Transmission losses

[paul gtir]

i am pretty sure you get 24% transmishion loss on a 4wd car and 14% on a 2wd these a est just look at a few dyno sheets and work it out

[ChrisSZ]

In that case I'm only making around 185 brake at the fly wheel - a fair bit down on the quoted figure!!!!

[chilli]

It is.

That's why you have to press the brake while you are stopped and in "D".

This friction that the brakes have to overcome is roughly the engergy wasted by the TC on idle.

Worth a few bhp.

 

It's more like X + Y(Power)

X = idle losses

 

you are right, there are some losses at idle - just not 30bhp worth of course. I was thinking the idle losses are small, just a bhp or two? thats a guess though. Main point is that the large part of the losses are related to bhp (and also revs too I think for frictional losses in the box and transmission? so it must be a fairly complex relationship)...

[ChrisSZ]

So the book says 220bhp, I (auto tx) get 163 rwp, Dave (manual tx) got 168 rwp. Still lost 50odd bhp somewhere? 14% = around 22bhp so where's the rest gone?? Or are the books wrong? (or have I got it completely wrong??)

[JohnA]

As a very rough estimate, I'd think that the auto TC losses on idle are equivalent to the horsepower of a moped needed to tow a supra and go like an auto supra leaves on idle and "Drive" if the brake is not pressed. A handful of bhp, no more.

The rest of the losses up the rev scale would be roughly proportional to the power transmitted.