To prove his point, 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. An LCD data logger on the dashboard reveals the air-fuel ratio from the engine computer. Approaching redline, the BMW's computer richens the mixture all the way to 9.5:1 as the underhood temperatures soar.

That's one thick mix, practically charcoal briquettes blowing out of the tailpipe. But then, with the M5 running in fifth gear (the 1:1 gear ratio with the least friction, preferred by dyno testers), the computer expects 159 mph worth of cooling wind blast around the horsepower peak. It's getting nothing, and it knows.

Now Dinan opens the hood and turns on a small Home Depot shop fan blowing about 10 mph worth of air. The M5 is allowed to shed some excess heat and then run again. This time the computer finds another 37 horsepower, or 371. Things are looking up, but the M5's output is still nowhere near Dinan's expected number of 415.

"I can't claim something I can't measure," says Dinan, so 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.

"I'd pick up four or five more horsepower if I came back tomorrow morning and ran it at 70-degree room temperature," says Dinan. The room is currently 81 degrees.

"Basically, what horsepower would you like? I can give you anything from 330 to 420 with the same car," Dinan says