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MG MGF Technical - BHP at wheels but what is it at the engine

I have had some mods done to the car and would like to get it rolling road tested. I expect the test to quote at the wheels. I would like to know what it will be at the fly wheel as quoted by Rover. I have a 1.8i and there must be a different formula to the VVC as this has lower gear ratios. Does anyone know the formula?

Steve, there are no hard and fast formulae for converting power at the wheels to power at the flywheel- there are simply too many variables involved from frictional losses at any point in the transmission chain from the tyres to the gearbox itself to the properties of the tyres themselves...

This is why rolling road quotes for power at the fly wheel are estimates only.

The most common way that I have seen these transmission losses estimated is through negative power runs- taking the engine up to max power, and side stepping the accelerator and disengaging the clutch to see how the transmission slows the roads' rollers. There are obvious problems with this technique, but then there are with any other one might want to try. Roger may want to comment on this as he has a wealth of experience with this kind of activity.

Given that rolling roads are fickle (giving different results depending upon atmospheric conditions and are potentially dependant upon the technician who is operating the machine), the only way that you can be sure that your mods are successful is to bench mark your car before the mods. Ideally all tests should be performed under the same conditions- but this is not always practical. It is important that the technician does not try to impress you with power gains that aren't really there too...

Good luck with the conversion and testing! New cams I take it Steve?

Rob Bell


You are spot on New Cams were fitted. Power increase was definately felt from 4000 upwards and engine does not run out of puf like before. Also good on petrol 30 plus and thats around town. I will not do any more mods as after this it gets really expensive.

Ah you couldnt resist it could you Steve! Did you go for the fast road jobbies like mine? And did you notice the deeper intake and exhaust tone?



It is the same as yours. The deeper note on the exhaust is sweet music. I found that the cams were the only mod that I must admit made a difference. I only have one small upset, the tick over some times goes down to 500 revs and then returns to 900 after I have put my foot down then suddelnly take it off. e.g some prat pulling out in front of you while you are accelerating. In this situation if I catch the revs when they are at 500 the engine may stall. Anyways this only happens now and then.

Hi steve

I was told it was a round 20% but then the rolling road tested said that he did that calculation for every vehicle. So 100bhp at the wheels = 120 at the engine.

Hi Steve,

Mine did that for about a week. Strange but true, I don't know the ins and outs of the mems but it is possible that it may have adjusted itself over that period. How long have yours been in?


I have had mine since mid march. Have you changed anything to correct this, serviced etc.

No - but what intake configuration do you have now? Could it be that the mixture becomes too weak for a moment when you come off the throttle. I seem to recall a similar thread previously on this problem. Is there anything in the archives perhaps?


Nice one Steve for going for the cams. Look forward to hearing about the rolling road results when you get them. Is the car other wise entirely standard?

Regarding the uneven idle, I wonder if the problem could be related to a mucky throttle body? Lloyd recently sent a group e-mail with this message, which may well be relevant to you Steve. Sounds like a very straight forward cleaning procedure too...

Over to Lloyd:


On stripping my inlet system this weekend (in order to gain access to my troublesome coolant hose) I inspected my throttle body and found large deposits of 'gum' around the butterfly sealing rim and spindle. On investigation I found this to be the main contributor to the throttle sticking problem afflicting our cars. Later cars 99MY on, have the throttle spindle made of a different material (other than brass) which has a lower coefficient of expansion. This elevates the problem even with the 'gum' build up, as spindle tolerances are more accurately maintained throughout the heat cycle.
Removal of the throttle body and cleaning however is a piece of cake, four 8mm bolts secure it to the plenum chamber, a pipe and throttle cable to disconnect. Once removed, clean with an old toothbrush and a jar of unleaded. Rebuild and your done. 15 mins tops if your busy. Take half an hour and you might get a brew from the misses. Result a sweet idle and fuel savings to boot!



Rob Bell

Rob, Nick

Apart for the Cams, it has the usual, K&N cone and SP exhaust. I have also put an Anti Cat on the Car not for power increase but because they are expensive to replace. I will let you know what the BHP is.


It will be interesting to see the comparison with mine. The differences being; no anti-cat, Enclosed Induction and Icon (2 years no probs!). Keep us posted Steve.


Hi Dave...a front engine rwd car will loose more bhp from engine to wheels than a rear engine or mid engine rwd car because of the ?inertia? of the ?propeler shaft?. So that formula of the 20% is not reliable!

Regards: Bruno Valadas
Bruno Valadas

Losses vary from RR to RR and then operator differences add more error. A rule of thumb I always use is 10% plus 12, which has provided a good measure for Peter Burgess's RR. Thus a reading of 100bhp at the wheels is plus 10% which is 10 plus another 12 which is 122. This happens to be the ball park figure that is seen with standard 1.8i MGF's on Peter's RR. Many other different standard cars have over many years supported this as the resyults fall very close to the manufacturers quoted figures.

The choice of 10% allows for different engine capacities and the 12 is representitive of most tyre and transmission losses irrespective of FWD or RWD.

Roger Parker

Interesting method of calculation Roger, I've always stated a figure of 155ps for my 130 RR readout, using my 'guestimeter'! and the figure is the same with your equation.


Thanks for the info Rog.

Inertia doesn't affect bhp. It does affect acceleration, but remember that the propshaft is on the output of the gearbox so it isn't spinning particularly fast (and it has a very small diameter). The inertia of the flywheel & clutch have a much bigger effect, especially in 1st gear - but on acceleration not bhp. If a rolling road measures bhp by letting the engine accelerate then the resulting figure can be affected by inertia (of everything including the wheels which will have a lot more inertia than the propshaft) - but the operator should set the acceleration to be low enough for this effect to be negligible.

Mike Bees

This thread was discussed between 05/07/2000 and 10/07/2000

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