I know its been discussed before but i can't find it. How do you calculate hp at the flywheel? I always see hp discussed using RWHP, but the factory gives numbers pertaining to the crank/flywheel. Example: 500 rwhp times X equal flywheel hp. What does X equal? Cecil.........

neuronbob

07-09-09, 11:51 PM

Simple algebra :D ....the calculation should be crank hp times X=rwhp.....or crank hp=rwhp/X. The X is percent drivetrain power loss. Try 15% just as a general number. :thumbsup: HTH.....

CTSV_510

07-09-09, 11:58 PM

X varies with different cars and different dynos. And you don't use the estimated drivetrain loss in the equation, you'll use (1.00-loss = 0.85).

So generally people would multiply crank hp X ~0.85 to get rwhp.

556hp X 0.85 = 472.60 rwhp

To go backwards, you divide.

472.60 rwhp / 0.85 = 556 hp

But if you dynoed your car stock at 460 rwhp, your X would be 0.8273. That is, of course, assuming your engine actually puts out exactly 556 crank hp.

To sum it up, you can really only make an educated guess.

tedcmiller

07-10-09, 12:55 AM

The HP at the flywheel is more than the HP at the rear wheel by an amount equal to the losses through the drive train. The loss will vary from car-to-car. It is definitely simple algebra if you know the loss for the car you are interested in. The correct formula is rear wheel HP = flywheel HP - loss. In this equation, loss is some percentage of the flywheel HP. anonfrank gives 15% as a general drive train loss (is it correct - I don't know). For example, if you have 556 flywheel HP, then according to anonfrank rear wheel HP = 556 - 0.15(556) = 556 - 83.4 = 472.6 HP. In your question you asked what is X in the equation 500 rwhp times X = flywheel HP. In the example above, X = 1.176 since 472.6 rwhp x 1.176 = 556 flywheel HP. Note that X does not equal 1 + 15% = 1.15

The rear wheel HP given in many discussions is taken from dynamometer readings. So, to get to the flywheel HP, add the drive train loss (if you know it) to the rear wheel HP.

X varies with different cars and different dynos. And you don't use the estimated drivetrain loss in the equation, you'll use (1.00-loss = 0.85).

So generally people would multiply crank hp X ~0.85 to get rwhp.

556hp X 0.85 = 472.60 rwhp

To go backwards, you divide.

472.60 rwhp / 0.85 = 556 hp

But if you dynoed your car stock at 460 rwhp, your X would be 0.8273. That is, of course, assuming your engine actually puts out exactly 556 crank hp.

To sum it up, you can really only make an educated guess.

If this is an educated guess, then i take it that all companys put out higher hp numbers then whats acually there, so that they can one up their competitors. To bad everyone doesn't go by the rwhp. Thanks for the info. Cecil..........

Gotham CTS-V

07-10-09, 09:49 AM

Manual trannies are usually like 13-15% drivtrain loss.

Automatics can be 15-18% drivetrain loss.

All wheel drive also causes drivetrain loss.

I guess we just need a little compilation of stock dyno runs and we can calculate what the drivetrain loss is on automatics and manuals for these cars.

neuronbob

07-10-09, 11:41 AM

There is a stock dyno thread floating around here somewhere.

musclesbmf

07-10-09, 11:49 AM

If this is an educated guess, then i take it that all companys put out higher hp numbers then whats acually there, so that they can one up their competitors. To bad everyone doesn't go by the rwhp. Thanks for the info. Cecil..........

If you mean all companies as in GM, etc, no they don't make an educated guess. they have engine dynos that dyno the actual flywheel horsepower. They do this with the engine out of the car. I know for my Z06, they dyno'd each engine to make sure it was @ 505HP. Then again those motors were hand built.

Mark

tedcmiller

07-10-09, 12:16 PM

Another factor to consider is the government requirement that actual HP numbers be quoted now rather than the previously rounded up HP numbers. For example the Cadillac DTS used to quote 300 HP (in fact, they put in big black letters on the engine cover) for the performance version. The actual number from the dyno runs was 292 HP. When the feds said no more rounding up, Cadillac quit talking about 300 HP and removed it from the engine cover. I don't think they even quote a number in their advertising for the performance version (292 doesn't sound nearly as good as 300). I guess the federal government is good for something.