Transmission losses

[Lefty]

Stupid question...

My car was dynoed at 345 at the crank with 280 at the wheels so 65bhp being "soaked up" by the transmission and drivetrain.

Does that mean that if it had 65bhp at the crank it wouldn't move?!

[Scrambled]

Seriously?

[kambites]

No; the losses in the gearbox rise with the load on it. You could turn an unloaded gearbox by hand if you tried.

[Jaldi]

Lefty said:

My car was dynoed at 345 at the crank with 280 at the wheels so 65bhp being "soaked up" by the transmission and drivetrain.

Does that mean that if it had 65bhp at the crank it wouldn't move?!

According to your figures it's losing 19% in the drive train.
So if it had 65 bhp at the crank it would have 52.75 bhp at the wheels.


By the way, your 345 bhp at the crank is just an estimate, worked out from the WHP which was measured.

[Lefty]

Jaldi said:

So according to your figures it's losing 19%
So if it had 65 bhp at the crank it would have 52.75 bhp at the wheels.


By the way, your 345 bhp at the crank is just an estimate, worked out from the WHP which was measured.

Yeah I know the dyno measures WHP and estimates engine BHP, I didn't know that the losses for a certain car (Impreza say) would always be the same in percentage terms whether the car was making 200, 300 or 500bhp (assuming the gearbox, diffs etc were all the same).

[Jaldi]

Lefty said:

Yeah I know the dyno measures WHP and estimates engine BHP, I didn't know that the losses for a certain car (Impreza say) would always be the same in percentage terms whether the car was making 200, 300 or 500bhp (assuming the gearbox, diffs etc were all the same).

I'm not sure I believe it either. Doesn't seem to make sense, does it. That's what they reckon though.

[Krikkit]

Lefty said:

Jaldi said:

So according to your figures it's losing 19%
So if it had 65 bhp at the crank it would have 52.75 bhp at the wheels.


By the way, your 345 bhp at the crank is just an estimate, worked out from the WHP which was measured.

Yeah I know the dyno measures WHP and estimates engine BHP, I didn't know that the losses for a certain car (Impreza say) would always be the same in percentage terms whether the car was making 200, 300 or 500bhp (assuming the gearbox, diffs etc were all the same).

They won't - the losses in transmission will vary by power transmitted, wheel/engine speed ratio, the temperature and condition of various bits of drivetrain and I'm sure a dozen other factors. But a half-decent approximation is a %age of the power being soaked up.

[BrewsterBear]

Here's the thing with the guessing though - 65bhp very roughly translates to 50kW. That loss will be almost entirely in heat within the gearbox/transaxle, etc. 50kW of heat is a lot. Whilst a gearbox does get warm with use if it had 50kW of heat saturating into it then it'd be glowing white hot after every journey. All this sticking of fingers in the air and saying "transmission losses are 65bhp" smacks very much of let's tell the customer what they want to hear.