Converting wheel bhp to flywheel bhp and vice versa
To reflect the fact that % losses are high for low powered cars and vice versa I use the following equations which have been found to correlate well with real world transmission losses.
FWD cars - add 10 bhp to the wheel figure and divide the result by 0.9
RWD cars - add 10 bhp to the wheel figure and divide the result by 0.88
4WD cars - add 10bhp to the wheel figure and divide the result by 0.84
To estimate the expected wheel bhp from a known flywheel bhp just reverse the equations
FWD - multiply flywheel power by 0.9 and then deduct a further 10 bhp
RWD - multiply flywheel power by 0.88 and then deduct a further 10 bhp
4WD - multiply flywheel power by 0.84 and then deduct a further 10 bhp
Remember, these percentages are not "gospel" - they are good realistic averages. The measured wheel bhp can change depending on tyre pressure, tyre size, suspension angles and other things which won't affect flywheel power - so the actual transmission loss % will also change. It pays to try and standardize as many of these things as possible if you intend to do a series of power runs over a period of time. Always use the same tyre pressure because this is a factor which can easily change from day to day and make sure the tracking is correct on a fwd car. Copyright David Baker and Puma Race Engines
Also please remember that the manufacturer's claimed power figures for a standard car are not gospel either. Even engines in perfect condition can vary by plus or minus 5% due to manufacturing tolerances. High mileage or poorly maintained engines can be well below the claimed output. It is no proof that a rolling road flywheel bhp estimate is correct just because it comes out as the same figure as the manufacturer's. Always compare with the measured wheel bhp to see if the transmission losses agree with the data above.
DYNO COMPARISONS
Imagine we take a car with a true 200 flywheel bhp engine to each of the various types of dyno. Assuming accurate dynos which is by no means always the case and calibration standards can be very lax then we would expect to see the following results.
FWD
Engine dyno - 200 bhp
Hub Dyno - 200 x 0.93 = 186 bhp
Wheel dyno - (200 x 0.9) - 10 = 170 bhp
RWD
Engine dyno - 200 bhp
Hub dyno - 200 x 0.91 = 182 bhp
Wheel dyno - (200 x 0.8
- 10 = 166 bhp