CheckmateFever
08-01-2007, 03:15 PM
Hey ya'll, i recently had my '86 21' Checkmate Enchanter converted from origional thru-prop hub exhuast over to straight thru-hull exhuast and also added a set of performance GLM aluminum manifolds as well. The engine is a MerCruiser 350/260hp sb chevy with an alpha one outdrive. After all was said and done i've lost about 300 rpms from last season according to my tac. I was turning 4300 rpms before the exhuast conversion, now i'm only getting barley 4000. I have a GPS unit in my boat and at 4000 it says i'm doin almost 60mph, does that seem right?? I've always used a 23 pitch prop, however i just put on a new 23p prop that is a little bigger in diameter than my old prop, would that make that big of a difference?? With the exhuast mods i would think i could turn a slightly bigger dia prop right? Where do you think my revs went? I've heard also that my engine my not be properly tuned for the straight thru-hull exhuast and that i would have to completly re-jet and re-tune the carb due to all the added breathing potentional of the new exhaust, would that be why i lost 300 rpms? Just to give you an idea, i checked my rpm's at different speeds, at about 2500 i was running around 34 mph, the shop that worked on my boat said that seemed a little fast for the rpms and that my tac my be off, what do you guy's think? If i could turn 23p before why can't i now dispite the bigger dia.? The book on my motor says my WOT rpm range is 4200-4600, i may try my old prop and see where that get's me. I also added a new K&N marine flame arrestor to to help the motor breath as well, i'm just plain stumped as to why i lost horespower! Any suggestions or ideas would help a ton, thanks!