All the stuff I've ran with the big timing has had the INDY CNC combustion chamber....Those #'s do NOT relate to any type of hand ported chamber as they can be any & all kind of shapes/sizes etc. It just seems the CNC chamber for whatever reason seems to make best power with alot of spark lead. There are alot of factors that go into what timing an engine likes, fuel, chamber design, piston design etc etc. But as generality they seem to like alot of timing, at least all ones I've ever dynoed have.