I have an 02 206 cobalt powerd by a volvo penta 320hp 5.7gxi 1.95 ratio dps drive and want to repower with an 420 hp 8.1 gxi. The boat hits 63mph on the speedo with the small block. What else will I need to change? Will this overpower the the boat?

I had a 21 Chris Craft with that same 5.7GXI (280hp) DP. (awesome equipment). The larger 8.1 will squat the stern and require a prop change at least. That hull probably wasn't made to run over 70 that I would expect from that setup.....also, Ins. Co will probably have a big problem with your plan. IMO you need to step up to more LOA.

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Cobalt: if you can, send me a private message with your e-mail and I can give you a bit of info. that might help.

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Going to the 502 or 496 would be cool, but getting more from you 5.7 may make more sense. If you are looking for 400 - 420 HP it can be done pretty easily with the small block. Some AFR heads, a little more cam and some good exhaust with a holley 750 - 800 will get you there. It would also be fun to pass an occasional BBC with the mouse powered boat!

I would tend to agree with WETTE VETTE. It's a fairly light boat and you get get good power out of the SB without adding the extra weight of a BB and you prbably don't "need" a lot of the extra torque you'd get.

But don't listen to me...I have a new 540 under my hatch.

WV and SR J , Thats cool but I would like to keep my mpi fuel setup . So what cam will do it and what heads? Ive been looking at stainless marine exhaust. I would like to go low 70's and be reliable. I would think thats alot to ask for with a smallblock.