Peak hp is not the only indicator of an engine’s performance. On the dyno sheets I saw, the Rotax was the peakiest of the TaG engines. The Rotax may have the highest peak hp, but only over a relatively short rpm range. The other TaG motors have much flatter power curves, so they are stronger at lower rpm ranges.
The maximum rpm that an engine can turn also affects performance. Some of the other TaG motors turn more rpm than the Rotax, so on a track with a wide range of speeds shorter gearing can be run on those motors for the slow corners and they still won’t run out of revs on the straight.
Finally, on track results are a real performance indicator. We saw several different types of engines running up front this year. If the Rotax was dominant everybody would be switching to it, and that certainly wasn’t the case.