DanielRicany

Quick question. When we dyno at the wheels and want to convert to crank horsepower, if we use for example 15% drive train loss, is our conversion result in net horsepower or gross horsepower?

MavsAK

I think the 15 percent conversion rule is a little optimistic in favor of crank hp personally.

But it would be a conversion resulting in a net hp estimate.

Gross hp is the power your engine makes with zero accessories (no alternator, etc attached).

aDigitalPhantom

That also assumes that the dyno is accurate, and not being fed wrong information by the operator.

vader86

When converting from rwhp to crank with the rule of thumb 15-16% factor in manual transmission (17-18% for automatics), you are coming back to NET crank hp.

Gross is what the engine would make without the additional drivebelt accessories, like PS or A/C.

rocco16

My experience leads me to put the power losses somewhat differently.
Manual trans: approximately 10% loss going through the drivetrain.
Automatic (without TC lockup): approx. 15% loss.

Tom400CFI

The 12, 15, 18% numbers for manuals seems like the stuff of day-dreams. If 15% were accurate, my stock LS2 C6 would be 414 chp (did 360 wheel) and my stock LT1 would be 320 chp (279 at the wheel).

We know that the LT1 was 300 chp, and the LS2 was 400, so 10% brings things a lot closer to reality.

also w/the outcome of the conversion being in NET hp. An LT1 rated in GROSS hp would be something more like 370 hp. Kooky numbers.

vader86

Does bear mentioning then that modern day drivetrains are more efficient than the older stuff was, magazine tests have showed it in several cases that 15% doesn't apply to a new manual transmission from the factory today. The manufacturers are chasing the efficiency for mileage, 0-60 times, etc.

But an old 4+3 or the like, that would be higher.

Has come up before that using a percentage is not entirely right at all for highly modded engines. It is more asymptotic and approaches a constant factor when youre in that 400-500+ hp range.

aminnich

So if mine dynoed at 386 hp at rear wheels what would that covert to at the crank? LS1 with 4L60E trans and some engine mods.

Tom400CFI

I'd say about 440-50 crank, since LS3 'Vettes put down ~370-80ish?

DanielRicany

So what does it go by after you get to that range?

five7kid

I would look at chassis dyno data as a tuning aid only. They are notoriously inaccurate dyno-to-dyno, and even worse brand-to-brand. If your operator knows what they're doing, they can produce run-to-run repeatability so you can see the effects of tuning changes.

There was a magazine article a few years back (which I can't locate now) that ranked the accuracy of various horsepower measuring choices as: 1) engine dyno; 2) drag strip performance and using formulas to convert to HP; 3) estimate based on parts used; and 4) chassis dyno.

vader86

Appears to settle out in the 50-75 hp range

DanielRicany

When you said 400-500+ hp, that was at the crank or wheels in which you were referring to?

Tom400CFI

Well, a QUICK SEARCH shows that C6 ZR1's put down about 540WHP so that is down ~100 from the crank rating. In that car, it seems like 16% is appropriate?

ANOTHER QUICK SEARCH shows CTS-V's making around 475 RWHP stock. Crank rated at 556, so also ~16% correction there too.

YET ANOTHER shows the stock LT1 with it's 455 chp rating making ~415 at the wheels, which is a 10% correction.

with five7kid that you can't very well use a chassis dyno to come to an accurate chp number. At best, you can get a guessed range.

DanielRicany

I guess I'll have to start using the word "about" when I tell someone my converted crank number. Haha.