O.S. actually states a max practical RPM of 20,000 which translates to around 2,350 RPM head speed. I have mine governed at 2,150 RPM head speed (17,850 engine RPM) in IU2. The engine will actually turn quite a bit more than 20,000 RPM if you just open it to WOT and let it run. I had entered the gear ratio incorrectly in my governor once and that thing was flat screaming. I tach'd the head speed and, if I remember correctly, it was turning at around 2,600 RPM (22,100 engine RPM).