Power @ wheels & Power @ flywheel

[Gadge20vt]

Gents, I wonder if you can help with a query about power figures. I have been told that the only way to measure a cars power at the flywheel is to remove the engine. Yet looking at some RR plots, they all seem to quote flywheel figures and wheels figures.

How is the power at the flywheel calculated?

[BenS1]

I think they do this on a rolling road by taking the car upto the red line in a gear and then dumping the clutch and releasing the accelerator... they can then see how long it takes the car to slow down, and from this they can work out the drag through the transmittion.

If you know how much drag there is through the transmittion then you can use the power-at-wheels figures to workout what the power at the Flywheel must of been.

For example:

If you find that at 4000rpm you are getting 200bhp at the wheels and the drag test shows that you are getting 25% loss through the transmittion then 200bhp is only 75% or the power being produced at the flywheel. So 200 / 75 * 100 = 266.7bhp at flywheel.

Calculating flywheel figures in this was is only an approximation.


Thanks
Ben

[ibizacupra]

Originally posted by BenS1
I think they do this on a rolling road by taking the car upto the red line in a gear and then dumping the clutch and releasing the accelerator... they can then see how long it takes the car to slow down, and from this they can work out the drag through the transmittion.

If you know how much drag there is through the transmittion then you can use the power-at-wheels figures to workout what the power at the Flywheel must of been.

For example:

If you find that at 4000rpm you are getting 200bhp at the wheels and the drag test shows that you are getting 25% loss through the transmittion then 200bhp is only 75% or the power being produced at the flywheel. So 200 / 75 * 100 = 266.7bhp at flywheel.

Calculating flywheel figures in this was is only an approximation.


Thanks
Ben Power Station do the overrun measurement as described, and read direct bhp loss, and add this loss to the At the Wheels figure to get the Flywheel figure.
Total rolling losses are measured, which includes tyres, and transmission. Tyre pressure, make, stickyness etc all have an effect on the rolling losses.
On the 19th, I will personally be looking at the relative @ wheels measurement more than the calculated flywheel figures. (relative to the other cars on the same rollers-standard and modified)

% calculated losses are very general I think.

I have seen my old Jetta loose 22bhp through the transmission and VR6's loose 40+bhp. Different gearboxes, and the cable change boxes do look like they have more transmission losses than say the old 020 rod change boxes. Apply the general % rule and it won't be accurate for both..

regards
Bill