what is the difference between closed chamber heads and open chamber heads?

Answer 1

If you are referring to Big Block Chevy heads.
It is the size of the combustion chamber in the heads.
Open chamber heads have a larger combustion chamber. the valves are not shrouded, it helps the engine breath better. These are the standard since early 70s. They require the proper pistons to match the heads to get the right compression ratio.
If you used CC pistons with OC heads, low compression. If you use OC pistons with CC heads, there would be interference and cause major damage.
Closed chamber heads have a shrouded exhaust valve. They do not breath as well, especially at higher RPMs. These were the original design for the Mark IV BB engine that came out in 66. They were used on different lower performance engines and truck engines into the early 70s.
Hi Performance, Larger displacement engines used OC heads.
CC heads, usuall had a round and smaller intake port. OC heads had a rectangular and larger intake port.
All Aluminum heads were of the OC design.