Squish in Kart Engines, What You Need to Know (and What You're Getting Wrong) - Part 3 of 3

Mechanical contact between the piston and the engine head is the most obvious limit, but this is not the parameter that establishes the minimum usable squish value. Long before physical interference occurs, other, more critical limits come into play, with direct effects on engine operation. The first is detonation: reducing the squish height increases pressure and temperature in the combustion chamber, and consequently the safety margin in terms of reliability rapidly diminishes. In many situations, it is precisely this phenomenon that dictates the operating limit, well before any contact between components. Detonation, in fact, can cause serious damage to the piston crown, sometimes so quickly as to compromise the component even in the space of a single driving session. Another limit is determined by dynamic conditions. When the engine is not "running," the squish has one measurement, while when it is in operation, its size varies due to the piston moving closer to the engine head as a result of expansion and deformations "under load," i.e., those that occur at high temperatures and high revs. The actual squish measurement of a running engine will therefore always be lower than that measured when cold, and this could lead to physical contact between the piston crown and the cylinder head dome under dynamic conditions, even if this does not occur when the engine is off. This is why particular caution and care must be taken when setting an extremely low squish measurement when cold. Another aspect to consider before worrying about possible contact between the piston and the cylinder head concerns combustion: increasing the squish values ​​to extremes can make it excessively aggressive or difficult to manage, with negative effects on engine smoothness.