OK, I am not a pro engine builder, so take my advice with a grain of salt if you like. I think you were on the ragged edge running boost to a stock Merc MPI motor (non-blue motor), especially with almost 400 hours. First of all, I am 99% sure that those engines were not equipped with inconel exhaust valves from the factory. I say this because several years ago, I was trying to answer that very question myself. I called up Mercury Racing and spoke with a tech - he told me that as far as he knew, only the blue motors have the inconel valves, because those were torn down by Mercury Racing, blueprinted and reassembled with better components, including the Manley inconel exhaust valves. He said the black motors just had whatever GM put in them. Maybe GM put inconel valves in their marine engines, but I was never able to establish if that is true.

Secondly, as others have touched on those stock head gaskets were not made to handle the extra cylinder pressure of a blower. A set of Cometic MLS gaskets would be your best bet, but MLS require an absolutely flat sealing surface, with the correct finish. Not sure if you can get that without having the heads and deck resurfaced. Second choice here would be a set of quality head gaskets such as FelPro or similar. Head studs would be advisable for a boosted application as well.

I guess the bridge you have to cross is if you want to continue to take chances with things like they are now, or if you want to back up and use the best parts to make this engine live a while under boost. Of course, nobody wants to hear that they are going to have to spend big bucks on their boat to go fast, including me! ;)

Good luck with it whichever direction you go.

Thanks for your advice. I have decided that my summer boating season is over at this point. Really only had one more event planned this summer and that was Hot Boat at Hardy Dam. I dont want to rush to get the boat back together and hope it runs good for just one more weekend. At this point I am planning on pulling the motor and assessing the damage. Then decide how I want to proceed from there. I really enjoy performing my own work but am not sure if that is the way to go with this project. I would love to learn how to rebuild/assemble a motor, but dont want to use mine as a guinea pig. I may take it to an experienced builder and have them advise and fix it for me.

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Finally tore the motor down. A lot of water in their. I am thinking that i will send it to a pro to have it rebuilt then re-install into boat myself.

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could you see any places head gasket had failed ?

From the first picture it looks like cylinder 6 blew out the fire ring.

yeap..see that now, thats 7 and looks like 3 as well but not as bad...

I was actually talking about the first picture in post #34. That is cylinder 6 and the firing ring is blown out. It does also look like cylinder 7 and cylinder 3is blown out in the third picture of post #33 looking at the head gasket sitting on top of the block.

This thing went full sputnik, that's for sure.

That is everything I would expect to see with stock head gaskets. I have seen that every time a blower was added to a 454-502 without changing the head gaskets. Guys that want my help, I tell them I won't do it unless they change head gaskets. A friend of mine has a whipple on a 525 and has never had a problem, but that probably uses a better head gasket than the 454-502mpi.
It doesn't look that bad, I would have a machine shop check the cylinders and pistons, but they are probably ok. I would just do a complete rebuild since it has 380 hours and start of fresh next year. OH, and lower the boost.

yeap...see #6 as well...killed them both for sure...