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 