When it comes to knock, compression is only part of the equation. Heat and flame travel/combustion efficiency also play a big part. Amuminum heads run much cooler than iron and that allows them to remain "knock free" at higher compression ratios than iron can. Getting good turbulence in the chamber helps in several ways: it helps keep a nice homogenous mixture and the circulation helps prevent isolated hot spots. Both of these guard against getting multiple ignition sources with resultant colliding flame fronts leading to knock. The hotter things get in the chambers the more likely you are to get detonation and also soften up the exhaust seats so the action of the valves can pound them into the heads. Good chamber turbulence mostly comes from having a good tight quench pad area: where the piston top comes very close to the flat part of the head as it approaches TDC creating a pressure wave that spreads across the whole chamber keeping things nice and "shook up". Fatter gaskets hurt this - a lot - especially on a closed chamber head like the 670 for example. Couple that with the fact that closed chamber heads tend to need more ignition advance because they already aren't as combustion-efficient as open chamber heads are, and you can get into trouble really quick. A D-shaped piston dish gets you the addtiional chamber volume you need to drop compression ratio without sacrificing the quench pad area.
If you lower compression with fat gaskets and in the process sacrifice "too much" quench, it's possible to find yourself in a situation where the motor is still detonating as much (if not more) than it was before.
Last edited by BearGFR; 07-20-2011 at 10:44 PM.