As you can see there is allot to this subject, Flusher hit the nail on the head as he usually does, I personally would stay away from off the shelf octane boosters, if you have an engine that has high cylinder pressures then you have to dig a little deeper in your pockets to now give it the fuel it needs to avoid damage from detonation. A big mis conception that many people have is that octane makes power, it does not, this number is only to show the fuels resinstance to detonation. Tests have shown and I too in dyno rooms testing engines and different racing fuels that if you feed an engine a very high octane fuel that does not need it (mild cylinder pressures) it will loose power. I would set ign. timing to at a safe area and try 91 oct. pump gas and see how it feels, chances in a boat you will not hear the ping of detonation, so if you know what your looking at reading spark plugs this could give you an indication of what's going on in the combustion chamber. If you see little black peppered specs on the porcelain this is the first sign of detonation. These specs are carbon particles that have been ratteled loose from coliding flame fronts. You may have to splash a bit of good fuel along with pump gas just to be on the safe side, but every combo is different with allot of variables. Using AV gas is fine, that's what I do, I mix a 50/50 mix for my boat BBC engine and I'm dead nuts 10.0:1 compression with ign. timing set at 16 inital and 36 total but I set my dist. up so full advance is all in at 1,800 rpm so she pulls hard out of the hole, it works for me and I don't have anything really radicle, engine dynoed at 542 hp with that fuel blend.
