I've had some issues with high compression engines and Powermaster starters. If you look at them carefully during operation you will see that the starter gear does not full engage the gear on the flywheel; it only goes 2/3-3/4 deep into the teeth. I've taken them apart in the past and machined the mounting flange so make it thinner, so that the entire gear engages the flywheel teeth. But I've had better luck with the RobbMc starters, which don't have this problem, and also offer the pinion support like a factory starter. I think if you are running 12:1 or more on a big engine, you should be leaning towards the RobbMc starter.