Eddy current dynos still place the bike’s rear wheel on a large rotating drum, but the dyno gathers information in steps as the engine moves through its rpm range, pausing at regular intervals for several seconds to have the power output measured. The engine is still running at full throttle, but engine speed is controlled by the dyno’s electromagnetic brake acting on a vented disc that looks like it belongs on a very large, heavy auto. The power output, as determined by the amount of force required to hold the engine speed constant, is then measured and recorded up to 30 times per second, providing a very accurate picture of what is happening. In the case of our Vulcan Classic, Salvisberg took five-second readings every 500 rpm from 1500 rpm to the 5500-rpm redline. Aside from gathering extremely accurate data at each step, holding the engine at each rpm step allows the carbon monoxide (CO) output to be measured, giving the tuner one more piece of data hinting at why the engine does or doesn’t perform well at a particular rpm.
Salvisberg, who manufactures and sells both kinds of dynos, cautions that the comparison between eddy current and inertial dynos is not an issue about which are superior. Instead, they are different tools designed to fulfill different needs. Similarly, since different manufacturers’ dynos generate slightly different results due to different manufacturing and calibration specifications, simply comparing dyno numbers without knowing their origin is less revealing than comparing baseline and subsequent runs on the same dyno.