I have an 89 Scarab with two Mercruiser big blocks. The engines have a fresh water cooling system. The engines are well built, with high lift cams, performance heads, and so on. I am a little worried with how close to the elbow the seawater mixes with the exhaust that comes up out of the manifold. My concern with the standard mercruiser exhaust is that between the shockwaves from the valves opening and closing, and the valve lash between intake and exhaust stroke, seawater particles will be sucked back into the engine and participate in the power cycle, which could be very bad for the engines.

I have a friend who recommends fabricating a custom stainless steel exhaust riser, which will mix the seawater and exhaust further back, and far below the elbow so the water has no chance to get sucked back uinto the engine. I still have a stock cast iron exhaust manifold, so there is a possible concern with electrolysis between the cast iron manifold and the stainless exhaust. My friend recommends a sacrificial zinc plug that threads into the exhaust manifold to prevent corrosion. Our concept looks something like the offering from http://www.stainlessmarine.com/.

Does anyone have an opinion or experience regarding stainless exhaust risers connected to cast iron manifolds with a freshwater cooling system? Or with the concept of moving the exhaust/seawater mix further away from the elbow?

If you'v got high ift cams as you say, it will for sure revert water at idle.
Get an aftermarket performance exhaust system. The exhaust you are running is robbing power as well.

what you are talking about is called reversion. it has been studied in the high perf boat world to a degree you wouldn't believe. it sounds like you wanna design and build your own exhaust risers. it's possible, if you have a machine shop and can weld stainless. it is also a huge waste of time and will probly take 2 or 3 tries to get something that is inferior to what is for sale commercially. first, find out what you have for cams, duration, lift, lobe sep angle. then search reversion on here. a couple of people make an extended stainless riser that fits stock merc manifolds, but it is widely considered a waste of money cuz they fit onto stock manifolds ONLY which are limited to about 350 to 400 hp tops regardless of what riser is on them. and i think they are around 800 bucks a pair. exhaust is one of the biggest challenges and expenses we face with high perf boats.

The word "reversion" is crucial! Thanks for informing me about that. The trick seems to be to match the cam and valve kit with the right length of exhaust riser. In extreme cases, you have to run "dry exhaust" as there is nothing that can prevent reversion at low RPM. Time to find out what my cam angles and valve lift are.

lets just hope they are marine cams. dragster cams have so much overlap that they will suck back to 2500 or so rpms. lobe sep angle is the crucial number. if you can get cam spec sheet or part number and trust that it is accurate, you will be in a better starting place. degree wheel and dial indicator is a PITA.