Dynamometers tell us how much horsepower an engine produces. You may have heard of dynamometers and probably looked at a dyno chart or two, but you might not know exactly how an engine or chassis dynamometer works. If you have already read our explanations of horsepower and torque, you're ready for this page. If not, please check out those pages first, because you need to understand those terms to follow this discussion.

Engine and chassis dynamometers

There are two types of dynamometers, engine and chassis and they mean what they say. Engine dynos mount to the engine directly which is separate from the vehicle. Chassis dynos allow the entire vehicle to drive on with the engine installed normally and the tests are then run with the drive wheels on a single drum or sometimes two.

Brake dynamometers and inertia dynamometers

Dynos are further classified into two broad categories, brake and inertia, and these will need a little more explanation.

A brake dynamometer, applies a measured braking force to an engine or drum to maintain a specific RPM. The measured force and constant RPM together give the necessary data to calculate horsepower. The engine can be held at any speed while other measurements or adjustments are made.

An inertia dynamometer uses a drum with a known size and weight which is then accelerated through the RPM range. A computer does the math based on the known mass of the drum and speed of acceleration to yield horsepower. The engine must be accelerating or there is nothing to measure so there are no constant RPM runs while adjustments are made. Often, this type of dyno also measures the deceleration of the drum to give an estimate of driveline losses by comparing how the drum decelerates without any vehicle load and then with the vehicle attached and clutch disengaged.

Characteristics of each type of dynamometer

Engine dynos found at major vehicle manufacturers and in high end speed shops or racing team facilities allow extensive testing of the engine alone. Complete access to the engine is possible and all sorts of configuration changes can be made long before a vehicle chassis is even available. More often these are "brake" dynos and engines can be held at various RPM's so complex fuel and ignition curves can be mapped.

Brake dynamometers, in general, are more complex than inertia dynos because of the braking mechanism which can be water, hydraulic or eddy current and all of the associated controls. Everything needs to be carefully calibrated or readings will be inaccurate.

Inertia dynamometers are much simpler and need essentially no calibration since a known quantity (the drum) is simply accelerated by the vehicle. When you know how fast the drum was accelerated, you know how much torque was required and you calculate horsepower. Remember, horsepower is calculated, not measured. You measure torque.

Extremely high performance engines, the kind often found in drag racing, that are designed to be run for very short periods of time are better suited to inertia dynos since a short run through the RPM range is all that's required. Continuous running at a steady RPM may very well destroy the engine. In the case of the nitromethane drag racing engines, the kind found in Top Fuel and Funny Cars,, no dynos are available to directly measure their output and calculations and horsepower estimates are made from other data and measurements.

Comparisons on different dynos and different days

Comparing dyno runs on different days requires careful consideration of and adjustment for weather conditions. Air density and temperature can have a dramatic effect on engine output.

Vehicle comparisons made on different dynamometers may be more accurate on inertia dynos since calibration is not required.