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i have a 21 ft searay i/o that needs more power and i dont really want to put through transom exhausts on it. Need to keep it as quiet as possible. It runs fairly well at 53 mph right now with the 220 hp 305 chevy engine but not quite good enough. Hull handles too well to stay at 53 mph.
If i kept the exhausts like they are now ( through the prop) , how much power in a sbc 400ci could i build before the exhaust would be just too restrictive.
 

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It depends upon your drive, Alpha = bad, Bravo = best, others in between. If you have an Alpha, you need thru transoms. Typical gains with a 350/260 HP is 4 to 6 mph. The 400 with, I assume decent heads and cam will kill the Alpha sooner or later anyway. I got away with an Alpha drive on a light layup 24' Daytona with a 350 with cam(280 comp), performer airgap intake, 750 AFB, and ran a whopping 50 mph. I then ran the exhaust straight out the back and ran 54+, then I bolted a set of bigblock risers (bigger inside and outlet), ran 61/62 mph and promptly puked the lower. The bravo has a much larger exhaust housing inside, and a ventilated lower so that all of the exhaust doesn't have to go thru the prop. It will also stand up to a bigger smallblock just fine. We like to build 427, 434 or even 454 small blocks for a lot of customers with bravos. A cheaper way to go is with a 383/388. There is a LOT of room for improvement, HP wise! Leave the stock mercruiser rocker covers on it, paint it all black, and tell the insurance man that its a stock 350. The most important part of this repower deal is make SURE that the engine builder KNOWS boat motors! Many of the parts and almost all of the internal clearances are different. TIMINATOR
 
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