RWHP to Crank HP reduction
 

I have been researching the percentage reduction for a manual transmission car between the flywheel/crank engine dyno HP vs the rwhp on a chassis dyno. I was only looking for empirically data and not "the rule of thumb". I never trust rules of thumb without some science! Knowing there lots of variables, starting with the engine dyno usually done in optimal conditions to every aspect of hp reduction points on the car. After many hours of searching the best I have seen with people trying to address this question is an 18.5% reduction. Most empirical comparisons were actually in the low 20%'s. Way more than the "rules of thumb". Any thoughts on this topic? What is your experience?

My first comment would be you can't work with just one % without addressing manual or auto tranny. An automatic is going to eat up much more power than a manual.

There is no magic number I completely agree, but I have always operated under the range of 12-18% loss for manual and 20-25% for automatics.

No "real world" experience here, but I do have experience in the NASCAR world, although It won't apply to street cars, because we built everything to last one race, and it was all low to zero drag components. Numbers there are unbelievably low, but it only has to make it one race...
What drivetrain are you contemplating? Not only the gearbox, but the rear end also. The rear end makes a bigger difference than you'd think, GM vs. Ford, that is. Like you already stated, there are a lot of variables.

Not trying to compare auto vs manual. I was only trying to understand the urban legend of 10-15% lose for manual. Can not find any facts to back this up and I think the low 20's is more realistic. I cannot even find an empirically supported 12 - 18%. The best is 18.5 with a lot of transmission work and very light axles. All the rest are the 20's.....

We have had all of our race car engines on the dyno ... and then the whole car on a quality Superflow chassis/wheel dyno over the last 8 years.

This is around 35-40 cars & engines ... some with manual trans & 9" Fords & some with direct drive & quick change rear ends.

For all the "standard" drivetrains, we measured 18.1-18.8% difference (loss) from the engine dyno.

For cars we reduced the parasitic losses in ... with REM polished gears, ceramic bearings, no drag seals, lightened gears, etc ... we saw 16.4-16.8% difference (loss) from the engine dyno.

But we never lightened up parts that would have reduced the reliability, like driveshafts, u-joints, etc. More could be gained, but the risk wasn't worth it to us.

We lightened driveshafts, and lost power due to shaft flex causing harmonics.

Sorry, I missed that in your first post.

Just for a reference, when I was in the Porsche world for a couple years. Engine to chassis dyno conversions were routinely in the 12-15% range.

I have read about sub 10% losses for NASCAR... any truth to it, Che70velle?

I have read there is also dimishing returns at higher horsepower engines. In other words, 15% (or so) at 550 - 650 hp, maybe a percent or two less over 750.

I have never seen proof of this theory though.

That's interesting Scott.

For our race teams, not finishing races due to parts failure was unacceptable.


So we went the other direction. Stronger driveshafts (3"), bigger u-joints (1350's not 1310's or 1330's), nothing "borderline".

I forgot ... we did run some powerglide automatics for 1 season & did chassis dyno test them.

These ran NO converter, nor clutch, so they were light & designed to lock the gearsets hard. With only 2 light gear sets & nothing else they used less power than typical automatics.

The numbers were 19.1-19.4% difference (loss) from the engine dyno.

But, I don't think we can compare these #'s to the automatics in street cars, even well built pro touring cars.