I have noticed that many people think in the following way when it comes to determining their crank horsepower: We'll use an A4 in the following example with an approximate drivetrain loss of 18%. Whereas in an M6 it's only like 15% loss.

"My car has 320 rwhp (as determined by dyno) and so therefore it has about 378 hp at the fly" They figured this out by simply adding 18 percent of their rwhp to the number to get a total for the crank. So 320hp+18%(58hp)=378hp.

I believe that the calculation should be a deductive process however. Example is as follows:

390hp X 82%(18% loss applies here still)=320rwhp.

So between the two, the differences are that the first example adds the 18% loss to the actual rwhp number (which is going to be less of course than by adding it to the total flywheel hp number).

The first one shows only 378 hp at the fly, whereas the second one shows a full 390 horse at the fly. According to much of the general consensus on this site, both ways are correct. That is not the case. I think the drivtrain loss should be deducted from the total crank horsepower. So try plugging in different crank hp figures until you come up with what your rwhp is. Assuming a 18% loss for A4 cars and a 15% loss for the M6 cars. Don't simply add 18% to your rwhp figure as this is only giving you a drivetrain loss on a number that has that already factored out of the equation. The drivetrain loss should be applied to the total crank horsepower when determining fly hp, not the other way around.

My drivetrain loss figures are generalities and you'll notice that there is not as much difference between M6 and A4s as in the older cars due to what I presume would be increased efficiency in our automatic transmissions these days.

Now you can only imagine how much differnce this would make in calculations when dealing with much bigger hp numbers.

If my information is correct, this should be stickied.

 

If you look at the new SS. The figure would be 425 X .85 (15%loss) =361.25rwhp. I believe that the A6 assumes a 20% drivetrain loss with the 4l80e due to heavier duty than 4l60e with it's 18% drivetrain loss.

 

So if you know what your rwhp is, there is no reason not to know your flywheel horse. However, if you only have the the manufacturer's claimed flywheel, then you can only apply the drivetrain loss to that figure and be close, but not really accurate due to possibilities of that number being underrated from the factory. Just remember to SUBTRACT the loss from various flywheel figures until you come up with what your (dyno proven) rwhp number is. That is the only way without having a crank dyno to determine with accuracy what kind of hp your engine is capable of.

 

At a certain point, this method is no longer accurate. The drivetrain loss is a constant figure, until or unless you do mods to the transmission or rear that will sap more power.

For example, if you manage to get an LS1 engine to the 1000hp level on an engine dyno, then bolt it into an otherwise stock F-body (stock trans, stock torque converter, stock rear end), you won't lose 180hp via the stock drivetrain (that would be your 18% figure). With a stock 350hp engine, that same stock drivetrain would only be sapping 63hp (much more accurate number). Just because the engine now makes 650hp more, doesn't mean that the same drivetrain will waste an extra 117hp getting it to the wheels.

The drivetrain does not become *less* efficent at transfering power as power goes up, which is what's suggested when percentages are used. In other words, this percentage value only works within a certain engine HP range, after which the numbers are inaccurate.

 

What you say makes sense. Also I overlooked the mods to the rear and trans. I guess it applies more to stock/near stock vehicles and is largely vehicle specific *(ie trans type used is particular vehicle). Thanks for clearing that up sir. But c'mon, putting a 1000hp through a stock trans and rear? I'd say the loss would be 100% due the grenade factor lol. Jk.

 

I wonder at what hp range the percentage theory stops working?

 

I've always used 16% for the Standards, and 22% for the Autos..

Take the Dyno Readings and divide by the decimal percentage..

EX..

Standard: RWHP/.84
Auto: RWHP/.78

 

Would bigger, heavier wheels and tires also skew the "RWHP" numbers?

 

Absolutely...... It is one big reason guys run light drag racing wheels. More HP to the ground...

 

Good formula, but when dealing with the 4l60e and stock 10 bolt, I really doubt the drivetrain loss is that high. By those figures that would mean that a stock ls1 car putting down 300rwhp (01s and 02s often can put this down with the ls6 intake) would produce 385 hp at the crank, stock! I know GM has underrated these engines, but I doubt they were that high in stock form. I would estimate that my car would be about 390ish at the crank AFTER all the mods, but not stock. Just an observation. Your 22% loss is more realistic on an older turbo400 trans from the back in the day, not now. Even the 4l80e would surprise me if it lost that much, let alone the 4l60e which is nowhere near the same. I still think 15 percent on an M6 and 18/19 percent on an A4 is realistic. That would make an M6 car that puts down 310rwhp approx 365 hp at the flywheel. A little more realistic I think.

 

Yes, which raises an interesting question for me. I am wondering how much heavier or lighter my rims are than the stock hawk ones were. Mine are one peice american racing, same dimensions as stock. I wonder if the lighter weight of a 16 inch rim would out weigh the lower traction of a 17? Guess it would depend on whether you have streets, and how much tire smoking power you have. If you don't spin much on 16s, my guess is that you'd do better with them.

 

I think 12 percent is better for our cars. 300/.88 = 340. Which is where a stock ls1 should be.

 

I would have a hard time believing any ls1 is under 350 at the fly, even with the ls1/egr intake. But that seems close to me. You may be right.

 

Well the ones that dyno on the low end (285) are prob not 350 fly.

 

Yes I guess on an A4 without the ls6 intake that low is possible for sure. So you're right there. But I think it's safe to say then that the "average" ls1 produces about 350 flywheel horsepower in stock trim. Agree?

 

Yeah probably since the gto and vette were rated at 350

 

+1


OP if you really want to factor it out..teh 'proper' mathmatical equation is:

320/.82 (18% loss)
320/.85 (15% loss).

 

dont forget wheels and stall converters can play a huge factor as well. and the % is only a guesstimate its not a perfect number. some cars are a bit better than others for whatever reason

 

http://www.stangbangers.com/01_Bullitt_Article2.htm


So do you guys think this car should have been rated at 380bhp??

 

Probably lol. I remember that article