Promoting the maximum possible turbulence in the air/fuel charge is critical for efficient combustion. In more modern engines this is usually done by shaping the inlet ports to encourage swirl and tumble as the charge enters the cylinder. In our elderly Holden sixes though, we are more reliant on the combustion chambers squish pad to stir things up. There are useful power gains to be had by optimising the chamber shape - specifically, at TDC we want to reduce the clearance between the piston and the squish/quench pad to the bare minimum. This is the flat area of the head alongside the chamber bowl, and it's primary role is to induce turbulence in the chamber as the piston approaches TDC. It does this by "squishing" the mixture out of the confined space into the bowl area, but there is a downside to this as well. Because the mixture in this area is in such close proximity to the relatively cool metal of the head and piston, it doesn't burn along with the rest of the cylinder charge when the piston is at or near TDC. It either doesn't burn at all, burns only partially or only burns when the piston is significantly past TDC and it is too late to derive any usable work from it.
How much power can be gained? Lets look at an example engine, a 202 with 11:1 compression that makes about 220bhp. Lets assume the quench area covers about a third of the head area, and we have a piston to head clearance of 0.070". In this example the volume of gas in the quench area at TDC works out to about 12cc, or roughly 22% of the total chamber volume. In other words, about a fifth of the mixture in the chamber won't be producing any useful power. Lets say we reduce the clearance to about 0.35", so that we now have only about 11% of our available mixture in the quench area. In theory at least, we have just picked up over 20hp. In the real world of course the increase mightn't be so dramatic, but at any rate there are good gains to be made for very little effort.
Remember that the piston crown is part of the chamber, and any dish in the crown will have to be taken into account. Wherever possible, it will pay to use a flat top piston, or at least try to limit the dished area to that part of the crown that matches the bowl area of the head. How tight should the piston to quench pad clearance be? Basically you want to run as close as possible without actually having the piston smack the head at high revs. And that depends on how much piston rock there is at TDC, bearing clearances, thermal expansion of the piston and so on. You should be safe at 0.040", but I'd be nervous about anything less than 0.30". You can juggle the clearance by altering piston height, deck height or gasket thickness. Obviously, if there is any sign of piston/head contact you'd want to get a thicker gasket in there pretty quickly.
This has a major impact on flow so it's important to get it
right. Both the 9 and 12 port heads can use a simililar
Use 5 angles for the inlet, a 70 deg cut to blend into the throat, a 60 deg bottom cut, 45 deg seat, 30 deg top cut & 15 deg chamber blend with a 1/4" radius. Cut the chamber relief .125" out from the edge of the valve (provided that doesn't take you out to the gasket with a bigger valve). On the intake valve a 30 deg backcut will help low lift flow.
On the exhaust use 3 angles. Make about a 3/8" radius off the seat for the bottom cut, 45 deg seat, 30 deg top cut, 15 deg blending/relieving cut with about a 1/4" radius. Like the intake, relieve about 1/8" out from edge of valve if possible.