Calculating engine HP from rear wheel HP #'s

[fekbmax]

I have heard a few different opinions on this and thought that maybe some of the guys here with dyno (both engine and chassis) experience could shed a bit of light on it.

Trying to closely estimate the engine HP from lets say ,,,
731 peak rear wheel Hp at 6700 RPM's, high gear, full clutch lock up.

[machoneman]

Keith, Wallace Racing has about the best calculators on the 'Net:

http://www.wallaceracing.com/Calculators.htm

[blykins]

My experience is that there are way too many variables to really nail down a correlation between the two.  Dynos are tools that should be used to show incremental testing and not really placed in comparison with other dynos. 

With that being said, the biggest variables seem to be the exhaust and transmission.  I've seen Shelby Cobras gain 100 RWHP by removing the sidepipes.

[jayb]

Just as a ballpark number, divide the rear wheel horsepower by 0.85; this is assuming a 15% loss in the driveline.  Could be more or less, of course, but that is in the general area.  In your case, flywheel horsepower is 860.

[scott foxwell]

Our experience is ~20% on the average. Definitely lot of factors involved.

[scott foxwell]

My experience is that there are way too many variables to really nail down a correlation between the two.  Dynos are tools that should be used to show incremental testing and not really placed in comparison with other dynos. 

With that being said, the biggest variables seem to be the exhaust and transmission.  I've seen Shelby Cobras gain 100 RWHP by removing the sidepipes.

The best Cobra ex system (Lo Back) we ever tested was 60hp @ the crank so that wouldn't surprise me.

[fekbmax]

Just as a ballpark number, divide the rear wheel horsepower by 0.85; this is assuming a 15% loss in the driveline.  Could be more or less, of course, but that is in the general area.  In your case, flywheel horsepower is 860.

Our experience is ~20% on the average. Definitely lot of factors involved.

LoL, the dude running the dyno said 18%.
Im guessing 15 to 20 % is in the ball park.

Thanks guys.

[fekbmax]

My experience is that there are way too many variables to really nail down a correlation between the two.  Dynos are tools that should be used to show incremental testing and not really placed in comparison with other dynos. 

With that being said, the biggest variables seem to be the exhaust and transmission.  I've seen Shelby Cobras gain 100 RWHP by removing the sidepipes.

Home made 2-1/8" equal length X 4-1/2" merged collectors.
Jerico clutchless shift. Hi gear pull, 5135 disk, loaded up on counter weight.

[Falcon67]

Just as a ballpark number, divide the rear wheel horsepower by 0.85; this is assuming a 15% loss in the driveline.  Could be more or less, of course, but that is in the general area.  In your case, flywheel horsepower is 860.

15% is the SWAG I tend to use.  Should be close - 122.8 @ 1675 lbs is around 470 RWHP.  Divided by .85 is close to 550 motor.  Which is about what I wanted.  I will put the motor on a dyno this winter sometime to work out new carburetion so we'll see how my guesses are.