Performance Racing Kawasaki Triples since 1978
690 Polaris Blvd. SE
Rio Rancho, New Mexico
(505) 410-8064 (mobile)
What are "Squish Heads" or
what is a "Squish Band"
Basically the name "squish" is just a
description of having the fuel mixture "squished" towards the center of
the combustion chamber. There are a couple of reasons that you want this
to happen. First it causes turbulence which makes for a more combustible
mixture, but more so, to help prevent detonation.
But before we get into the mechanics,
I want to define pre-ignition and detonation. They get interchanged a
lot, but are two totally different things.
Pre-ignition is the fuel mixture
igniting from a "hot spot" in the combustion chamber, the two usual
causes are built up carbon deposits and the sparkplug being too hot of a
Detonation is the fuel mixture being
ignited by the heat and pressure in the combustion chamber, usually at
the outer edge of the piston. When that combustion "event" collides with
the ignited fuel from the sparkplug you get the "marbles in a can sound"
from the motor. Detonation is very hard on the bearings as the loads on
the bearings are greatly increased. Also the action of the collision of
flame fronts, strips off the thin boundary layer of mixture (about .020"
thick) that actually protects the metal parts from the intense heat.
Signs of detonation are the aluminum parts are pitted, and usually metal
flakes on the spark plug. In bad cases a hole will actually be burned
through the top of the piston.
And now to "squish" bands.
To decrease the chance of
Detonation, what is needed is to "push" the fuel mixture away from the
outer edges of the piston and also use the boundary layer to our
The squish angle is the degree of
angle difference between the angle of the piston dome and the angle of
the combustion chamber. If a piston dome angle is 13 degrees, then the
head angle can be from 14 to about 17 degrees depending on what the
motor usage is. The smaller the angle the faster the MSV (mean squish
Next there is squish width, which is
set (basically) by the percent of the area of the combustion chamber.
This can be anywhere from 20% to 70% again, depending on the motors
usage. But usually ending up around 40 to 50%. The higher the motor is
loaded, the wider the squish width.
Finally the squish clearance. This is
the clearance between the piston dome and the head. Now we are back to
the boundary layer. If we reduce the squish clearance to around .040"
the cool boundary layers on the combustion chamber and piston dome
intersects creating a cool area around the edge of the piston preventing
heat from igniting the mixture, thus preventing detonation. The
clearance is dependant on a few factors, including "stretch" of the rod
at higher rpm's.