this is long but detailed. If you just want to formula, skip to the last paragraph.

I've done some work here to come up with a realistic conversion factor to take RWHP, as measured on a chassis dyno, and convert it back to an estimated equivalent flywheel horsepower. While everyone knows that rear wheel horsepower (or front wheel on a FW car) is all that really matters, our cars come from the factory with manufacturer's ratings given at the flywheel, most people think of power in terms of that, so it is nice to keep apples compared to apples. But you can't measure flywheel HP easily, only RWHP, so how do you know what your various mods really produce in terms of flywheel HP? You can only measure their improvement at the rear wheels.

Rear wheel HP is always less than flywheel rating, except in rare cases where the manufacture deliberately significantly under-rates the engine for whatever reason. Thus, a new C6 with the manual 6-speed will put down about 355 HP at the rear wheels, even though it is rated at 400 HP. The difference, 45 HP, is due to driveline and tires losses -- energy spent overcoming friction between gears, u-joints, and in the tires, etc. Maybe it's surprising that that much power disappears in the drivetrain, but if you ever feel your transmission after you've driven a while, you can believe it --its very hot, and that heat represents a lot of energy! Incidently, almost none of those losses are due to water pump, alternator, etc.: the flywheel rating is supposed to account for those (although not the AC, its assued you turn that off when you want max HP).

Anyway alot of aftermarket tuners use a figure of 15% losses as a correction factor: "Our MonstrosoBlock Corvette with our special elasticized cam produced 500 RWHP. Using the standard industry driveline losses factor of 15%, we rate it at 1.15 x 500 = 575 flywheel HP!" Similarly, alot of people trying to sell aftermarket parts use the same method of rating the improvement their equipment makes "Our Blow-hard Supercharger added 100 RWHP in back to back dyno test, that's why we claim it adds 115 HP measured at the flywheel."

Three problems with this: First, the people doing this always want to sell me something - so they have a vested interest in calculating the number as high as they can, I' not saying they lie, but they will certainly push a claim as far as possible.
Second, that arithmatic is not quite right, if driveline losses really are 15% of the flywheel rating, then the correct math is: 500/(1-15%) = 588 HP. Actually more,
But second, and more important, are the losses 15%? 15% is widely accepted but no one has really proven its accurate, and in fact, on Corvettes, at least late-model, M6 'vettes, its not even close.

So, here are some actual data, in each case I use the average of three or more cars of the same type, all stock Corvettes in nearly new condition.

C4 LT1 265 RWHP, 300 flywheel -- 11.7% losses
C4 LT4 293 RWHP, 330 flywheel -- 11.5% losses
C5 LS1 306 RWHP, 345 flywheel -- 11.3% losses
C5 LS6 358 RWHP, 405 flywheel -- 11.6% losses
C6 LS2 355 RWHP, 400 flywheel -- 11.3% losses
C6 LS7 445 RWHP, 505 flywheel -- 11.7% losses
Average for six types of M6 vettes 11.47% driveline losses

This is a very tight set of data - very consistent and believeable. Not only that, but it covers a 40% range of data (i.e, the lowest RWHP, 265, is 60% of the highest) meaning its consistent over a pretty wide range of HP.

All of this to get a formula, which I believe is legitimate in estimating the flywheel HP for a modified corvette, at least if it has the M6 transmission: flywheel HP = RWHP/(1-11.47%). Thus, that Monstroblock Corvette is actually only 565 HP.

Another way, which is just as good, is to ratio readings as in: my 400 HP C6 made 350 on the dyno, stock. With mods it does 385 at the RWs, so 385/350 times 400 = 440 HP at the rear wheels.

 

What an awesome thread Lee. Thanks for taking the time to share and explain that for us in such great detail. I always suspected different make/models of cars would exhibit different behaviours around the actual constant for the algorithm.[8D]

 

I was with you until the last sentence. Isn't that 440 at the fly wheel?

 

Opps, typing too fast!!! You are right., that 440 at the flywheel.

 

Thanks Lee,
Great usefull post!!!

 

Thanks Lee. That was a LOT of work. Great post, as usual!