Dynamometers, in general, are a category of testing devices that measure the power or torque produced by a machine. An automotive dynamometer, more popularly known as a dyno, is a specialized machine that calculates how much horsepower and torque an automobile can generate. The automotive dynamometer is popular among car tuners and mechanics, both as a diagnostic tool and as a way of measuring the results of performance modifications.
The there are two main types of dynos, and the difference between them is whether they test power at the engine or at the wheels. Power put to the wheels will always measure lower due to a phenomenon called drivetrain loss. A dyno that measures power at the engine is predictably known an engine dynamometer. One that measures power going to the wheels is known as a chassis dynamometer. Chassis dynos offer a more accurate gauge of real-world power — that is power actually being transmitted by the wheels into velocity.
Though not always, engine dynos are often simply one element of a larger diagnostic system known as an engine test stand. Used mainly by car and engine manufacturers, these systems also house testing devices for things like emissions, fuel efficiency, and overall stress capacities. They are often used to determine the final specification numbers that are put out through official channels when a new car is unveiled. An engine dyno works through electronic sensors placed through the motor itself, that detect and translate output into common units of measurement, like horsepower.
By contrast, the basic setup of an chassis automotive dynamometer machine consists of a ramp and platform upon which a car is placed, with rollers underneath the drive wheels that allow them to spin without the car going anywhere. Most dynos have only a pair of these rollers, which means four- or all-wheel-drive vehicles will not test correctly. For these cars, a special four-wheel dyno must be utilized.
The rollers on a chassis dyno may either exert a resistive force against the wheels to measure power, or, instead, be constructed to approximate the forces that a vehicle is subject to when accelerating on a flat surface. These sub-designs are known as brake dynos and inertia dynos, respectively. The process of testing a car with a chassis automotive dynamometer of either stripe is relatively straightforward, however.
Once placed upon the test bed, the car is strapped down and given a general inspection for obvious defects or faults. An electronic pickup is placed on a spark plug wire to measure engine revs per minute (RPMs). If it is a two-wheel-drive vehicle, chocks are placed in front of the non-drive wheels to ensure they don't move. The car is then started and brought up to about 10 miles per hour (22 kph) on the speedometer, to align the rollers. Then, the engine is brought up to testing speed, usually somewhere around third or fourth gear.
The moment of truth comes as the accelerator is then pressed down the to floor — a position known as wide-open throttle (WOT) — until the tachometer hits its redline, at which point the pedal is lifted and the wheels naturally slow down. Typically, a test involves three of these runs, with the final power numbers comprising the average of each run. A variety of factors can affect total power on a given series of runs, and tuners invariably highlight the maximum numbers achieved, however, rather than the average.