Squish: What it is and how to check it

Squish: What it is and how to check it

By High Output

This is another topic I have seen repeated inquiries about so maybe a little discussion would be helpful to some..........

"Squish" is a relatively narrow circumferential "strip" around the periphery of the combustion chamber that comes in close proximity to the outer piston edge at exact top dead center. On two strokes it is USUALLY of consistent width while on four strokes it is quite common for the width and symmetry to vary and it may be referred to as "Quench". The purpose in each case is the same.........

1) to reduce the size of the "pocket" or deeper centered area of the combustion chamber (thus reducing the required "flame travel" distance to consume the trapped fuel/air charge). Reduced flame travel distance is always a good thing as it reduces the time needed to burn the fuel/air mixture and reduces the likelihood for the more distant gases to be preheated and possibly spontaneously combust before the flame gets there (detonation).

2) to force the end gasses at the bore's edge into the "pocket" and impart turbulence which will promote a more homologous mixture of the fuel and air thus reducing fuel droplet size and exposing more fuel surface area to the upcoming spark event. This will improve the quantity of fuel/air mixture that is combusted with less waste and more pressure yield.

Generally speaking, a "tighter" squish clearance at top dead center is desirable because it reduces the quantity of fuel/air mixture that is stored away from the flame source. The tighter the squish, the more turbulence imparted and the higher the velocity of the mixture as it enters the pocket. But because you must have enough clearance to deal with slop in the main, big end and small end rod bearings, piston to cylinder wall clearance, piston expansion, etc. without allowing the parts to actually touch, there are limits as to how tight the squish clearance can be. (It also has been found that too high a value of squish velocity can encourage the onset of detonation, too.) This requires that a person be able to measure accurately his or her actual squish clearance to determine how their motor rates. It is incredibly simple to accomplish.......

You will need about a 10" length of solder of a diameter that is thicker than your current squish clearance. Common available diameters are approximately .065", .090" and about .140". Start with the .065" and if it is not thick enough because your squish clearance turns out to be greater, just go up to the next size. Bend the solder into an upside down "U" shape with the "legs" about 1/2" apart and parallel to eachother. Now cross the "legs" over eachother (in the shape of one of those ever popular politically correct symbolistic ribbons that we're instructed to wear by our fearless leader evrytime we have a national tragedy........ whoops! sorry! I'll stop the political innuendo....). You should now have an inverted "U" of solder with "legs" crossed in an "X" pattern. Remove spark plug from your engine. Bring the engine up to top dead center (follow the piston up the bore with another piece of solder stuck into the plug hole....... if it's a four stroke, make sure both valves are closed as you approach TDC. If not rotate the motor through another 360ø and they will be). Now back the motor up opposite normal direction of rotation until the piston has dropped about an inch back down the bore. Insert your two "legs" of solder into the plug hole while holding the solder by its' "U". Push the "legs" left and right across the piston dome INLINE WITH THE WRIST PIN AXIS. Do not push the "legs" fore and aft (front to back) in the bore or the potential for the piston to "tilt" on the wrist pin in its' cylinder wall clearance could give you false readings. You'll be able to "feel" the tips of the solder as they move left and right out to the cylinder wall limits. When you feel them touch the cylinder wall, stop pushing and hold what you've got with one hand while rotating the motor up to TDC with the other hand. If the solder is thicker than your squish clearance you will feel resistance as you reach TDC. Roll the engine through TDC and allow the piston to descend enough down the bore to easily retract the legs of solder back out the plug hole. Keep track of which tip was at which side of the bore so you'll know which is which if there's a difference in squish clearance from one side of the bore to the other. Differences in clearance from one side to the other are common and can be caused by a "tilted" squish band in the head, "tilted" piston crown, etc.. Now measure the compressed tips of the solder and record them. This is your actual squish clearance at TDC.

Optimum squish clearance is decided by squish angle vs. piston crown radius(and how close they are to paralleling eachother), squish width and area as a percentage of bore area, cylinder bore size, piston manufactured style (cast or forged, which dictates expansion and required piston to cylinder wall clearance), uncorrected compression ratio and most importantly dynamometer testing. These are all subjects that will require addressing on another day. Hope this helped and made some sense. Have a great day!