I am running a 350 that dyno'd at 404hp. I am running it thru stock merc exhaust manifolds and risers with 4" thru-hulls. My question is how much power am I losing thru stock merc parts and who's performance manifold and risers have given the best results. I would like a riser that is about 4" taller than stock.

jhnrckr,

My guess is that you're going to lose A LOT from 404hp assuming that you went from tubular dyno headers to the stock Merc crude exhaust manifold. I would suggest an aftermarket type exhaust system from someone like Stainless Marine or even Lightening. Stainless Marine will custom make your tailpipes/risers to fit your engine/boat application that you are interested. Dennis Moore, Budah or someone else who has lots of experience with the SBC could probably give you some good additional advice as well. But as far as I know the SBC really responds well to a good flowing exhaust system and you are certainly NOT going to get it from the stock, crude cast iron exhaust that Merc provides that's for sure!

Dennis has some advertised on his site. I don't know if they are "tuned" or not.

Imco Thumpers look good too.

Stay away from the stock Merc manifolds. Several years ago I had the stock replaced with EMI thunder manifolds and the difference was unreal. The engine responded better, and gave a much nicer growl. I'm sure there are even better products out there that will do your boat more justice.

JP