Engine 850: B5234TR/2.3/20v/T5R: Dyno Run 850R 1996 with RICA

[s70t5chris]

Originally Posted by s60ben

Hi Guys , as you know in Europe we quote BHP at the flywheel (ie the power coming out of the engine) rather than the power measured at the road wheels. Obviously the latter will always be a lower figure due to power being lost through the gearbox,tyres etc.But which is the best and most reliable figure to quote? In the US the guys often use wheel horse power figures. The reason being ..(1) this is the actual power being put through to the tarmac and (2) loses through the engine do not need to measured or calculated or guessed!
OK this looks at first glance of being the better option , but if you look further you realise that flywheel figure is the only reliable figure....
The main problem with wheel horsepower (whp) is the tyres. Firstly the tyre is squashed at 2 points on the rollers not one (as it would be on the road) so the drag is increased . For every rotation of the wheel the rubber is squashed throughout its entire circumference not once but twice! Also consider the gear you are in ; if you do a run in third gear and then again in forth gear ,the 'length of rubber' that gets compressed in third gear will be less than in forth. -So you will get more WHP in third than forth.
Taking this further you will also get more WHP with narrower tyres (assuming no slip) higher tyre pressures, a smaller diameter tyre , a later MCS (with its lower gearing) ,etc etc.
From this you can now see that quoting WHP is pretty meaningless due to so many things that will effect the measured output.
This is why we need to MEASURE the losses and add this figure to the WHP figure to get a true indication of the engines power.Using a figure and just adding it to the WHP is laughable. At a rolling road event a while ago I spoke to another Mini tuner who said he just measures ast the wheels and adds 20-25% !
OK, the measuring is quite straightforward and just involves (after reaching beyond peak power) knocking the car into neutral and letting it coast down. At this point the the inertia and weight of the rollers are driving the wheels and gearbox,- not the other way round. This shows as a 'negative' horsepower .eg at 5000 on coastdown it may measure 'minus 20bhp'. This 20bhp will be added to the WHP at 5000rpm to give the 'flywheel BHP ' at that engine speed. This 'minus' bhp is measured and then added on to the WHP at all engine speeds from peak revs down to maybe 3000rpm ,at which point the RR operator gets bored and applys the footbrake.
Now because we have measured the ACTUAL losses it makes no difference to the FLYWHEEL BHP what gear/tyres/tyre pressures you used as the losses however big or small are added accordingly.
As an example lets take a stock Cooper S with 16" wheels 50psi tyre pressures,
and run it up in third gear on the rollers. Lets say the WHP came out at 160bhp.
We then take the same car put 19" wheels on it with 15psi in the tyres and run it up again in forth gear. This time the WHP comes out at 130bhp!
From this its clear the WHP although necessary to measure is not the 'final figure to be quoted'.
What we now need to do now is apply the run down losses we measured on each of the two runs;-
On the first run (16" tyres pumped to 50psi, third gear...) the losses measured 10bhp.So 160 WHP + 10bhp losses=170 bhp @ flywheel.
On the second run (19" tyres 15psi tyre pressure, forth gear...) the losses measured 40bhp.
So 130WHP + 40bhp losses=...........wait for it........170bhp.!

Thanks for that Ben. It explains what I already knew, but hopefully will help others to understand why WHP is all a bunch of BS. Unfortunately people will only believe what they want to believe.