Engine horse power..

[MICKSVRS]

...vs flywheel horse power...what are the differences??

Thanks

[faboka vrs]

Do you mean Wheel vs flywheel horsepower?

Flywheel is how much power the engine makes.

Wheel is how much power the wheels make after going through the transmission system.

[KenONeill]

Or possibly something like BHP vs PS? In which case BHP ~ 0.99*PS.

[ap0gee]

One's a scientific measurement, the other's a complete stab in the dark?

HTH

[rurwin]

Brake Horsepower (BHP) is horsepower into a brake. The engine is removed from the car and connected to a controllable load, which can be a simple manual brake, but these days is usually either an electric motor (which can drive the engine as well as brake it,) or an eddy-current device (which can only brake.) These are called dynamometers (dyno).

Every part of the transmission from the engine to the wheels has losses, and this means that the power at the wheels is always lower than the power actually coming out of the engine.

Without removing the engine from the car, you have to use a rolling road, this works in exactly the same way, except the dynamometer is connected to the wheels instead of the engine.

The power at the wheels is more meaningful than the power at the engine, simply because it is the power at the wheels that makes the car go. There is no use having all the power in the world if it is all wasted heating up the clutch, gearbox, differential and wheel bearings. However the bhp is bigger, and therefore more fun ;-)

It is possible to estimate the losses in the transmission fairly accurately by opening the clutch and letting the car coast to a stop. If there were no losses then it would keep going forever. Of course it is also necessary to know the losses in the dynamometer so you can subtract them.

Also the figure "torque at the wheels" is questionable. The quantity measured on a rolling road is force. You can calculate the torque by multiplying by the radius of the wheel, but the numbers quoted above are very accurate. To get figures of that accuracy the radius must be measured to within a quarter of a mm.

So in conclusion, the measurement of bhp is done with the engine out of the car, and is accurate. The measurement of power at the wheels is also accurate, and is arguably more interesting. However the calculation of power at the flywheel from a rolling road is only as accurate as the calibration of the testbed and measurement of losses and sizes - which even on a well maintained and operated testbed would involve a series of error terms that would make it markedly less reliable.