Is there a formula for converting between the two. Or is it a simple percentage. And if so, what's the formula?

 

depends on wether you have either a manual or automatic transmission. drivetrain loss for a manual is like 14% and an auto is around 17% so take your RWHP and multiply it by .86 if you have a manual or .83 for an auto. please someone correct me if i'm wrong

 

You are sort of wrong. The %'s you have are correct, but you need to divide rear wheel HP by .86 (or .83) to get flywheel HP.

Example....

300rwHP / 0.86 = 349 flywheel HP

The 14% loss for an M6 is probably OK for relatively stock HP cars. Using direct comparison engine and chassis dyno tests on my engine, I found that the M6 losses were about 12.8% at 500 flywheel HP, and only 12.2% at about 800 flywheel HP

Automatics can go the other way... when I converted to the TH400, I found that with a loose, high stall non-locking convertor, the losses were 19.5% at 500 flywheel HP and 21.5% at 800 flywheel.

 

I think you numbers are pretty close if not right but actully, some guy on another board said the math is wrong

You dont multiply, you divide

For example
400rwhp x 1.14 = 456fwhp
400 rwhp / .86 = 465 fwhp

I have heard the division is the math matically correct way

 

ah there we go. thanks

 

Thank you very much.

 

How about torque....or is torque the same at the flywheel as it at the drive wheels ....or does it depend on tire size, rear gear, trans, etc.?

 

I know tq is dependant on all that stuff

I remember on moster garage the rock climer the built out of a old bronco had and enigin with like 190 hp/ 240 ft/tq and the estimated with the gears it had it was making like 20,000 ft/tq at the wheels or something liek that

 

I posted about this in the lounge (because it isn't specific to a powerplant). Injuneer, I'm glad you posted your answer. You confirmed my thoughts. It isn't a percentage at all. 14 to 15 percent is a good rule of thumb for a stock-ish car, but it isn't really a percentage. Your driveline takes a certain amount of horsepower to move (tranny, ds, rearend, axles, wheels/tires). If it takes 50 hp to move all of that stuff, your stock 300 hp LT1 will dyno 250, right? Then, you mod it so that your dyno reads 300 rwhp. Well, the driveline bits haven't changed, so you've added a total of 50 hp, all of which shows up at the wheels. So, if you now dyno 300 rwhp, you can say you make 350 at the crank (not 300 rwhp/0.85 = 353 or whatever). See the difference? I've thought this for a while, but haven't done a proper engineering analysis of it. But Injuneer's response seems to support this. At higher numbers, the losses show up as less of a percentage, because the actual AMOUNT of hp required to move the driveline is the same. So, say it takes 50 hp to move the driveline. 50 hp out of 300 is a bigger percentage than 50 hp out of 600, right?

Any thoughts?

 

Not sure but sounds good to me.

 

I suspect that's not what Fred would have said. You've got a fixed portion of the loss, and a variable portion of the loss. As you go up in horsepower, the constant portion stays the same, the variable portion increases. So the net is that if you calculate the percentage loss (in a manual tranny) as the power goes up, that percentage will drop. But it's not like (in your example) you have a 50hp loss regardless of how much power you put through it.

Dave

 

Total losses are not constant with hp. If you look at Injuneer's numbers, 12.2% of 800 is about 98 hp, but 12.8% of 500 is 64 hp. The losses are closer to being proportional to total power transmitted than a constant. IOW, a percentage loss is a lot closer than a constant loss.

Ring and pinion losses are proportional to power transmitted thru them and are a big part of total loss. Bearing friction and tire drag are pretty much constant, but vary with rpm, of course.

Rotating inertial is pretty constant for the same acceleration, but a more powerful engine accelerates the inertia dyno rolll (like a Dynojet) quicker, so there are more inertia losses as hp goes up.

This interesting topic is addressed regularly on various forums here.

 

Yeah i think he was trying to say the same thing as you. I think he was just using 50hp as a way to explain it.

 

how much would a 12 bolt and a dennies nitrous driveshaft make the % goto.

 

Drivetrain losses have both a FIXED and a VARIABLE component.

The rotational inertia items... gears, clutch, driveshaft, axles, wheels/tires are FIXED. They do not change with engine HP. They will change if you change the mass or diameter of one of the rotating components. If you measure the FIXED losses - and ws6.com does this with a "coastdown" dyno test, you will find its a fairly small part of the losses. If you solve my "loss" numbers as simultaneous equations, to seperate the FIXED from the VARIABLE, you find the FIXED portion is only about 18HP. That number is going to remain constant, no matter how much HP you have.

The "friction" items are VARIABLE, and vary with the amount of torque being transmitted. Double the HP at any RPM and you have doubled the corresponding torque. That torque forces the friction components together harder.... gear faces and bearings see the load increased, and loss = friction factor times load, so the frictional loss is VARIABLE with engine HP.

Throwing in a torque convertor on an automatic confuses things even more. Losses in the convertor, and slip throw things all different directions. I won't even attempt to clarify those losses. As you can see from my measurements, losses in an automatic can increase as a % with HP.

As far as the specifics of the 12-bolt vs. 10-bolt, I doubt there is much difference, even though the diamter of the gears is increasing. And ws6.com also measured the affect of steel vs. aluminum, and you are only talking about a couple HP at most. Using a 9-inch will hurt you significantly, to the tune of 3% greater losses, because the lower pinion shaft placement causes higher internal friction losses.

My numbers for the M6 were measured with:

-McLeod steel flywheel/street twin
-stock T56
-Mark Williams 3" chrome moly DS
-Strange 12-bolt, 3.73 gears