How do closed chamber heads work on marine motors?

they will raise the compression to a level that might not be good in a boat without some good fuel...

if you have flat top pistons they will work fine,almost the same cc combustion chamber of the merc 525 efi cylinder head..i would say their good for 50 ponys..
+1 make sure you run good gas and take it easy on the timing advance,,32* total..

They tend to shroud the valves resulting a loss of airflow. Some heads can be ported to help. Which heads are you considering?

Those heads are 118cc's. They are Edelbrock heads made for Mercruiser.

Not for my setup, it's on a friends boat.
Just hadn't heard of anyone using closed chamber heads on a boat motor
It's set up as 10.5 - 1 with ported closed chamber heads

hey if they are early model BBC closed chamber heads (rare) and in good shape $$$$

I just built a new motor it has gm 290 big ovals they are closed chamber had big valves added and bowl and runner work done on them