What sounds like mild disagreement here would soon disappear if you guys were all hanging out around the same bench, pointing at the same innards...
There is a bit more to BWalker's story about the purpose of the squish band, however. It all dates back to the 1920's and a fellow named Recardo who was doing research into understandin preignition and controlling detonation in engines...at the time there was almost no understanding of the difference between the two. The engines of those days, in combination with the fuels, were only able to use about
4:1 compression ratios without serious consequences.
Preignition is too far advanced ignition, caused by advanced timing, glowing deposits, etc. Preignition causes spark knock when all the fuel is ignited too soon, causing a pressure spike too far before TDC. Preignition also drives too much heat into the piston, cylinder head, and other engine parts, and thus can cause damage, even if knock isn't experienced.
Detonation is caused by the advancing flame front (and attendant pressure rise) trapping pockets of unburned fuel such that the fuel ignites explosively from the pressure...the fuel in the trapped pocket does not burn progressively but all-at-once. A look at a cylinder pressure graph will show an angled line up to a point where it turns into a near-vertical spike; the pressure rises to unsafe levels in literally no time at all, and the total pressure level reaches very high levels. The effect is almost like the piston crown being struck very hard by a large hammer.
Preignition is a common cause of detonation, of course, but Recardo learned that combustion chamber design has a great deal to do with controlling detonation. The molecules in the fuel/air mix that are in contact with the relatively cool combustion chamber surfaces almost never combust, and when they do, do so inefficienlty, forming CO, etc. AS the flame front in the burning mix reaches the boundary areas of the combustion chamber, the flame is quenched even though there are still a few unburned molecules; also, as the cooler molecules are squeezed out of the QUENCH areas and mixed with the burning ones in the mix, flame propogation is slowed at the critical point in the burning sequence, as well as eliminating small, dense pockets of fuel/air mix that can detonate. For this reason, some motorheads call the squish area of a cylinder head the "QUENCH"
When fuels resistant to detonation are combined with well-designed combustion chambers, ignition can be timed optimally, pressure levels rise predictably, and motorheads are happy.
