Wheel power or engine power?

Websites are full of more or less accurate dynamometer pull results. It is common to use dyno results (dyno graphs) for various cars to compare their capabilities, potential accelerations and driveability. In most cases, these results are comparable, at least according to the standards used (most popular are SAE in the USA and DIN across Europe). Still, the painful fact is that they are widely incomparable across continents: in America, wheel power is king. Europe relies on engine power.

There are many methods of "converting" one power to another. Some advocates of simplicity suggest adding 20% to the wheel power results to get engine power. But truth be told, even the most advanced and physically well-documented conversion methods miss one simple truth: wheel power depends on the tyres, gear ratio, tyre pressure, weight of the car and, especially, on how the car is strapped.

Tie-down force deforms the tyre and directly affects the friction between the tyre and dyno roller. Therefore – especially for cars with soft tyres (winter tyres, slicks etc.) - one notch up on the ratchet results in a WHP difference of up to 5%. Even two dyno runs will not be comparable with increasing tyre air pressure (due to increased friction). Even the type of tread pattern can affect the final dynamic measurements by up to 3%, while keeping the wheel size, diameter and weight the same. This is obvious when comparing winter tyres with normal summer tyres.

Mechanical friction of rotating parts varies from car to car. It depends on the type of oils and lubricants used, the age, the total wear and tear of the engine and gearbox, and other unpredictable factors. All of these factors will affect wheel power, and there is no method to predict the extent of distortion.

Measuring engine power with a modern European dynamometer solves all these problems because it also records the coast-down phase. Most of the traction and friction losses are measured. These losses depend on speed and are usually comparable (similar value) in the same range, regardless of whether the car is accelerating or slowing down.

There is no problem for such a dynamometer to also show wheel power. So an engine power measurement (European style) can be an extension of the usual wheel power measurement.
It is recommended (also to our conservative American friends) to measure engine power and compare it instead of WHP. This is much more reliable and less ridiculous than using one correction factor for this, another for that and adding 1.5% extra if the car runs on thinner oil, and getting three different results on three different dynamometers.