What is an Engine Performance Dynamometer?

For any car performance enthusiast having access to a dynamometer is a must to get the most out of your vehicle modifications. Engine dynamometers measure how much horsepower and torque an engine is producing. This is invaluable information when tweaking for the maximum power output. The process of tweaking an engine for maximum performance is known as dyno tuning.

There are many dynamometers on the market, but they all serve the same purpose, to measure an engine's power output.

How Does a Dynamometer Work?

The basic operation of a dynamometer is a pretty simple concept.

Dynamometers work by attaching the engine to a load, then using the engine to accelerate the load. By monitoring how the load reacts to being driven by the engine the engine's output horsepower and torque can be calculated.

What are the Different Types of Dynamometers?

The two main types of dynamometers used in engine performance tuning are engine dynamometers and chassis dynamometers. Engine dynamometers measure the power at the engine's crankshaft or flywheel while chassis dynamometers measure the power at the wheels.

Engine Dynamometers

Engine dynamometers, sometimes called flywheel or crank dynamometers, are the most popular type of dynamometer for engine builders. They are accurate and relatively easy to use. The engine is mounted on the dynamometer and all ancillary systems such as cooling, oiling, and fueling are plumbed in. The flywheel is then attached to the dynamometer input shaft.

Chassis Dynamometers

Chassis dynamometers are a more expensive option, but they have the advantage of being able to test the engine in the car. This is important for two reasons. First, it allows you to test the engine under real-world conditions. Second, it allows you to make power measurements without having to remove the engine from the car.

Applying the Engine Load

Different types of dynamometers use different methods to apply a load to the engine.

There are water brake (hydraulic) dynos that use water to apply the engine load. In this type of dyno, the dynamometer churns water inside the dyno. The amount of water flow is used to measure the engine's power output. Water brake dynamometers are usually engine dynamometers.

When it comes to chassis dynamometers, there are two main types that use different methods to provide the engine load for testing.

The first main type of load used for engine performance dynos is what is called eddy current. An eddy current dynamometer uses a metal disk that passes through a magnetic field to provide a load for the engine being tested.

There is one other type of chassis dynamometer that uses a large drum that is very heavy as the load. These are called inertia load dynamometers.

Chassis dynamometers work by attaching a large roller to the car's rear wheels. The car is then driven onto the dynamometer and the roller turns, simulating road conditions. The dynamometer measures the power at the roller, which is then converted to power at the engine.

There are also dynamometers for all-wheel drive cars. These dynamometers have two rollers, one for the front wheels and one for the rear wheels.

No matter what type of dynamometer you use, engine power measurement is an essential tool for any performance enthusiast.

Drivetrain Loss

To get the engine power output at the flywheel with a chassis dynamometer the vehicle drivetrain loss must the calculated. Drivetrain loss is the amount of power that is lost during the transfer of power from the engine's crankshaft to the vehicle's wheels.

In most vehicles, there are a lot of things soaking up the engine's output power before it gets to the wheels. Even in a vehicle with a manual transmission, the friction between gears in the transmission reduces the power transferred through the transmission.

Vehicles with a traditional automatic transmission have even more internal components, such as a torque converted and pump, that steal power from the drive wheels.

There is also the differential gear ratio to consider. The higher the gear ratio, the more power is required to turn the wheels.

All of these factors must be taken into account when using a chassis dynamometer to measure engine power output. However, once you have the drivetrain loss figured out, a chassis dynamometer is an excellent and convenient way to estimate engine performance.

Keep in mind, there is no practical way to get the calculation 100% correct, so with a chassis dynamometer, while it can measure the power output at the vehicle's wheel, it will only allow you to estimate the actual power output at the engine's crankshaft.

Another important thing to keep in mind is that different types, brands, and even different dynamometers of the same kind can, and usually will, give a different power reading for the same vehicle.

Because of this, when you need to compare multiple dyno runs, for example, when making performance tweaks to a vehicle, it is best to use the same dyno for comparison.

If you don't have one already, or if you are using a dyno you have not used before, it is best to do a baseline run on the dyno to make sure you will be able to more precisely measure any power differences as you make changes.

Atmospheric Conditions

There are some additional things that are out of your control that will affect the horsepower and torque readings you get from a dyno. These are the atmospheric conditions present when the run is made.

These are things like the current temperature, humidity, and barometric pressure.

In the end, engines are basically air pumps, and these atmospheric conditions affect the engine's power output by affecting the number of air molecules pushed into the combustion chamber.

More air means more fuel is needed, and that, in turn, translates into more power output.

Corrected Power

Fortunately for us, there are formulas to take into consideration these different atmospheric variables and correct them to a set of standard conditions. This helps when comparing different runs on the same dynamometer under these different conditions.

You will see dyno run numbers mention "corrected" or "SAE" when the power output has been standardized with one of these formulas.

There are several formulas available, and you can check out our article about corrected dyno numbers if you would like to get more details on these.

Final Thoughts

Now that you know what an engine performance dynamometer is and why you need to use one you are probably wondering how much a dyno run costs.