Squish band in a combustion chamber


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There are quite a few things that can be tweaked in the combustion chamber to change performance (notice the word "change"..... not necessarily "better" or "worse".... everything is inevitably a compromise).

 - cc's
 - squish (usually measured at the corner)
 - squish angle (relative to the piston, of course)
 - dome shape
 - blending radius between the squish and the dome
 - width of the squish band

Please note: the dimensions on this drawing are not intended to be taken as literal. I merely drew an approximation of a Yamaha combustions chamber to illustrate the various terms associated with squish bands.

Note the area that is shaded black. This is the "squish band".

The effective squish angle in this drawing is probably a few degrees (this would be considered tapered squish).

Some pistons (like a Yamaha) have a radius on the crown (as in this drawing), while others are machined conical (ie: the crown would appear as angled straight lines, with a small flat area at the top).




*Generally* anything you do to increase combustion chamber turbulence seems to increase bottom-end/midrange (with a corresponding tendency to heat more there). Remember: heat is power is heat .......... no way around it !!!

Things that increase turbulence:

 - reduce squish
 - reduce squish included angle
 - reduce blending radius between squish and dome
 - wider squish band / smaller dia-deeper dome


Of the three things mentioned above, the most overlooked is probably the blending radius. When the squish gets "down" to under .030, this becomes fairly sensitive! Since our hands are tied a bit [here in the USA] at 11cc's (as far as spec piston port motors are concerned), there's only so much that can be done with most of this.

Gasoline motors tend to "like" a couple of degrees of squish angle (IMO). Running parallel squish on gasoline can be asking for trouble, especially on a highly-tuned engine. On a restricted-exhaust engine (can), it's much easier and more common to run parallel squish.

Road race motors tend to "like" a bit more open combustions chamber, ie: narrower squish band, bigger dome, decent size radius between the squish and the dome.

All these things go HAND in HAND with: pipe, fuel, timing, gear, the driver (in the seat<g>), etc, etc.