Percentage power loss through auto

[Gary - K7GLD]

OK guys, here's a question on logic for ya...



IF it takes, say, 30 HP to rotate an auto transmission at 2500 RPM behind a 160 HP engine - Roughly 20% - WHY would it then take *60 HP*, same 20%, to rotate that SAME transmission behind a 320 HP engine?



Percentage of power dissipated in a transmission is a CONSTANT, based upon RPM, and independent of the HP of the engine doing the twisting... So trying to apply a SINGLE percentage equation equally to all engine configurations is elusive, and likely full of errors...



The only real (and usually unknown) variable, is the LOAD, which then potentially creates SOME addition friction losses in the drivetrain...

 

[JLEONARD]

Gary,

I agree... . but I think we are trying to consider a general rule of thumb here.

This generality would apply more accurately to those of us that are nearer the stock hp/torque level than those with more radical mods.

It would be nice to know closer to what it takes to spin the rear wheels of a 4x4 vs 4x2, manual vs auto.

Then we could be accurate.

Of course there's also differential ratio, tire size, gear ratio, and a host of other factors to consider... . I won't even mention dyno type.

 

[y-knot]

The general rules are based on stock HP numbers, or what is a normal HP rating. Obviously it it takes 15HP to turn your auto transmission in it's stock form, it will not require any additional power when you up-grade the engines HP. Also the parasitic losses by all the engine accessories such as alternator, and so forth will remain the same regardless of what additional power you add to the engine. Just your percentage of actual loss will go down.

 

[Gary - K7GLD]

WELL, the reason I bring this up, is because SOME guys after modding their trucks extensively, and then running on the dyno, STILL factor that 20% or so back in when stating their power...



"YUP, I got 500 HP, and with the 20% power loss thru the transmission, am REALLY getting *600 HP* at the flywheel...

 

[JLEONARD]

Gary. . another point well taken.

I dynoed on the past at 165 rwhp... but I have a really tight transmission that has a 50% loss... I swear it does.



Well, I for one would be interested to know what the actual hp losses are thru the various components... .

Anybody have any good knowledge on this?

Jay

 

[BushWakr - RIP]

Ahhhhh..... cerebral exercise... ... . gotta love it.....

There are a number of factors all working together/against each other here... .

As HP goes up so does stall on an auto... . if stall rises then losses and power transfer changes, caused by and resulting in heat build up oil breakdown, slippage etc... .

Much of this rests with the big items like the TC efficiency, line pressures which hold clutch components together (or not), I think that a shift in torque band and HP banding will alter losses because of 'torque multipication values/equations' (non-lock up sytems).

I'd leave out the alternator, ps pump, AC etc : 1)if they remain a constant they don't count when determining transfer losses and, 2) "line losses" relate to the activity from the crankshaft to the rear wheels anyway.

If losses are say 10% at torque stall eg:1800RPM. . then as the HP rises at the same RPM stall moves upward as well and the loss percentage will be a variable .....



Yes/No/???



Pastor Bob...