|Inertia dynamometers are essentially large free-spinning barrels, that work in conjuction with a computer to precisely measure horsepower and create horsepower to rpm charts. The barrels are spun as the vehicle accelerates, and the time that it takes to go from a set range of speeds determines the horsepower of the vehicle. |
The torque number is algorythmically calculated from the horsepower number, time, and several other variables. While this is not the most accurate way to calculate torque, it is within reason, and good enough for most applications.
Inertia dynos work properly under load only. It will not calculate your horsepower while holding steady at a fixed RPM. Although inertia dynos aren't the greatest for fuel tuning at xxxx RPM, they will give an extremely consistant, and accurate readout based on wide open throttle pulls. This is because a proper inertia dyno does not have any calibration associated with it. It's purely mathmatical, thus eliminating a lot of error margin.
Inertia dynos are especially useful for back to back runs intended to measure improvements from slight adjustments like plug gap, intake temp, lubricant changes, and other minor adjustments.
The primary function of the inertial style dyno is simplicity and dead on accuracy, which it achieves superbly. Even the most subtle change, such as turning ones headlights on, can show a difference on an inertia dyno - now that's precision.