I ran out of solutions other than making a dry to almost tip tails. I took a perfectly good running fresh 383 I used last summer .i put about 50 hours on it. Block was zero decked.cheap pro comp heads and a decent cam ,cmi headers.made 420 hp on dyno .sold motor to a friend that cracked his block last winter .i swapped cam out from my buddys 300 hp vortec motor and had bad reversion with stock manifolds.and risers . Had him replace them because they were looking a little old .still had it. Added 3" riser extensions still didn't fix it. The dry tails fixed it . maybe mercury has a custom ground cam to except the longer stroke Idk ? I doubt mercury calls comp cams and orders their over the counter marine cam designed for a 350 ,maybe I'm wrong but I do know what I just went through to fix it.

Many thing can contribute to water reversion.

Topic of cid came up. More cid's pulls more air, no ? Since the anwser is yes, it can also pull more water in the exhaust too.
During ATDC of overlap, both valves are open and the piston is going down Air/fuel from intake and exhaust/water/air are all trying to fill the increasing void left by the piston going down. Larger this void is, the more the atmosphere tried to fill it. Remember, again, during overlap the atmosphere can enter from the exhaust too.

So, as we know, cam overlap, internal exhaust design, it's rate of fall, where water is introduced to the exhaust stream, intake manifold design, cyl head design, engine rpm, and etc,etc all contribute to how much exhaust reversion happens.

thanks SB ! You hit on my point exactly and then some, it just took trial and error for me to come to that conclusion.

You shouldn't of need dry tails though. Not with a cam that made 300hp in a 350, unless it was some dumb cam like the GM 290hp/350 cam.

Something else was probably fubared.

The cracked motor was a Michigan motorz 300hp vortec motor. After I kept getting reversion I called comp It ended up with a cam that comp cam said "should" work but didn't .i left it in.
grind # CS xm270hr-12. It also had the intake off his vortec motor on it also. Only things different from when I ran motor last summer was cam,carb and intake.and of course the exhaust both the cams I tried were both smaller than what I was running in it. The cam I was running went in to my new build

I can't follow your typing and I live in the dirt, so I'm used to bad english. LOL.

I thought you said 300hp cam ?

The XM270hr-12 has been used in a bazzillion marine 350's and 383's. Cam of choice actually for really good, but nothing wild, performance. Yes, exhaust needs to have inner pipe extended a bit. Tons of threads on this.

sorry for not being clear .i couldn't find a clear answer ,probably using wrong key words.I got it figured out tho ,thanks