RobsZ

If 10-12 RWHP are realized from a mod, what would the engine HP that was gained be? I know that there is more loss through the drive train with an automatic, which I have.

Critter

10 to 12 * 1.12 = 11.2 to 13.44 i think. thats with 12 percent loss. of course i just pulled that out of my butt.

Bayer-Z28

15% drivetrain loss on a Manual. 20something % on an automatic.

You would need to get a baseline HP reading to find out accuratly what that mod produced. Before and after Dyno tests.

If you had 340 RWHP, after a 15hp mod... 355 RWHP (guestamite) ... FWHP would be 426.



Mine, I have 336/346 RWHP/TQ, My FW hp/tq would be about 386fwhp/397fwtq (prolly a little more now since I added the 160 t-stat)

I was running 343fwhp before my LS1 Edit. so I gained 43hp from my tune... (I forgot my TQ #'s)

RobsZ

How do you like the 2900 stall?

Critter

it blows, its a slp stock restall and only stalls up to 1700ish, i think i got took. guy said it was 2900 but i'm thinking it was a white lie. o well i should have done my research. DON'T BUY A STOCK RESTALL!

Bayer-Z28

*ahem.... ^^... Don't uy SLP, more like it.. Except a lid or small stuff..

RobsZ

Thanks for the heads up!

JakeRobb

Just to be a bit picky on the math side of this...

If you're assuming a 12% drivetrain loss, you need to divide by 0.88. Multiplying by 1.12 is equivalent to an ~11.8% loss.

Not like it's going to change your results much, but it's not that hard to remember the right way, so you might as well do it.

% loss on a stock-ish A4 is probably more like 15%, BTW. AFAIK, 12% is low even for an M6.

Capn Pete

I'm with Jake on this one .

RWHP = Flywheel HP x drivetrain efficiency (expressed as a decimal)

ie: RWHP = 350 HP x .88 (assuming 12% loss) = 308 RWHP

And then "backwards": 308 RWHP / .88 = 350 Flywheel HP

I also believe that the drivetrain losses are not as high as some say. Safely around ~10 - 12% for a manual, ~15 - 18% for an auto. And I've got a couple examples!

My car, A4 w/4.10's, dyno'd 289 RWHP. Divide by .82 (for 18%) = 352 FWHP.

Now with M6 w/3.23's, dyno'd 317 RWHP. 317 / .90 = 352 FWHP. If I went with 12% loss, it would be about ~360 Flywheel. Considering I have a lid & catback, that's probably more accurate, so we'll "agree" on ~12% loss!

rskrause

Different types of dynos produce different numbers. This applies to chassis dyno and engine dynos as well. Depends on the design, how they are set up, what correction factors are used and so on. As far as automatics, the losses through the converter can be quite high depending on the design. Certainly over 20%. But whatever the exact number is really misses the point. think of the chassis dyno as a tuning tool. It gives reproducible results when used properly, so if you make a change and it's reflected in the dyno numbers, you have learned something. Calculating "backwards" to FWHP is really just an educated guess. Back to back engine and chassis dyno numbers on high stall automatic equipped cars can show up to 25% drivetrain loss. OTOH a very efficient drivetrain (MT, lightweight components, etc.) might lose less than 10%.

Rich

Stephen 87 IROC

Guessing at a percentage loss is only a wild guess. Having a 250 hp engine with a 20% loss through the drivetrain means you lose 50 hp and have 200 hp reaching the wheels. Do some engine upgrades to make 400 hp and through the same 20% loss, your drivetrain now eats 80 hp and you have 320 reaching the wheels.

How can you lose 30 more HP through the drivetrain with only an engine upgrade?

It's better to know exactly how much power is lost through the drivetrain because that will stay static unless you do drivetrain modifications. Differentials take a specific amount of power to turn. Transmissions take a specific amount of power to turn. Unless you modify them so they use up less power, how much they use won't change if you change how much power is put into them.

I only have 3 tranny power usages
Powerglide use 18 hp
TH350 use 36 hp
TH400 use 44 hp

That's all assuming the tranny is stock. There's also a weight consideration. A PG is less than 100 pounds while a TH400 is about 130 pounds. I can't remember what a 4L80E is but I think it's 250+ pounds.

Theoretically, if you do a mod to the engine to make an honest 10 more HP and don't change anything on the driveline, you should see an additional 10 HP to the wheels. Don't expect claimed increases with bolt-ons. The only way to normally see bolt on improvements is on a dyno.

Injuneer

You got 1/2 of it right . Divide rwHP by (1 - drivetrain losses) to get flywheel power.... or divide rwHP by 0.88 when there's a 12% drivetrain loss.

But multiplying rwHP by 1.12 is not the equivalent of an ~11.8% loss, it would correspond to a ~10.7% loss.

100 rwHP X 1.12 = 112 fwHP

100 / 112 = 0.893

1 - 0.893 = 0.107 = 10.7%

I actually measured the losses on my M6, and yes, they were as low as 12.1% but that was at 765 flywheel HP.

Losses are a combination of "fixed" and "variable" components. Inertia losses are a function of the mass of the rotating components, so they do not change with engine HP..... with the exception that the faster you accelerate those rotating parts (as a more powerful engine is capable of) the larger they will become. Coast down tests or using simultaneous equations to solve for fixed and variable loss components will show that the fixed value for drivetrain loss is probably in the range of 20HP or less for the kind of vehicles/drivetrains that we are working with.

The variable losses are the loads that are proportional to engine torque, including the friction forces on gear faces and bearings. Increasing the torque transmitted increases the normal loading on the gear faces at the contact points, and in the bearings.

The numbers below are the results of running the engine on an engine dyno, with the full intake and exhaust system, then installing it in the car and doing corresponding chassis dyno pulls. Drivetrain consisted of a steel flywheel Street Twin, stock T56, 3" chrome moly DS, Strange 12-bolt with 3.73 gears and HD Eaton posi, and 17x9.5 wheels with 275/40-17 GS-C's.

(RWHP / FWHP) - 1 = calculated losses

425 / 486 = 0.874 = 12.6% losses
557 / 635 = 0.877 = 12.3%
671 / 763 = 0.879 = 12.1%

Same setup, but with a TH400, 4.10 gears and 15x10 Pro-Stars w/ 28" QTP's:

486 / 390 = 0.802 = 19.8% losses
507 / 635 = 0.798 = 20.2%
602 / 763 = 0.789 = 21.1%

Note that in the auto, with a non-locking convertor, the losses increase with HP..... a function of the hydraulic coupling.

teke184

sweet...

now that is some killer information.

i typically use 12% with an m6 and 15 with an auto.

but as stated...it's just a best-guess anyway.

Capn Pete

12% (manual) and 15% (auto) seem to be the closest to "accurate" for stock, or close to stock calculations for the LT1/LS1. As power is increased, those percentages will likely change.

JakeRobb

That was a typo on my part. I meant to write 10.8%. I truncated rather than rounding nearest, hence the "~".

Thanks for clarifying.