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What Is “The Squish” In Engines?

Many racers overlook what happens inside a combustion engine. For the most part, people understand that fuel and air are mixed, which causes a mini ‘explosion’ that is then, in turn, used to power the wheels. But what is “the squish” in engines, and why is it important?

The squish in an engine refers to the air that becomes trapped when a piston is in motion. The squish zone refers to the 1mm gap where the piston stops. This is where the air is pushed out at a high rate, causing turbulence in the combustion chamber.

It is important to know how this works, as it can affect the performance of your engine. It is critical that you know how to measure and adjust the squish in your engine if you want to get the most out of it, and below we’ll give you some tips to do just that.

What Is The Squish?

Not all engines are built with the squish effect in mind. You will mostly find the squish effect in side valve, overhead valve and overhead camshaft engines. Overhead valve engines can be found in GM SUV and pickup truck models. However, they have also been used in performance vehicles such as the Chevrolet Corvette and the Dodge Challenger.

Overhead camshaft engines are also quite common as they can provide a car with more power and more speed. These engines can be found in vehicles such as the Ford Mustang, Alfa Romeos, and many other modern cars.

Position Of The Pistons

Engines have something called a Top Dead Center, and a Bottom Dead Center. This is essentially just the position of the piston. In other words, Top Dead Center means that the piston is at the top of its stroke, and vice vera.

When the piston reaches Top Dead Center, the piston crown comes extremely close to the piston head. Normally there is a gap of 1 mm or even less between the piston crown and the piston head. When this happens, the gasses (an air and fuel mixture) become trapped, and ‘squished’ inside this small gap.

An Essential Process

Needing somewhere to escape to, the gasses are suddenly released into the combustion chamber. This creates turbulence which then causes a more thorough mixing between the air and the fuel. This is even better for the combustion process and more power can be extracted from the engine.

Engine squish can be found in many different types of internal combustion engines, including 2-stroke and 4-stroke engines. Engine squish also happens with different fuel types, including diesel and petrol.

Why Is It Important?

The squish is crucial for engine performance and reliability. Having the right amount of squish means that the engine is capable of using all of its power while running as smoothly as possible. It’s therefore very important to set it to the correct amount.

Lower squish means that the piston crown comes closer to the piston head. This means there will be more turbulence since the gasses are put under more pressure and they will be squished out more violently.

The opposite is also true, and higher squish means that the piston crown stops its motion further away from the piston head. This means that there is more space for the gasses that are trapped, and a less violent squish will happen, resulting in less turbulence.

Lower Squish Is Better

In other words, the lower the squish, the more turbulent the air is that is released into the combustion chamber. Higher air turbulence is equal to higher performance from the engine because the fuel and air are better mixed and there is overall better combustion.

On the other hand, if the squish is too low, you could have a situation where the piston crown will make contact with the piston head. This can cause the engine to overheat and ultimately fail. If the squish is too high, you will be compromising your combustion of the fuel mixture, causing a massive loss of performance.

Clearly it is a very delicate window of operation, with mere millimeters separating optimum performance from disaster. It’s advised that you seek the advice of a mechanic or an engine professional if you want to adjust the squish of your engine.

Squish In Modern Cars

Most modern ‘average’ engines are built with a lot of precision, but they will be built with a higher level of squish. The higher levels of squish might not bring out the best of the car’s engine performance, however it will reduce the room for error and decrease reliability issues such as overheating and engine failure.

On the other hand, performance engines are built with much lower squish. This would be the difference between a standard Mercedes-Benz engine and an AMG edition engine for example. This allows them to squeeze a little bit more power out of the engine.

However, it could also bring with it some reliability issues. High performance engines require much more maintenance, and they could suffer from overheating and engine failures because of their lower squish.

How Do You Adjust Squish In An Engine?

Adjusting the squish of your engine is an extremely precise and delicate process. A simple tenth of a millimeter can mean the difference between optimum engine performance and complete engine failure.

If you’re planning on adjusting the squish of your engine, you do so at your own risk. It is highly recommended to get help from someone who knows how the engines work and how to make these adjustments.

Method 1

There are two main ways to adjust the squish of your engine. The first of these is easier to do than the other, and if you are a DIY mechanic, we recommend you use the first method. Remember, before you make adjustments, first measure the squish of your engine and make sure there is some room for adjustment.

The first method is to change the cylinder base gasket to a thinner one. This will actually bring the piston head lower as opposed to bringing the piston crown closer to the piston head. However, it has the same effect at the end of the day.

Method 2

The second method to lower the squish of your engine is to replace your cylinder with an aftermarket cylinder. These come in different sizes, and you will need to find the correct one for your vehicle.

This process is much more difficult and there is a lot more room for error. It is highly recommended to let an engine specialist do this for you instead of doing it yourself. Using a larger aftermarket piston will lower your squish and using a smaller piston will increase the squish.

Final Thoughts

Squish is an element of the combustion engine that many people overlook when it comes to performance. However, optimizing it can be an easy and cheap way to maximize your engine performance. Remember that lower squish equals more turbulence and higher performance, and higher squish equals less turbulence and lower performance.