How do you figure flywheel horse power when you know the rear wheel horse power? What horse power do they advertise? i.e. 310 hp for 2002 Z28 If you were to show the horsepwer on the car would you use fly wheel or rear wheel?

 

Well, there are a couple loosely "agreed" upon #'s that can help you calculate between flywheel HP and (rear)wheel HP. That's the drivetrain loss. I'm going to give a couple #'s that seem to hold up pretty well for stock, or stock-"ish" F-bodies:

A4 (auto) 15%
M6 (manual) 12%

SO, here's what you do. Knowing the flywheel HP (say, 350 HP) then you multiply that by either 0.85 (A4 ... that's 1 - 0.15) or 0.88 (M6 ... 1 - 0.12).

ie: 350 HP x 0.85 = 297.5 RWHP (A4)
or 350 HP x 0.88 = 308 RWHP (M6)

Considering the LS1 engine is rated around ~350 HP, those #'s are pretty typical of what an LS1 F-body would dyno at the wheels.

Now to do the opposite (figure out flywheel from RWHP #'s) then do this:

RWHP / percentage loss = flywheel HP

ie: 295 RWHP / 0.85 = 347 HP
or 320 RWHP / 0.88 = 364 HP

What you have to keep in mind is that those %'s are just "rough" figures. And autos can/will have even higher losses, especially if they're running a high-stall converter. Losses of 18 - 20% or even higher aren't un-heard of.

Consider this. You could dyno your modded A4, and get ~375 RWHP. Now if you figure ~15% loss, that would come out to about ~440 HP (flywheel). But if that car has a stall, and the losses are closer to ~20%, then that would be about ~470 HP .

As far as the factory 310 HP "rating" ... that's supposed to be "engine" (flywheel) HP, because it's lower than the Corvette's 345/350 HP rating. Then if you had the SS/WS6 with upgraded intake/exhaust, the "ratings" were higher ... 325/335/345 HP. Again, flywheel. BUT in reality, dyno the cars (even a stock "310 HP" Z28/TA) and you just might get ~310 HP at the wheels . Many do dyno that high ... mine put 317 the way it is right now ... so that "rating" is a crock .

 

you also have to watch out with those percentages.

say you take a stock 6spd LS1 and dyno 300rwhp. figure that's about 335hp at the crank....or 35hp lost through the drive train.

now if you put a heads and cam on it, dyno 450rwhp and don't change the drive line than the hp lost through it should remain about the same.

so by using the percentage calculation you would estimate 500hp at the crank...or 50hp lost through the driveline.

but why would that lost power iincrease if everything behind the engine was the same??

see my point...

basically there is no calculation to determine what you are asking...just best guess estimates.

and if you're gonna do some kind of badge for the car....i would calculate a conservative crank hp level from a dyno proven hp number...and then round down.
better to advertise a little less and have more...than the other way around....cause you don't want to be accused of doing ricer math

 

the above is the easiest method, however, if available, you can have the engine dyno'ed and then dyno it again in the car for rwhp. that would be the most comprehensive way of seeing your drivetrain loss.

 

You appear to be saying that if a given drivetrain loses 50HP (15%) when the engine is at 334HP, it will always lose 50HP, even if you increase the flywheel HP. That is not correct.

There are two components to drivetrain losses......

FIXED losses include the HP lost to the inertia of the rotating parts. That is a fairly constant figure, and should not change with HP, but in fact, if you add HP, you increase the rate at which the mass of the rotating parts is accelerated, so you increase the HP lost to inertia. Typically, you are looking at 20HP or less for this portion of the losses.

VARIABLE losses include the HP lost to friction, and friction increases in direct proportion to the load (torque) applied to the part. Sliding friction losses in the gear faces, and bearing friction losses are directly proportional to torque being transmitted, so the HP lost increases in direct proportion to torque (which is mathmaticlally tied to HP).

As you increase HP in a manual trans drivetrain, it is logical that the % loss would decrease slightly as HP increased. But the absolute number would continue to increase. With an automatic tranny, and its hydraulic converter (assumed to be non-locking) the % loss may actually increase with HP, since the slip involves physical process that relate to the square and the cube of the slip.

I've posted my actual dyno test numbers (chassis dyno vs. engine dyno) in the past, and with the M6/Street Twin/chrome moly DS/12-bolt setup, the losses ranged from about 60HP (12.6%) at 486 flywheelHP to 92HP (12.1%) at 762 flywheelHP. Percentage went down, actual losses went up.

Switching to the TH400/loose converter increased the losses to 96HP (19.7%) at 486 flywheel, and to 162HP (21.6%) at 762 flywheelHP. Percentage and actual losses went up.

 

hmm....interesting...

i didn't realize that component of it was that high....

i stand corrected...

Fred...sometimes you're too smart for your own good...make the rest of us look bad....and i do that just fine by myself

 

It's ok Teke ..... he's proven me wrong before too!!

 

One of the benefits of actually being a mechanical engineer is the ability to relate the fundamentals of engineering to the real world, and the things that interest us

 

Wow, if you think about it, that's 162 HP simply to spin the trans, rear axle, etc

 

^ ^ Imagine that ... a little Civic engine would barely move the car!