View Full Version : Chassis Dynomometers explained...

12-04-02, 10:06 PM
Hot Rod (http://www.hotrod.com) magazine has an article in the January issue entitled "10 Ways to Estimate Your Engine's Power." Among the various methods used was a chassis dyno. The following is an excerpt from the article:

Chassis dynos measure torque at the rear wheels. All vehicles will see losses through the drivetrain between the flywheel and tires. Losses through a manual trans are less than those through an automatic trans. Collectively, Harold Bettes of Superflow says manual-trans cars typically will see a 50-80hp loss through the drivetrain, with 70-120hp the norm for most automatics. Full-race automatic losses will be on the high side because their loose converters have more slippage. Late-model vehicles with high-tech trannys and rearends with synthetic lubricants mat see slightly less losses. In any event, try to run the trans in direct 1:1 drive to minimize any losses through the "extra" gears (which could be as high as another 20hp).

Even with these variables, a reasonable relationship can be established between the rear-wheel output and engine flywheel output, depending on the sophistication of the dyno and operator skill. Old-style Clayton chassis dynos use a water brake to measure torque at the dyno's rear wheels. They don't have computerized data recording or software that compensates for drivetrain losses and can't make a quick acceleration pull. Data must be manually recorded at each checkpoint, and the old-tech dyno may not be up to holding a high-powered car on the rollers without slippage. A step up is the inertial cassis dyno, which as the name implies uses pure inertia to estimate torque and power. Although usually computer-controlled, they rely on a set of preprogrammed assumptions rather than direct measurement, and may underestimate torque and power during a quick acceleration pull, particularly with a heavy vehicle and/or an engine with a large internal mass. (Can you say "classic big-block musclecar?")

At the top of the pecking order are Eddy-current chassis dynos, which use opposing electromagnets and sophisticated computer compensation. They are much more accurate and can withstand extremely high torque levels. Most importantly, they use direct measurement to derive torque and horsepower. Some even take into account vehicle frontal area and coast-down time to home in on actual drivetrain losses.

Check out Hot Rod Magazine (http://www.hotrod.com) for more information on how to measure your car's power. :upthumbs

_ken :w

12-04-02, 11:00 PM
How would you convert rwhp to flywheel HP?.. I turned 324 rwhp..

Thanks Steve