What is Quench

Google was my friend on this one...
While the terms “quench” and “squish” are often used interchangeably by many manufacturers, quench and squish are not the same thing, nor are they produced by the same set of conditions. The Society of Automotive Engineers (SAE) has defined squish as the gases trapped between the piston dome and head that are ejected across the combustion chamber at high speed by the near-collision of the piston dome and head, causing turbulence and mixture homogenization. For our purposes, if the squish area is too close, there is a pumping loss and if the area is too far apart there will be lower squish velocity and less turbulence.

Quench on the other hand, is the ability to lower temperature of the end gases trapped between the piston dome and head by conduction. This prevents a second flame front from igniting the air/fuel mix prematurely. Members of the SAE acknowledge that for motors with 3.5” to 4.5” cylinder bores, a quench distance of 0.035” to 0.040” work well and result in near zero clearance due to thermal expansion, rod stretch and piston rock-over.