Squish

What is the magic Squish Clearance? Squish clearance is the distance that remains between the outer perimeter of the cylinder head and the piston when the piston is at Top Dead Centre (TDC). The use of a piece of solder is a common method of measurement.

The term “squish” is actually a general term for the mixing through turbulence of fuel and air within the combustion chamber. Any engine requires a specific amount of fuel and air for the burning process to take place. The fuel and air wants to be at a ratio in the region of 13 parts air to 1 part fuel. Ideally each particle of fuel requires 13 parts air to surround it and for all the particles to be distributed evenly throughout the combustion chamber for the charge mixture to burn efficiently.

Altering the squish clearance is one method of manipulating the amount of turbulence within the combustion chamber. The degree of turbulence is also referred to as squish velocity.

Engines are designed to give a certain level of performance. For example an engine with high durations of port timings is designed to give higher RPM performance. An engine such as this would inevitably have a short exhaust pipe and other parameters to suit those characteristics. The squish clearance should also be matched to the character of the engine. To give an engine like this a tight squish clearance would effectively be de-tuning the engine to be more favourable to lower the RPM’s at which it makes its best power.

The following also have to be taken into account as all will help determine the level of tune and what squish clearance would best suit the particular design of the engine.

Compression
Engine speed
Stroke length
Exhaust port timing
Combustion chamber design

Altering the Squish clearance compliments those design parameters, stroke length etc and gives us an easy method by which we can alter the engine speed at which MSV occurs. By altering the MSV we can change the power characteristics to some extent. If we increase squish clearance we would raise the point at which MSV occurs and vice versa thus altering the RPM’s at which the engine makes its power. The easiest way to think of it is to remember that a wide squish clearance favours power later in the rev range than a tight clearance.

So to answer the question “what is the magic squish clearance? There isn’t one, it is what ever the rider prefers and what suits the particular design parameters of the engine!