V6 is making 185 whp and GT is making 240 whp stock, is there any way to figure this out for other cars? Just wondering..

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Old 11-02-2008
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There is no magic formula, because there are just too many variables. SOme include geraring, condition of lubricants, mass of rotating parts, manual or automatci transmission and a few others.

Usually, you will see a 15% loss from crank hp to whp on a manual and 20% on an auto. This is just a rough estimate, but tends to be a rule of thumb.

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Old 11-02-2008
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Like Kyle F said.......generally a 15% drivetrain loss. I remember seeing something on a site where they were calculating actual HP from rear wheel HP by dividing the RWHP by 0.85 (which would account for the 15% loss).

300 BHP x 0.85 = 255 RWHP (approximately)

425 RWHP / 0.85 = 500 BHP (approximately)
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Old 11-02-2008
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+1 on the things said above, 12% loss manual 15% on autos approximately, but you also have to consider the stock numbers are approximate some cars get a good bit more and some a good bit less
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Old 11-03-2008
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For 2005+ Mustangs a very good "estimate" is 12.5% Manual loss and 19% Automatic loss. Typically automatics run about 6% percent greater loss than the manuals. Many simple use 15% (manual) and 20% (auto), and while convienent, that does tend to slightly over estimate crank horsepower. Don't forget that each dyno, type of smoothing, gearing etc (and even the operator) can have a significant impact on the final numbers. Dyno number really should only be used as a baseline for YOUR car in determining performance gains from upgrades and NOT to compare it to other cars (ie; paper racing). Hope that helps.
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Old 11-03-2008
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i think 20% loss for an automatic is too high.

I would say 12% manual and 15% auto.
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Old 11-03-2008
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I've heard the auto on thew 05+ was more along the lines of 17% but it really isn't that big of a difference.

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Old 11-04-2008
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Everything I've seen matches up with 12% manual and about 15% auto but every car is a little different.

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