an open chamber is a more-or-less smooth bowl the full diameter of the cylinders. a closed chamber can be any shape that doesn't cover the full cylinder diameter. this leaves 'squish' area where the piston closely approaches the head.
Aha! OK these are closed heads. But this was a turbo motor of likely-insane boosts, so I imagine customary rules of thumb don't apply (though physics still does :-)
Can anyone guess what sort of boost pressures might be required to get 700 hp from 182 cu in? Two TE06's with 1.54 A/R. It's 3.85