The best way to build a dynamometer is to first decide on the primary type of engine testing that the unit will be used for. Engine-only dynamometers are simple to construct, do not take up much space and can handle a wide variety of engine types. For at-the-wheel, in-car dynamometers, the construction of the test equipment will require a greater attention to detail than an engine-only dynamometer typically would.
The primary advantage to an engine-only dynamometer — beyond its simplicity — is that engine-only dynamometers require less room, less equipment and, usually, only the assurance of proper ventilation. To build a dynamometer that tests engines only, the builder needs to purchase either a desktop or laptop computer with dynamometer software that is suitable for testing traditional or commercial engines. The builder must also purchase the dynamometer attachment that connects to the flywheel of the engine and connect that attachment to the computer. While designing the dynamometer, the builder must provide both fuel and cooling for the engine, while ensuring that the environment is either well-ventilated or that hoses direct exhaust gases out of any interior space.
The major drawback to an engine-only test unit — and a good reason not to build such a dynamometer — is the fact that engine-only units measure horsepower and torque at the flywheel, not at the vehicle wheel. The difference between the two readings is usually quite significant, especially if the purpose of the dynamometer is to fine-tune a high-performance or race vehicle.
This drawback is problematic because at-the-wheel power metrics are more useful and test the performance of a vehicle's entire drive train. Performance metrics taken at the flywheel of the engine test the direct output of the engine only and are often only useful for quality-testing engines prior to sale or installation. Such dynamometers are not accurate enough to test the true performance of a vehicle on the road or at the track.
It takes a few more pieces of equipment to build a dynamometer that tests at-the-wheel horsepower and torque. A builder will need to clear out space for a drive mechanism that propels the wheels of the vehicle during the test. It is not advisable to build a dynamometer of this type entirely indoors, as it is nearly impossible to provide proper ventilation of the exhaust from a fully operational vehicle during a dynamometer test. To build a dynamometer that tests at-the-wheel performance, a builder will still need to purchase laptop or desktop computer with dynamometer software capable of comparing multiple test-runs.