BHP -Wheel horsepower v Flywheel horsepower

[roland2003]

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.!
Best Regards Roland GTT

[RVW]

Interesting write up Roland
I am I to surmise from this that truthfully Rolling Roadsare really a waste of time, and that they can only give an indication of output. Also if you intend to use one for tuning purposes it is best to use the same equipment and operator for each test.
Loads of other factors can give unexpected readings i.e air temp, moisture content, time of day, fuel used etc.
The only true way to measure output is to remove the engine and place it on a dyno rig.

[roland2003]

...Even a dyno is not perfect, ( eg air intake tract etc may not be the same as fitted in the car)
A properly calibrated RR ,used accuratly and correctly 'SHOULD' be accurate. Ulfortunatley more often than not its not. As a comparitor for checking changes in output (after mods) it fares slightly better but again too many variables to rely on it.
To summarize:-
-Flywheel BHP is what you need to measure (ie WHP +measured losses added on) . The WHP on its own is meaningless, as is a WHP figure with '20% just slapped on top ' used as your Flywheel HP.
-Very large differences in WHP & Flywheel HP could mean binding brakes etc.
-Most RR's are capable of recalculating & changing the power figures depending on the ambient pressure & air temp. (to a DIN standard or whatever) Therefore it shouldnt make any difference to the figures you get presented to you. In reality some places just bang in an air temp (say 18Deg C) and leave it there come snow or shine! So if its colder than that you get a higher reading and if its hotter you get a lower reading.

Best Regards Roland GTT

[MINIAC]

Horsepower figures quoted in the UK are also slightly higher than those in the US because they are expressed in terms of metric horsepower.

1 metric horsepower = 0.986320073 horsepower

[roland2003]

..yes its true new European cars power outputs are expressed in PS , though in the uk (aftermarket tuning) we use the old unit more. God knows why?
Best Regards Roland GTT

[Fin]

PS or Pferdestarke translates as 'horse strength' if I am not mistaken. Which I don't really see as being correct, since it is the rate of work done by the engine, and therefore the power that is being measured. Rather than the strength.

We should just deal in kW and be done with it

[GTT 280]

If 10bhp loss = 16" rims, and 40bhp loss = 19" rims...

then does this mean 20bhp loss = 17" rims, and 30bhp loss = 18" rims ?

or is it more based on pressure or even width, or is it 33/33/33 against these three attributes?

Cheers,
K.

[roland2003]

...these are just figures off the top of my head used as an example .I could have used 20bhp loss on Run 1 and
30bhp on run 2. The point being that the bigger tyres (greater circumference=more length of squashed rubber) lower pressure (more wall squashing) and 4th gear (more rotations =greater length of squashed rubber) will give a lower WHP reading,but with loss measurement /correction it should always result in the same power @ flywheel whatever wheels/gear/ pressures you have.Best Regards Roland GTT

[roland2003]

Hi Guys,i'd bring this up as theres alot of discussion on the R56 tuning forum about exhausts that gain (claimed) lots of power on the RR. A lot of that thread is to do with drivetrain losses and an assumed percentage loss......
Losses on any decent modern RR are measured on the run down; theres no guessing needed.
To assume a percentage loss is crazy, yes on a 200-250 bhp Mini it so happens the losses are around 12%, but if you double the output of your Mini to 450 BHP the losses are (in BHP terms) virtually identical therefore the 'percentage loss' is now only 6-7%.
Measure it don't guess it.
Best Regards Roland GTT

[DaFlake]

Solid writeup Roland.

[Snowie]

Excellent write up - this has now taken the mystic out of RR and HP calcs for me.

This should kill some of the bull I have seen elsewhere


Snowie

[roland2003]

Thanks Guys, yes RR's are open to more Bu......it than most other topics put together.
Best Regards Roland GTT

[Gardus]

One of the known trick used by tuners to increse the figures of their cars is to lightly press the brakes when decelerating to increse the loss correction factor and have more flywheel hp...

Nothing new but interesting reading Roland.

[roland2003]

true,....or to give less power (competitors product for example) , you dont put the pedal fully to the metal on the power run. Theres so many fiddles that can be done.
The main post here is to explain why a Flywheel power figure (carried out correctly ,) is the only meaningful figure.
Best Regards Roland GTT

[DaFlake]

True, it is nothing new. However, this is an important thread as new people come to the mod scene. Many fall for the marketing side of it and with guys like Roland (GT Tuning) and Don (DMH Motorsports) giving cold hard facts, it clears up a lot of the grey area. This gives us folks that don't know all about the stuff the ability to get to the meat of what we are trying to do.