Squish Band Explained

[Jack Klarich]

A squish band is where a piston and head surface come in close proximity of each other at TDC. There are several types of squish bands. Some heads have an angled surface machined or cast into the inside perimeter of the head gasket surface. The piston has a matching area. In the case of a stock twin cam, the head has a bathtub shaped combustion chamber. The flat sides of the chamber extending from the head gasket surface make up the squish area. As the piston (flat top for a stock twin cam) reaches TDC, the AF mixture that was in this area gets forced out. The amount of squish effects how violently it is forced out of that area. When it is forced out of the squish area and into the combustion chamber the AF mixture creates turbulence in the combustion chamber and helps promote a more complete and quicker burn as well as reduce chances for detonation. The squish is determined by 2 things in a stock twin cam configuration, the deck height and the head gasket thickness. For example, if the stock deck height is .010" down in the hole then the piston is .010" below the head gasket surface of the cylinder. If the head gasket is stock it measures about .045". Add those 2 numbers together and you have your squish, in this case .055". Now if the deck height is set so the piston is .005" down in the hole and a .030" head gasket it used, the squish is now .035", a more desirable number.

 

[FishHunter]

I will add that the closer you can put the piston to the cylinder head w/o contact during normal operating temps, the less chance of detonation one will have, and also will create a more turbulent combustion chamber.
If working with an engine that is so equipped, spark plug gap needs to be snugged up, particularly if the ccp is at/near 195-200 psi.
This can be witnessed while up on a dyno.