Many thanks to Frank, who scanned this article (“Tech Stuff: Is Your Dyno Lying?”) out of the May 2004 issue of Car And Driver, then emailed it to me over dial-up! But tonight I found a link to it on the Car And Driver web site!
The article was sent to me by Frank, “in defence of the JCW mod” (see YarrowSport Dyno Day) …
Horsepower is like good luck. It can’t be seen, touched, or tasted, but you know when the inventory is low. How much do you have? Turns out that different horsepower dynamometers will spit out different results on the same car …
… Dinan bolts to his Dynopack one of his 2003 Dinan M5s, heavily tweaked to make a claimed 470 horsepower at the crank (he expects about 415 at the wheels). With the hood closed and no external fan blowing air into the radiator, the car wheezes out just 334 horsepower at the wheels … Now Dinan opens the hood and turns on a small Home Depot shop fan blowing about 10 mph worth of air … This time the computer finds another 37 horsepower, or 371 …
… the crew then wheels out the big gun: a $7000 electric fan that looks like it should be hanging on the wing of a Boeing 737. It blasts 38,000 cubic feet per minute of air at 75 mph down a narrow duct, right into the M5’s radiator. The fan roars, the M5 howls, the computer twinkles, and the graph paper ticka-ticks out of the printer. It says 411.4 horsepower, the best run of the day …
[Not that I ever expected >400 out of my MINI ;)]
M&I / GBMINI 
I see what they’re getting at in the article, and I don’t mean to knock the performance of your car, Ian, but the different bhp that Dinan can measure on one M5 is not quite the same as what we saw at Yarrow.
In our case, all the cars were run in the same room on the same day with the same fans blowing over the same factory fitted sensors, so car-to-car comparisons should be as fair as it’s possible to be. Now I agree that a specific bhp figure from the chassis dyno doesn’t necessarily mean that is what the car is really producing, but they should be relatively the same.
I think the place where the effects that article talks about might be more noticable is for the people who’ve done chip mods to the ECU. They might have different outputs based on a set of inputs compared to factory spec or JCW spec ECUs. How did your car perform compared to Joe’s..? Was yours the only factory spec JCW there..?
BTW, I highly doubt that even an M5 can measure the ‘bow pressure effect across the front of the car’. I know the F1 cars have pitot tubes to meaure this kind of thing, but I’ve never seen them on a road car..! More likely it has simple pressure sensors in the air inlet which could be fooled by a powerful fan blowing into the duct.
It would be an interesting experiment to hook up a variable speed fan to the chassis dyno to see how much the ECU really ‘knows’. Vary the fan speed to match the ‘road’ speed and see if it helps the bhp numbers…
My previous comments here & on mini2 are based on this article.
I doubt any road car has air pressure/speed sensors. But that can easily measure the entrance and exit temps, or efficiency, or the radiator, and retard engine performance if it isn’t cooling as effectively as it would be at high air speeds.
Because you were comparing mini to mini, this should not matter, especially because it was all done on the same dyno and the same day. But never take it as a true HP reading.
Gavin, I agree – “in defence of the JCW mod” was Franks phrase, not mine.
I remain happy that COMPARATIVELY we got reasonable results on the day, no matter what the actual figures are – of course there are bound to be some MINI-to-MINI differences.
The interesting results on the day were a 5hp increase on GBMINI by allowing it to cool, and a further 10hp by blasting it with cold air – these results hint at what this article discusses.
Yes, but I think the article is probably assigning too much intelligence to the car’s ECU. Cooling GBMINI removed the heat soak from the engine block and the intercooler that had built up on your ride down. (It was a warm day, so I doubt any of the cars cooled significantly in an hour parked in the sun). Blasting it with compressed air on the dyno cooled the intercooler more efficiently. That really only proves how susceptable supercharged cars are to inlet temp, not how smart ECUs are as the article seems to claim…