I'm no expert .... I use McCoy 59 & 9's on 65% nitro. Its cold up here (so no testing), but I've read that plug selection is one of the factors is getting an engine to run at good operating temps for a boat (@200 degrees). So run whatever you plugs have, and (if you can) then measure the head temp's after running. If it is running cold - go to a hotter plug - if its hot, go to a colder plug. Am I wrong in thinking this? (BTW Rod G restricted my head's water outlets on my .21 & .12 motors last summer but I havent measured the engine temps yet). Curious about this....