Wheel power or engine power?
It is common to use dyno results (dyno graphs) for various cars to compare their capabilities, potential accelerations and driveability. Internet pages are full of more or less accurate dynamometer pulls results. In most cases, these results are comparable at least according to standards used (most popular are SAE in USA, DIN across Europe), but painful fact is, that they are widely uncomparable across continents: in America wheel power is king. Europe relies on engine power.
There are many sophisticated methods of „recalculating” one power to another. Some advocates of simplicity propose adding of 20% to wheel power result to achieve engine power. But to be honest – even most advanced and physically well-documented transition methods fall with one simple truth: wheel power depends on tyres, gear ratio, pressure in tyres, car weight, and especially – on the vehicle tie-down method.
Tie-down force distorts tyre and has a direct influence on friction between tyre and dyno roll. Thus – especially for cars with soft tyres (winter tyres, slicks etc), one step-up tooth at ratchet causes difference of whp as big as 5%. Together with arising inflation pressure (due to increased friction) even two pulls won’t be comparable. Even type of tread pattern can influence final dyno pull result up to 3 percent, maintaining same wheel size, diameter and weight. It is especially visible if winter tyres are used to compare with regular summer tyres.
Mechanical friction of rotating parts vary from car to car. It depends on type of oils and greases used, it’s age, age and total wear of engine and transmission, and even other unpredictable factors. Wheel power will be affected by all these, and there is no method of predicting scale of distortion.
Measuring engine power with modern european dynamometer solves all these problems due to recording also free (unpowered) roll off phase, where most of traction losses, and friction losses are measured. These losses depend on speed, and are generally comparable (similar value) within same speeds wheter car accelerates or decelerates.
There is no problem for such dyno to show wheel power too. So measuring engine power (european style) may be named a kind of extension to ordinary wheel power pull.
It is highly adviced (also for our conservative American friends) to measure engine power and comparing it instead of whp. It is much more reliable, and less ridiculous than using one correction factor for this, second for that, and add 1.5% extra if car runs with thinner oil, and see three different results on three different whp dynos.