I was looking for the post about calculating your horsepower with the 20% drive train loss-
but i can't find it..

any help is appriciated..
Thanks.

 

Flywheel HP x 20% then subtract that from flywheel HP = RWHP

 

no - thats not what i was lookin for... i want to calculate rear wheel hp...

 

Flywheel hp X .8 = rear wheel horsepower assuming 20% loss.

300 fwhp=240rwhp (300 X .8=240)

This is again assuming 20% drivetrain loss.

 

That's what I give you,

like Lo-tec put

300fwhp x 20% = 60hp loss

300fwhp - 60hp loss = 240rwhp approximate

 

The products of American Public School Systems at its best.

 

Wow... thats pretty funny...

Im going to assume it was a temporary mental lapse... (ill give yu benefit of the doubt...

 

I see spelling isn't too high on the list either.

 

where do you get 20% drivetrain loss?

is that with an auto? its not much with a standard is it? but what about a lighter weight flywheel? aluminum driveshaft?

 

All these are approximations, and the percentage loss will vary depending on who you ask. 20% loss for auto, 15% loss for stick is a close guess for calculating flywheel hp. A lighter flywheel and driveshaft will lessen the drivetrain loss due to the lighter rotational weight.

 

so- assuming that the calculation from you two is accurate- and with the 20% loss for stock auto drivetrain-

my 380fwhp engine should be around 304 rwhp-
correct....

Is there any way to figure torque ?? it was 420lbft flywheel

 

Torque would be 336. Again, these are all just approximations. Transmission type, rearend type, driveshaft weight, flywheel weight, rear gear ratio, etc etc all change the fudge factor a little. If you want to know the power to the wheels, put it on a dyno. If it's been on an engine dyno and is tuned in correctly, there really is no need. They dyno is just a tuning tool; the track is where it matters.

 

agreed- but, im going to put it on the real dyno when i get it finished-.. im tired of my buddy bragging about his mustang GT and im going to show him how sorry it really is....

 

I've always hated the percentage drop rule since it's not very accurate.

Assuming the 20% drop then taking out a 300 hp engine which will lose 60 hp and replacing it with a 500 hp means it now loses 100 hp through the drivetrain from just an engine swap. How do you lose 40 more hp through the drivetrain with nothing but an engine swap?

Drivetrain loss is a fixed constant so knowing exactly how much hp each component eats up needs to be calculated.

For example a TH400 tranny uses up 44 hp. A TH350 uses 36 while a simple little powerglide only eats up 18 hp. In any tranny the amount of HP it uses can be changed from different internal modifications or even by using synthetic oil.

 

I have to disagree on that. Inertia eats up the extra HP.

Quick example: If you do a dyno run with a 300 hp engine it takes say 10 seconds for the pull. Replace it with a 500 hp engine and it now takes 8 seconds for the pull. It takes extra hp to accelerate all the components in the drivetrain faster. That's why a gear change from 3.42 to 4.11 (faster acceleration) loses a couple hp, or changing from a 30-lb steel flywheel to a 13-lb aluminum one (less mass to accelerate) picks up a couple hp.

 

ok but i heard aluminum flywheels arent a good choice,but to stay with steel.

how much HP does a 700r4 take up? what about a t5 or t56? i plan on going back to standard since auto sucks and id like to get some TQ back.

it sucks if you make 375HP will only produce 300 at the wheels?

20% is that with an all cast iron motor (heads/intake/pulleys, etc) with auto/heavy flywheel, stock driveshaft etc.

what would you guess say with

aluminum heads/intake/pulleys/water pump/driveshaft, it wouldnt be 20% right? i was hoping more like 10% haha

 

heads, intake and pulleys could contribute to extra hp, but have nothing to due with drivetrain loss (although the alum heads and water pump would shed a couple pounds). The only thing on your list that would make a difference would be an alum driveshaft, and that would only be a minor difference (wag of 2-3 hp).

 

That isn't a calculation; it's a guess. It just combines various numbers you guessed at, using arithmetic. That doesn't make the result anything other than a guess.

There is NO SUCH THING as "calculating" RWHP from FWHP, or vice-versa. There are far too many variables. Every detail about the car the engine is installed into will affect it. Everything from the exhaust system, to the fluids that are used, to the tolerance the transmission clutches were stacked to, will change it. Even the drive shaft angle affects it.

If you want to know how much power you're getting at the wheels, go to the chassis dyno. But even then, one dyno will give you a different reading than another, depending on how the operator makes "compensations" for whatever. The ONLY thing it's good for is before/after testing & tuning.

There's a formula for calculating RWHP from track times; but even that makes some assumptions.... like, perfect traction (we all know how realistic THAT is), perfectly optimized gearing and torque converter, and so forth. Again, it involves a bunch of guesses. But it's the closest thing you can get to reality.

An aluminum drive shaft only makes HP higher if measured during acceleration. If you held the RPMs constant, it would have no effect whatsoever. In the real world, the car accelerates, which means the RPMs aren't constant though; so it does in fact effect the car's ET. In other words, it skews the reading, but in agreement with a real effect.

 

Sure, there's a calculation you can do:

1) Put the engine on an engine dyno, obtain the peak horse power. That's the EDHP.

2) Put the car on a chassis dyno, obtain the peak horse power. That's the CDHP.

EDHP - CDHP = drivetrain loss, DL. (DL/EDHP)x100 = % drivetrain loss.

3) Put the car on a different type of chassis dyno, obtain the peak horse power. That's CD2HP.

EDHP - CD2HP = drivetrain loss #2, or DL2. (DL2/EDHP)X100 = % drivetrain loss #2.

4) Repeat #3 until you have exhausted all of the chassis dyno possibilities, and don't have any hair left because you now realize how qualitative all this really, really is.

FWIW, it has been proposed that RWHP calculations based on track times are actually more accurate than chassis dyno data. There are a bunch of reasons for that, no need to get into that now.

 

Problem is, to get "drivetrain loss", the engine has to be set up on the dyno EXACTLY THE SAME as it would be in the car... same air intake source, same belts & accessories, same exhaust, etc. etc. etc. Otherwise, you're not going to get "drivetrain loss"; instead, what you're going to get will just be "difference between motor on a stand and what makes it to the wheels". Which will ALWAYS be a greater number than "drivetrain loss".

The problem is much much deeper than just the differences among dynos. It is, that you CANNOT accurately duplicate the real-world chassis conditions for a motor on a brake stand.

So yeah, you can calculate all kinds of stuff from dyno readings; the answers you get just won't have any particularly solid relationship to reality.

It might be interesting to hook up an electric motor to the transmission input shaft of A COMPLELTELY ASSEMBLED DRIVE TRAIN EXACTLY AS IT WILL BE EXCEPT WITHOUT THE ENGINE IN FRONT OF IT, and measure the HP it takes to spin it at various road speeds and engine RPMs. Now if you did that, you could then do the rear wheel thing, subtract those readings, and get something accurate, or subtract it from an engine brake dyno reading and come up with RWHP; except for, if you use an inertial type chassis dyno like a Dynojet, you'd still have the error being introduced from rotating mass in the drive train, which a steady-state power reading would not measure (since those losses are only incurred while accelerating the drive train and not while spinning it at a constant RPM).