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Old January 17th, 2004, 05:20 PM   #1
 
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Formula to calculate CHP/WHP

I know that this question probably varies by vehicle, but is there a set formula to figure out CHP to WHP (and vice versa)? I see ALOT of WHP numbers from dynos and I would like to know if you can convert that back to CHP (for comparisons). I have heard different percentages from 10 - 20 loss from CHP to WHP. I would think that a stick and automatic would have different equations (sticks having less of a loss). Any thoughts/input would be appreciated!!
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Old January 17th, 2004, 05:37 PM   #2
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There are no set figures. We use estimates. All cars are different. Your best bet and most accurate way is to throw the car on a dyno. Crank horsepower isn't really an accurate figure anyways. I see it more as a marketible spec.
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Old January 17th, 2004, 05:56 PM   #3
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On the industry standard Dynojet 248C dyno...

wheel hp/tq will be about 22-23% lower than actual crank hp/tq with an automatic.

wheel hp/tq will be about 15-17% lower than actual crank hp/tq with a manual.

The reason I say "actual" is because a lot of manufacturers are playing number games these days with their power ratings. What matters is what shows up on the dyno, regardless of what the crank rating is. And those "loss" figures above really won't vary too much. If something is coming back with a ridiculous 10% "loss" on the dyno then it doesn't mean that the drivetrain is incredibly efficient. It means that the engine was probably under-rated. And there's also a zillion variables, too. And some dynos read higher (or lower) than others, especially the Dynapack dyno. It reads about 6% higher than the industry standard Dynojet. So that's something to beware of also.

So if you dyno a car at 200 whp on a Dynojet, and then somebody else dynos their same car at 212 whp on a Dynapack, it doesn't mean their car is more powerful, just that that type of dyno reads higher and that's it.

Dynos are a great tool, but the information they provide can easily be misinterpreted and end up creating a lot of confusion in the end.
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