what is the difference between a closed chamber head and an open chamber

what is the difference between a closed chamber head and an open chamber

is it a cc size issue or would the actual heads be advertised as either one

well open chamber heads are usually large cc, and closed small cc. But also i think the term open is because the area around the valve is just open and shaped like a trapazoid, as opposed to a heart shaped style closed chamber head where the chamber kind of forms around the valves. Also i've heard that open chamber heads down let the spark travel as well, but i'm not sure if thats true, and if so, how significant that is.

The terms come from the Mark IV Big Block Chevy heads, which in addition to coming in oval & rectangular port configurations, also had small and large combustion chambers. The open chamber was the big one, the closed chamber was the small one.

Relief around the spark plug. Closed chamber heads the plug are flush/ flat with the cylinder head. Open chamber heads the plug actually looks like it is raised when in reality the area around the plug is recessed. Open chamber heads flow better than closed. Closed chamber heads were higher compression heads.

Why do you ask?