But if you want to use a true (measured from exhaust-closing) compression ratio much over 6.5:1, on a high-output engine, combustion control beyond that afforded by a non-squish cylinder head will be necessary. Considerable variation is possible, but a
good rule to follow is to make the cylinder head's squish band about 50-percent of the cylinder bore area. For example, in a 3-inch bore -which has a total area of 7.07-inches2 the squish band would be wide enough to represent an area of just about 3.5 in2. Assuming that you have centered the combustion chamber proper on the bore axis, then your squish band would be a ring having the same outer diameter as the bore, and an inner diameter of just over 2-inches. The combustion chamber itself, to meet the previously-stated minimum surface/volume requirement, would again be a spherical segment - with a radius that provides the total volume, added with that from the clearance space between piston and squish band, to give the desired compression ratio.
