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MG MGF Technical - rolling road mystery

As I couldnt make the trip up to Emerald for the mass RR shoot out I took my VVC down to Brooklands HP for a power run on their rolling road. Very odd results. To begin with the RR was only recording an rpm of 6200 when the car was showing the limiter cutting in at 7200. The guys at Brooklands thought that the tyres might be slipping on the rollers and so we put a weight in the boot and I got in the passenger side. This led to the RR recording max revs of 6800 against the 7200 shown in the car, so some improvement but still below max revs. We checked the rev counter in the car with a timing light and it seemed ok and the power printoff showed the power climbing with no drop off near the top. Max power recorded was 150 bhp at 6750 rpm and max torque 164 Nm at about 5200 rpm. As my F has the 52mm TB, a piper cross filter in a cool air box, Janspeed 4-2-1 manifold and down pipe and a supersports exhaust I was expecting at least 160bhp and more torque. The test showed that the car was running lean (aprox 4%), could this have accounted for the poor performance? Or was there something wrong with the RR? Can tyres "slip" as suggested? Seems odd as they did a mini cooper before the F with no problem and that was a much lighter car. Any views?
marc hanson

A bit odd - how does the RR measure your RPM?
Ted Newman

no idea!
marc hanson

The only thing to slip I can think of is the clutch.

Johan Slagter

Engine rpm is calculated from the speed at the rollers Ted - so any tyre slippage could cause problems with the calibration. Equally, incorrect tyre pressures can also cause headaches for similar reasons...

Marc, to some extent, this does sound like a calibration issue. As Power = torque * rpm, if the RR is under reading the peak rpm, then calculated power at the wheels (not to mention the derived estimated power at the flywheel!) will be artificially low...

It's a shame you couldn't make Emerald - perhaps next time?

Regarding what power to expect: VVCs often produce around 143bhp as standard: if you've released about 15bhp (which with your mods, you probably have), then you should be looking at something in the order of 155-158bhp...
Rob Bell

thanks rob. emerald are sadly just too far away. i'll have to try and find a half decent RR in london / surrey. any ideas?
marc hanson

Thanks Rob - that seems a strange way of getting the RPM when a much more accurate way would be to use the cars own electronics - or is this just a means of cross checking?

Ted Newman

Marc, no personal experience of RR in the SE - although there are plenty to choose from. I think that there is a RR in Essex that the Caterham 7 guys use - don't have the details to hand, but when I come across it, I'll let you know.

Ted, you're right, you could very easily make use of the car's own electronic signal to provide the rpm data. The reason I suspect that this isn't done is because every car is configured differently (different ECUs) - and until recently, there wasn't an industry standard diagnostics port to plug into. And that's assuming that the car on the rolling road has a ECU in the first place!
I guess it is simply convenient to derive all the numbers from the road itself - although clearly, this does create its own problems.
Rob Bell

You could always use a spark counter clipped onto one of the spark leads!
Ted Newman

Austec have a very good reputation in the TVR circles. They are in Crawley if that is not too far

>> You could always use a spark counter clipped onto one of the spark leads! <<

That doesn't work for cars with wasted spark ignition systems (like the VVC, and all MEMS3 cars) - you'd end up with an rpm twice as high as you might think! But having said that, the error would be pretty obvious and easy to correct for...
Rob Bell

Marc, yes - Southern Carburettors in Wimbledon - the r/r engineer is Dave McGivern who is race engineer on my mate's Jaguar XJS and did me a really good setup on Max's race midget last season. I won't go anywhere else now. Mention my name if you make a booking.
David S

thanks david and I live in Wimbledon too!!
marc hanson

This thread was discussed between 01/03/2005 and 04/03/2005

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