Legend: HP = Rear-Wheel HorsepowerTQ = Rear-Wheel TorqueRPM = Peak Horsepower @ RPMHP @ Flywhl. = calculated horsepower at flywheel*Gross Score = total of horsepower (HP) and Torque (TQ)
Like an engine dynamometer, a chassis dynamometer measures rear-wheel torque and horsepower and is calculated by the dyno software. Translating the columns, after displacement comes rear-wheel horsepower (HP) and torque (TQ). The engine horsepower rpm peak is identified in RPM. Finally, the Gross Score is the rear-wheel horsepower and torque added together.
Note that the chart includes a horsepower at the flywheel column (HP @ at Flywhl). The dyno software calculates a default 18 percent power loss from the flywheel through the drivetrain, to the rear wheels. Notice too that the flywheel horsepower ratings don't match any of the factory rating numbers. For example, Brian Pallas' '64 Corvette, with an L76 327, was factory rated at 365 hp. As you will see, the true way to measure drivetrain power loss is to dyno your engine on an engine dyno. Install it in your Corvette, then run it on the chassis dyno. Subtract the numbers, and you've got your drivetrain power-loss percentage.
We thought the horsepower figures noted in our chart would be quite revealing. Like those bench-racing sessions where owners claim 11-second quarter-mile elapsed times, factory horsepower ratings are far from real-world figures. But with a little bit of tuning, those outlandish factory ratings can be achieved and beaten by simple hot rodders' technology. Happy Dyno Day to you.
Power: Where Does It Go?Talk about engine power and rear-wheel power, and you're talking about two different things. From the engine flywheel/flexplate, you know the drivetrain will extract a percentage of power loss before it reaches the rear wheels. But the question is, how much?
For some additional insight, we suggest getting a copy of Jeff Smith's cogent article, "The Brutal Truth," in the Nov. '03 Car Craft. In it, the pros at Flowmaster Mufflers dyno'd two engines, installed them in cars, and ran them on Flowmaster's SuperFlow chassis dyno.
One of the test cars was a restored '63 Mercury Comet with a 357ci Windsor with aluminum GT-40 heads, and a 216/220-degree duration (0.050-inch) hydraulic flat-tappet cam. The drivetrain was an AOD and a 9-inch with 3.50:1 gears.