It's been said that a man with a watch always knows the correct time, but a man with two watches is never really sure. Unfortunately, there are no proverbs about a man with two knowing how much power his car really makes. In this article, we'll examine two popular chassis dynamometers and explain why each will give us a different rear-wheel horsepower for the same engine. We test both the Dynojet model 248 chassis dyno located at Strope's Speed Shop in Washington, Pennsylvania and the MD-250 Mustang chassis dyno located at Speed Nation in McDonald, Pennsylvania. We also performed testing at Keystone Raceway in New Alexandria, Pennsylvania, using a dyno system

from West Automotive Performance Engineering.

Dynojet

The Dynojet chassis dyno is referred to as an inertia-type dynamometer, because large drums provide an inertial load to the drivetrain instead of a friction brake. The working end of the Dynojet includes two 48-inch diameter drums that are mostly below the surface and driven by the vehicle's drive wheels. In the photos of the Dynojet, notice how the rear wheels are centered on the drums and there is one drum per wheel. This will become important later.

The vehicle is typically run in the transmission gear closest to 1:1 (Forth gear for manuals and Third gear for automatics) to

provide the most efficient drivetrain setup. The inertia of the large drums loads the drivetrain. The Dynojet software measures the horsepower applied to the drums and calculates torque from engine rpm via an inductive probe. The Dynojet provides both the torque and horsepower curves versus engine rpm. This unit also has an airbrake that can be configured to provide a constant load >>