Inertia vs. loading dyno information

Here we come to very interesting collision about chassis inertia dyno versus chassis loading dyno. Simple question which type of dynamometers is better for tuning cars? Or maybe in what cases should be used inertia or loading? Is it possible to tune car at partial throttle using inertia dyno?

Before answering that question let's look at the main difference between an inertia dyno and a Dyno Dynamics eddy current dynamometer:

An inertia dyno has a very heavy roller(s) and a speed measurement system. The car is run at full throttle. The more power the car has the quicker it can accelerate the heavy roller. Knowing the inertia of the roller and the acceleration it's a simple calculation to obtain power.

A Dyno Dynamics eddy current dynamometer has very light weight rollers. It also has a speed measurement system. In addition the Dyno Dynamics has an electromagnetic retarder that apply a load to the rollers. This load is adjustable and it works at steady speed as well as when accelerating. The dyno operator (technician) can vary the load from almost zero to very large. Knowing the inertia of the rollers), the acceleration, and the load applied by the retarder it's a simple calculation to obtain power.

OK, so what are the main differences from the motorist point of view?

An inertia dyno cannot apply load unless the vehicle is accelerating. This makes it impossible to test a car on a level road at steady speed on flat roads or up hill, or slowing down due to hills etc.

A Dyno Dynamics dynamometer can easily test a car on a level road at steady speed on flat roads or up hill, or slowing down due to hills etc. A Dyno Dynamics dynamometer can easily hold the vehicle at a say 50% throttle and 2,500 RPM while investigating a misfire that occurs under these conditions.

Mapping the vehicles fuel and ignition computer is part of tuning any performance vehicle. How does an inertia dyno compare with an eddy current dyno?

In it simplest form, “mapping” a modern fuel injection computer involves:

• holding the engine at a particular RPM point (say 3,000 RPM)

• holding the throttle at a particular position (say 90% throttle)

• observe Air:Fuel ratio

• power

and adjusting fuel and timing to achieve best results. This process is repeated at a large number of RPM points and throttle openings. when mapping is complete, the engine will have optimum ignition timing and fuelling at all points in it's operating range from idle through to max power. When steady mapping is complete, mapping whilst accelerating can be performed.

Does this mean that mapping at full throttle is possible on an inertia dyno, but an eddy current dyno is required for mapping the entire engine range?

Mapping at wide open throttle is possible on an inertia dyno in a limited way. For example, lets consider full throttle testing two cars with the same 150 HP engine. One car weighs 2,000 kg, the second car weighs 3,000 kg. On the road the heavier car will accelerate much slower than the lighter car.

• On an inertia dyno both cars will accelerate at the same rate.

• On a Dyno Dynamics dynamometer the operator can choose the rate (or let the software calculate the rate).

The rate of acceleration affects not only the engines requirement for fuel etc, but also combustion chamber temperatures vary considerably with acceleration and time under load. These variances required different mapping values.

As for atmospheric engines it seems clear, but what about forced induction? Is it safe to tune forced induction car on chassis loading dyno? Another side, will inertia dyno create enough resistance for engine to fully boost and produce maximum power?

Accurate and repeatable dyno test results depend on accurately simulating conditions. With forced induction engines this is even more important. For example, a Dyno Dynamics dynamometer enables the operator to stabilize conditions prior to each graph test. If the engine is simply run to the desired starting speed, than given a foot full (sorry, wide open throttle suddenly applied), then conditions at the start of the test will vary and so test results will vary. Turbo lag is a very important factor when testing turbo engines. For accurate results the acceleration rate during graphing is critical. Inertia only dynos have no means of stabilization at the start, nor controlling acceleration during the test.