i've heard 20%,15%,etc..........

does anyone really know how much it is for an automatic?

 

automatics are higher than manuals, a good number is 20% but there are so many variables that it is impossible to give any exact number.

 

Powerglide, TH200, TH350, TH400, 700R4, 200-4R?
Dexron, Type F, synthetic oil?
10 bolt, 9 bolt, 12 bolt, 9" diff?
Grade and type of gear oil?
Disk or drum brakes?

All these combination of parts can affect how much power can be lost in the drivetrain.

Putting the car on a chassis dyno can determine drivetrain loss by decelerating and having the power recorded while slowing down. Speed over time can determine how much power is lost through the drivetrain.

 

I have heard 24% but i forget where from.I think it was classic chevy magazine.

Jeff

 

since there is nothing mechanically linking the engine to the rear end until the torque converter locks up, unless you are a hydraulics and mechanical engineer and know some formulas and different coefficients of friction for different materials (refer to steven 87 iroc's comment) there is no way to generalize power loss for automatics, it will vary for each and every individual car. The hydraulic lock created by the fins in the torque converter is also dependant on input shaft rpm's.. and shoddy fluids will "shear" sooner than others, friction clutches from different companies all use at least a slightly different friction compound,etc,etc,etc. The answer is no.

 

As a guide, Dynojet testing uses these numbers, (NOT WRITTEN IN STONE).
MANUAL TRANS---15 TO 18 %
AUTO TRANS-------20 TO 22%
These are for 3rd gen cars and remenber these are guidelines.
I use the 22% number for my Auto Trans.

 

But also keep in mind that the number could be higher, I've seen as high as 40%. Just too many variables to get a set number. I like to stick around 20% to be conservative. That way after a dyno session I'm not saying that the engine makes 450 hp instead of 400. Heh, there's a variable no one mentioned, a high power motor can cause more slippage in the tranny, so more power loss.