The hemi refers, IMO, to a bowl shaped chamber with no squish band at the edges. Anything with a squish band should be referred to as a squish band head. This nomenclature gets really confusing, because merely cutting the inside corner off the squish band and rounding it can be called a hemi conversion. Other common head types are a wedge, like a stunt fox, seen with baffle pistons, a trumpet, like a Cox tee dee, and a double bubble, a squish band with a first bowl inside the band, and a second deeper bowl inside the first. For our purposes, squish band and hemi are what is most common. The hemi shape seems to give a slower burn, and a smoother break. As originally configured, the squish band head was used for racing. The idea was to get the deck height small enough to make the piston come within 10 thou or so of the head, and force all of the mixture to rush to the center bowl at the instant of ignition. This generated extreme turbulence and rapid flame travel, which made the combustion rapid and the break somewhat harsh. Stunt engines typically have more deck height, say 15 to 20 thou, but the harsh quality of the break seems to persist as log as the squish band is left square. Tom H.