Squish Band & Deck Height Calculator

Definitions:

The Squish Band or "Quench" is defined as that area between the flat of the piston and the flat of the cylinder head at top dead center (TDC). On the compression stroke, as the piston approaches TDC, the compressed mixture of fuel and air is "squished" to the remaining space of the combustion chamber where the spark plug and valves reside. The "squeezing" of the mixture creates turbulence and is expected to promote a better and more complete combustion.

Typical figures for this measurement are in the range of .040" to.045" which allows for rod stretch, carbon build-up and other variables. RB Racing's Harley Big Twin ORCA motors are designed with .035" squish to accelerate the turbulence and to further concentrate the mixture in the remaining combustion chamber while leaving a small margin for carbon build-up. Pure race engines with short duration applications may reduce this figure to .025" as some builders aren't happy till the pistons "kiss" the cylinder head. At .025" when you factor in high rpm rod stretch and piston "rock" at TDC you effectively reduce squish to zero.

It's a simple fact: The closer the flat of the piston to the flat of the combustion chamber, the more power you will make. In a race engine with all the factors considered you practically want the two surfaces touching one another. In F1 they machine each piston individually even for the spark plug relief.

For those of you who have gone backward in time, seduced by bathtub "hemispherical" combustion chambers, this calculator won't do you much good as their "squish is defined by the peripheral intersect of the spherical piston dome and the dome of the combustion chamber...There is no "flat" area of the head and piston to measure. Whereas we all love "Hemis" and all this word stands for, from Richard Petty and the Superbirds to all the current supercharged Rodeck, Keith Black, Ed Pink, and Arias hemis, it's not the chamber design we see in modern pushrod engines like in NASCAR. Swirl, turbulence, squish, quench, smaller combustion chambers, and higher compression ratios with narrower valve angles are the norm these days...not whacking big relief pockets into domed pistons. This is the era where we want minimum surface area on our piston tops and where we wish to direct our "burn" to the exhaust valve, not to the far reaches of the combustion chamber. Hemi designs compromise the combustion process.

Deck Height: The distance between the flat "quench" surface of the piston and the top of the bare cylinder (no head gasket). Typically an engine is set to zero deck height with the head gasket (compressed value) defining the "squish band" value. If the value is negative this indicates the piston's "quench" surface extends above the top of the bare cylinder.

This calculator allows you to define the various "variables" under your control like cylinder length, gaskets, and piston pin to crown dimensions. It is imperative that you physically check all dimensions as there are always variances to deal with.