An engine dynamometer is used to measure the engine’s power. Dyno tests can be done different ways, but the most common method in the racing world is a sweep test. A sweep test is where you slowly (or quickly) accelerate (or decelerate) through the RPM range with the engine at full power. This must be done somewhat smoothly and slowly, taking about 5 to 10 seconds for good accuracy.

When you do an accelerating sweep test, some of the engine’s power is going into accelerating the engine’s and dyno’s rotating inertia. The power recorded at the dyno will be lower than during a steady state dyno run, where you set for, say, 2 seconds at each RPM. The faster the acceleration rate, and/or the more inertia (heavier crankshaft, larger dyno impeller, larger engine flywheel, etc), the more the power loss. During a decelerating test, power is being released from the rotating inertia, so results will be measured higher at the dyno. This is one reason engine builders like decelerating tests more, because it produces higher numbers.

The Pro and Enterprise Editions of our Dyno DataMite software let you correct for these inertia effects. The goal is to produce the same power whether accelerating or decelerating, whether fast or slow, whether lots of rotating inertia or not. When “Correct for Engine Inertia Effects”is turned On in the program, the settings which effect how much inertia the engine has is set in the Engine Specs screen, as shown below.



A Dyno DataMite was using the Engine Inertia correction and wanted to know how much this effect was changing his power levels. So, like with many things you may want to investigate, you should first make a copy of a test under a new name. He could save his “10” test under the name “10 no inertia”. Then set the “Correct for Engine Inertia Effects” in Test Options screen to No and compare the results, as shown in the figure below.


You can see from the graph above and columns of numbers down the left side, this accelerating test gained about 20 HP (402 peak HP down to 382 HP) with Engine Inertia Correction turned On, or about 5%. This engine accelerated at about a rate of 300 RPM per second. If it had accelerated more quickly, like 600 RPM per second, the effect would have been about double. The idea would be that both the 300 and 600 RPM per second test would produce about 402 HP with the Engine Inertia correction turned ON. However, with the Engine Inertia correction turned OFF, 300 RPM per second accel would produce 382 HP and a 600 RPM/sec would produce about 360 HP. Obviously you want results which are repeatable, independent of the acceleration rate.

Click on the blue link to review our FAQ on this topic which goes into some more detail:  FAQ on Dyno Corrections for Engine Inertia Effect

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