The squish band has been a very important technical solution for two-stroke engines. It has increased performance, reliability and combustion efficiency (also reducing fuel consumption and pollution). When tuning an engine we can surely start with a good check of the squish band.
Types of squish
Squish bands can be of different shapes, like oval or circular, can vary in the width of the band and in the inclination of that same band respect to a horizontal line. Finally the minimum distance of the squish band from the piston crown is also an important parameter that can be varied. The first parameters are generally fixed by the engine builder, except machining the cylinder head, which is something quite complicated and that can lead to irreversible errors. Usually it is better to have different cylinder heads proposed by the engine builder. On the other hand the minimum height of the squish band can be varied easily, even though measures must be extremely precise.
Measuring squish height
Take your engine as it is, with the cylinder head well tightened, as indicated by the engine builder, to the cylinder. Take off the spark plug and start measuring the minimum height of the squish band. To do so take a tin wire of about 1,5 mm of diameter and no less then 1,2 mm. Bend the wire and insert it inside the hole in the cylinder head where the plug is usually positioned. Make sure that the end of the string is touching the internal part of the cylinder and has not entered inside any port. Some take measures at the exhaust port side, some laterally, on the lateral transfer ports side. I have always preferred the exhaust port since to me it is always the more critical area. Anyway take measures always on the same side so you will be able to vary the squish band measuring always the same parameter.
When the wire is inserted properly, turn the crank-shaft so the piston goes upwards to the top dead corner, the wire is squeezed between the cylinder head and the piston crown. Take the wire out and you will find a small edge (thicker part) on the extreme end of the wire, that is determined by the wire going in the open zone between the piston ring and the piston crown extreme external edge. After such thicker part you will see that the wire has its narrowest zone, that is what you must measure.
Take a calliper, possibly an electronic one, that will give you measures up to a hundredth of a millimetre. Use the narrow area of the calliper to make your measure. Repeat such procedure two or three times to check that values are all the same. Be careful non to squeeze the wire when measuring with the calliper. Also repeat the whole procedure by cutting the wire on the edge eliminating the already squeezed area and squeezing a new part once again.
Values of the squish and effects
Values of the squish vary from engine to engine and can give important effects to engine performance. Always check with the engine builder the indicated values from which to start. Acceptable values can go from 0.70 mm to 1.0 mm. Values over or under such range can determine reliability problems and limits in performance. For example a value superior to 1.0 mm can reduce the squish band effect and result in bad combustion and insufficient performance. Reducing the squish height to less then 0.7 mm can on the other hand determine over-heating of the engine and hitting of the piston crown on the cylinder head.
In fact when the engine is running, especially at very high revs, the deformation of the con-rod, the crank-shaft and crank-case, and of course the small plays of the bearing, especially the crank-case ones, can reduce the 0.7 mm to less then 0 mm. The hitting of the piston crown on the cylinder head will block the piston ring and engine seizure will be extremely possible. If not so, surely engine performance will be greatly reduced since the piston ring will not be able to work properly and seal the combustion chamber from the crank-shaft. Usually interesting values of the squish are around 0.7 – 0.85 mm for 100 cc kart engines, and around 1.0 mm for 125 gearbox kart engines.
Varying the squish value
To vary the squish value we must act on some copper circular gaskets present between the cylinder and the cylinder head. Such gaskets are generally of 0.1 or 0.2 mm of thickness, but for really precise regulation you should find 0.05 mm gaskets. So after a first measure of the squish band that is for example 0.9 mm, you can open the engine and take away a 0.1 mm gasket to try to have a 0.8 mm squish band. Always measure the squish band again after having taken away or added any copper gasket, since the final value of the squish could eventually be different from what you expect by simple calculations.
Also you could take away another 0.05 mm gasket if you were looking for a final 0.75 mm squish value. To ensure good measures always try to tighten the cylinder head onto the cylinder with the same strength, which means the same couple screwing the head nuts. Badly tightened heads of cylinders can determine great variations of the squish value and bad performance or reliability of the engine. Be careful that a ruined cylinder head, with a spoiled squish band, can give wrong indications for squish measures.
A value and effect
Values of the squish closer to low measures determine better performance especially at low and mid revs, whilst higher values help at high revs. Also low values determine higher engine temperatures also because reducing squish band also acts as an increase in compression ratio. Higher temperatures of course always means more critical functioning and less reliability of the engine.
Also the effect of smaller values of squish height is that combustion works better because fuel is mixed better with air, so more gasoline is burned, better performance is obtained and, as already said, temperatures increase. This also asks for a richer mixture when regulating carburetion compared to a an engine working with higher squish values.