dyno question, re: % power loss thru drivetrain

jerry S said:

......make me want to strut down the street like John Travolta in Saturday Night Fever.

Click to expand...

Please don't.

if you c-6 is a built automatic, by that i mean shift kit and high stall you won't actually lose 255 through your drive train. My brother in law was reading a article just lately it might have been in the car craft magizine maybe a mopar muscle anyways they tested a bunch of auto's and sticks to see how much lose of power was in the various tranny's. turns out a built auto doesn't lose much more then a 4 or 5 spd. well according to the article. Mind you this is all third hand but hope it helps. anyways i think the auto's on average lost 20% if built.

I have never understood the percentage thing. I mean if you take a C6 out of a 200fwhp car, where it was sucking out 50hp (25%) and stuck it behind a monster motor with 600fwhp why would it start sucking out 150hp (25%)?

The loss is from friction and rotational mass (plus other physics i don't care to discuss) but I don't see how/why that tranny would triple the amount of loss it exhibits in that example.

funny you should use that example. When my brother in laws and i were discussing auto's we came to the same conclusion. why does it take more and more power to drive the damn oil pumps in a auto. the only thing we could come up with is there has to be a point where your power lost has to hit a ceiling well at least thats our theory. i think because the average engines are making 300-400 ponies the percentage thing works for us muscle car guys. it would be interesting to see if the percentage works on 4 bangers the same never really given it much thought though. does a honda with a v-6 and a auto lose the same percent? probably a question i will not find a answer to because i dont want to read about hondas. lol

If you are running a 9", you are giving up 2-3% over an 8.8 also. 10 to 15 hp on a 500hp engine. what about synthetic oils? There are a lot of parasites.

HP is a function of torque and RPM. By definition, torque must be similarly affected, assuming that peak power points are similar on engine and chassis dynos.

I think the stall has a huge part in an auto also. The looser the stall, theless hp you will have at the top from pure slippage.

the whole percentage thing is a vast oversimplification of this situation. The transmission/valvetrain does not eat a set amount of power, nor does it eat a certain percentage. If you have a stock 200 hp motor, the C6 will probably take a relatively large chunk to turn, here, we'll say 50. This is where you might see a 25% loss. However, once you get up to, say, the 400 hp level like you're approaching, while the total amount of horsepower lost through the drivetrain increases (due to heat, friction, and other increases that naturally stem from more agressive combinations), the percentage drops. At 400 hp, you might lose 75 hp, for example, which brings your loss % to 19 or so.

Cliffnotes: A lot of rednecks create a lot of "rules of thumb" for various automotive calculations. While they may be decent approximations in most situations, they are hardly scientific and often miss large chunks of the equation, and end up being wrong. This is not to insult the redneck method (i know we have plenty of you guys on here, with far faster and better built cars than I), because their methods are time-proven to provide at least decent results, such as the famed 15% loss in manuals and 20%+ in most autos. But these approximations do have their drawbacks, and their limits. Whats actually going on is a mixture between the "it takes x amount of power to turn the transmission gears" and the "a certain % of power is lost through the drivetrain" camps. Even this is probably an oversimplification.

Uh, my cliffnotes ended up being as long as the orginal paragraph, and covered a bunch of other stuff too... shoulda organized that better... shrug...

However, 25% is a lot of power to lose for even weak engines with heavy duty transmissions. You're making a lot of power, but stick with what you know... how much you make at the wheels. Its highly doubtful you're making over 500 ft-lbs at the crank, but it doesn't really matter anyway, you have a fast car. Only way to truly know the flywheel numbers is to take the engine out and get a dyno on it directly.

I think a lot has to do with a higher HP drivetrain assembly will have "beefier" parts so to say, meaning bigger and heavier. As you go higher up in hp I think the percentage thing still works only its taking more hp to turn those bigger parts I.E. 8" to a 9" rear. Just my 2 cents.

a RWHP figure doesn't just take into account losses through the drivetrain, the rolling resistance of the tires is also accounted for.

if you do the calculations for losses due to gears, etc, your figure will be different than say 15% (for a manual transmission) and won't match the "rule of thumb".

to be honest i do not know the exact correlation, but it should be relatively simple to calculate the mechanical efficiency of the drivetrain. from there you can get a rough idea of losses due to other factors.