What was achieved overall? The high point was Aprilia’s last 125cc single, giving roughly 55 hp at 13,000 rpm. To make that power at such rpm, the engine was filling its cylinder every revolution as well as a highly developed four-stroke racing engine of the same displacement can achieve only every two revolutions. In both cases, that degree of cylinder-filling maximally exploited intake- and exhaust-wave action. As Rob “Mr. Superbike” Muzzy once said, “The harder you tune on a four-stroke, the more it begins to act like a two-stroke.” It’s the same air, the same fuel, and the same laws of physics at work in both cases.