Re: "12 port head with no chamber"
This is also called a "plank" head (since the gasket surface is flat).
The valves are exposed immediately on cracking open, but if the engine has quench they're still shrouded by the "O" or "D" cup in the piston dome B & ATDC during overlap. How quickly the dome "runs away" from the opening intake valve is a function of the cam event (early intake opening or late exhaust closing = worse), the stroke length (short = worse), and the rod ratio (high = worse).
Obviously, after the piston's position passes the cup depth (just a guess: .200" to .300" minimum representing the valve lift during overlap), the valves have radial clearance all around up to the cylinder walls.
A non-quench engine will have no shrouding, since the piston will be flat with whatever negative deck height is needed for the static CR chosen, but IMHO this is a bad exchange.
