open chambers mean the combustion chamber is basically the diameter of the cylinder bore and has a significantly larger volume per chamber
closed chambers have a chamber that is usually a little larger in perimeter than the valve seats. The rest of the head area in the cylinder is a flat roof.
closed chambers can be more efficient because that flat area lends to more controlled combustion (as opposed to using a domed piston in an open chamber head......the flat area in closed chambered heads (known as the "quench") contributes to higher comression without causing slower, less controlled combustion like many "pop-up" (domed) pistons do.....the domes tend to make the flame front "irregular" and are usually more likely to cause detonation than a quench area.
BTW, from what I've read, compression doesnt make but a few percent power difference with modern cams....back in the day when manufacturers didnt know as much about cams, high "static" compression did more for a strong, broad powerband........now days, carefully designed cams allow more power than back then AND use lower octane gas....
I'll stick with about 9:1 (unless I build a supercharged engine....then I'll go with 7:1)
