A harmonic Dampener is something that reduces engine vibration throughout different RPM ranges. Now my ME skills are coming back. Basically, an engine has a certain vibration to it and when you increase the RPMS, that vibration has different characteristics, meaning that it will increase or decrease. The engine has a certain harmonic frequency and when it is lower or in balance, you are not losing energy and you will save more HP, just like adding a light weight flywheel, you free up horsepower. The harmonic dampener is meant to counteract vibrations and provide a smoother revving engine throughout the RPM range thus saving you HP.
I would think it would only help under extreme power and load conditions, these units are pretty common for high hp drag racing applications. There is a difference between engine dampers and crankshaft torsional dampers. the crank shaft dampener is designed to minimize the slight wobbles that can happen at the crank under the right conditions. Engine dampening is an entirely different issue.
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