Crossflow dyno results

[powderbrake]

 

adjusting power for wind resistance doesn't make any sense to me, unless he's trying to run your engine at very light load to simulate on the road fuel economy. power at the rear wheels is what it is, even on a windy day

 

Yes I agree, it doesn't make sense, but he says tha'ts how the Mustang dyno software works he said he HAD to enter that data. He said it simulates the horsepower required at speed. I didn't have time to discuss it with him, as he was busy running the cars thru the dyno. I also agree that power at the rear wheels is what we are interested in, so why the offset?, I don't know.

 

Once again, don't dyno your car unless you have a problem, or are going to use it as a baseline for modifications.

[slngsht]

hmmm... it would make sense if the number he was looking for represented rolling resistance of the drivetrain, which would be different.

[xflow7]

Frictional losses within the drivetrain should be wrapped up in the 15% losses number which leaves rolling resistance of the tire.

 

IIRC, on a normal road surface, passenger tires typically have ~2% rolling resistance.

 

Assuming a combined 600lbs on the rear tires gives 12lbs rolling resistance which, at 50mph I work out to be ~ 1.5 HP, so 12 1/2 HP still sounds high to me. Having said that, a dyno roller may well inflict considerably higher rolling resistance since the fact that it's cylindrical will cause more tire deflection.

 

Also, now that I think about it, there could be an additional component due to the slip ratio of the tire in generating the tractive effort on the dyno roller. The slip ratio could be several % which would show up directly in the HP number if you're measuring roller speed and the wheel is actually spinning several % faster.

 

If it's 5% slip ratio that would be 0.05*176HP=8.8HP, or 10.3HP for rolling resistance + slip ratio correction, so maybe that's where it comes from.