Ok so here is what I have going on with an issue new rebuilt engine and need some guidance on how to resolve it.

First the basics, motor started out as 502 Mag MPI and has been reworked with the following:
Bored .040 over to straighten cyl walls, Eagle crank, Carillo rods, JE pistons with titanium rings, Brodix aluminum heads that have been ported and flowed, mild Comp cam and roller rockers, etc. estimated HP is somewhere between 600-650HP

Motor was broken in for about 17 hrs at lower rpms letting the boat plow and build heat and then letting it cool down for a while, and only hitting WOT for a couple seconds every once and a while during this time.

The fuel system is the stock MPI set up with ECM retuned.

The problem I am having appears to be a venting issue. The way the breathers were originally routed were the one from back of port side valve cover was teed into the one on front of starboard valve cover which in turn just had a little 2" piece of hose that went up against the air cleaner.

This would in turn cause crank pressure to build and blow dip stick out of tube and push oil out after a hard run.
Now they are routed like a buddies stock 502 mpi from the same year which is the one from rear of port side valve cover has PCV valve in it and goes to fitting under plenum just behind throttle body and the other is just routed to bottom of air cleaner like it was with a plug on the side where hose from other cover used to go.

My issue is that the one going to the plenum is apparently sucking oil and inside of plenum is coated with a light oil film which is in turn being burnt off but is also what I am guessing is to be where my oil consumption issue is coming from. IF you run boat at 4K or less, it really doesn't use any oil. its only when you do a few hard WOT runs that I believe is when its pulling oil from valve cover and sucking it into the intake.

So what I am trying to figure out is, is this happening because I changed the motor drastically enough that the stock breather set up doesn't work very well? Do I need to install a puke tank instead of routing vents like they are or just put bigger breathers in both valve covers and let vent to atmosphere?

Any help and guidance would be greatly appreciated as I have ben searching the forums on here for similar issues but really haven't found the answer I guess.

Stock 502 Mags can burn 1qt oil for every 10 hrs use and its considered normal. I have always found a film of oil (sometimes quite a bit) in the plenums when pulled.They dont build enough velocity in the intake to remove the oil that puddles in the plenum.Some say the low friction ( metric ) oil ring package doesnt control oil well enough. Ive gone through about a dozen 502's in commercial app and some just burn more than others. Good Luck!!! Mercruiser service manual says 454/502 Mag can use 1qt every 1 - 5hrs if run hard.

Do you have stock covers ? Probably not. Do you have oil deflectors in your valve covers ? Many afmkt do not.

Your increased hp and rpm motor needs more breather capacity.

You getting any oil on the plugs ? Have you done compression and leak down ?

I have heard that and been told that as well that 502's will use some oil and figured part of it was some blow by the first few hours due to rings not seated yet.

And yes it has the stock valve covers which just cleared all the rockers and only had to trim 2 of them for the covers to fit.

When I checked plugs at about 15 hours, they looked good and no oil on them and I have not done compression or leak down since it went back together.

Been testing props the last couple weekends and have one more for this weekend and will see what happens and go from there

The stock 502's will use oil. Why? GM doesn't hone there motors with plates. Your new motor should not have any blow by. It doesn't take 17 hours to break in a motor. Maybe 17 minutes. Either the machine work is extremely bad in your new motor or you have a stuck ring, burnt piston from bad tune or some type of mechanical failure. At this point you need to do a leak down test on all 8 cylinders. You should not have to put a puke tank on a 600 hp motor. We have motors out there 1000hp+ that don't have puke tanks and don't use a drop of oil...

How well do titanium rings seal