I personally think a 12X8 prop is a bit much on the ST 75. I usually run a 12X6 or 13X6 depending on the type of prop on that engine. Mine is an older italian version, but it runs great. I doubt that a 12 inch prop will give unlimited verticle on the four star, but running a 13 or 14 inch prop that is pitched accordingly would do a better job in the verticle department. I don't use a tach all that often, but if I remember correctly, I can get about 12,000 rpm on a 12X6 prop.