I asked this on another thread but never got a response (I think I killed the thread
) What is "squish"?
Short answer first.
In this instance it's a bit of a misnomer, what we are actually talking about is piston to head clearance, or more precisely, piston to
squish band clearance.
The squish band is the area of the head nearest the cylinder wall that closely follows the piston crown shape, having minimal clearance between the two. The combustion chamber is then set somewhere in the middle of this band. The chamber can either be centralised or offset, depending on what the design engineer is tying to achieve, but that band around the outside of it is still a squish band, even though it may vary in size.
In simple terms, the narrower that gap, the more efficient your combustion process will be and it dramatically reduces the chances of detonation occurring (very unlikely in a saw anyway) as the end gasses have been 'squished' back into the chamber, not lingering out near the cylinder wall where they may self ignite, a long way from where ignition is supposed to start occurring.
The reason for the gain in efficiency is that you are squeezing the maximum amount of charge (fuel/air mixture) into the combustion chamber where it can be ignited by the plug.
If the squish clearance is too great, anything up to 10% of your charge will be wasted, igniting far too late to get any real work out of it as it's so far from the plug and chamber proper.
This squish clearance can also be too little, leading to piston and squish band hitting as things stretch under the extreme forces or even through just a bit of carbon buildup.