Basic theory of engine compression

In this article, we'll try to explain the basics of compression in 4-stroke engines. We've always heard that higher compression means more power in the same engine. Let's see if this statement is true.

What is compression?
The compression ratio (CR) is a measurement obtained by dividing the volume of the combustion chamber (and cylinder) with the piston at bottom dead center (BDC) by the volume of the chamber with the piston at top dead center (TDC). In other words, it is the ratio of the air compressed each time the piston moves up.

For example, if with the piston down (BDC) we have a volume of 10cc and when the piston is up (TDC) we have 1cc of volume in the chamber, then we can say that the air has been compressed in a 10:1 ratio, this would be the CR of our engine.
Just for the record, we refer specifically to "mechanical compression" in this article. There are also other types of compression measurements (corrected, static, effective, cranking, dynamic, etc.). Different camshafts also greatly influence the compression ratio.

Advantages and disadvantages of increased compression
The internal combustion engine is an air pump. We try to introduce as much air as possible, then compress it and finally "ignite" it with the help of a little gasoline. The rapid expansion of combustion will cause a downward force on the piston, which is responsible for the movement of the crankshaft and directly related to engine power.
For the same amount of air, the more compressed it is in the combustion chamber, the more energy it will release upon explosion, and at the same time, the greater initial compression will favor the ignition of the air/gasoline mixture, all of which results in more efficient combustion.
This will result in greater mechanical stress on the moving parts and an increase in operating temperature, so we must always seek the ideal balance between increasing the compression ratio (CR) and the lifespan of engine components.

Compress your engine
It should be noted that the original engines of cruiser motorcycles generally come with a significant reduction in compression, as they must perform well in all types of markets (gasoline quality can differ between countries) and under all types of conditions.
If we have access to quality fuel such as 98 octane or higher or use octane boosters, we can increase our engine's efficiency by slightly raising the compression ratio.

Because mechanical clearances will be reduced, there is no need to overexert the engine under these conditions by letting it warm up before accelerating aggressively. This way, the metal in the components will have expanded enough to maintain the recommended values and prevent mechanical failures.
It is also important to know that increasing compression must be done on a completely "healthy" engine; there is no point in trying to correct a defect or power failure by increasing compression; the initial problem must be solved first.

Different ways to achieve the same result
There are several ways to slightly increase the compression of a stock engine, we'll list some of the most commonly used ones.

• Thinner cylinder head gaskets: Some manufacturers develop special cylinder head gaskets that slightly reduce the combustion chamber size, increasing internal pressure.

• Machining the cylinder or cylinder head: Some of these components are reduced by a few thousandths, resulting in a smaller combustion chamber. For experts only.

• Special cylinder heads: Some manufacturers offer cylinder heads with combustion chambers designed to optimize combustion and increase compression.

• Big Bore Kits : When we install larger cylinders and pistons, we're increasing compression, since we initially have more air in the same combustion chamber. These are preferred because, aside from the improvements from increased compression, nothing beats an increase in displacement for more torque and power.

• Taller pistons: Several manufacturers offer pistons with a higher crown, resulting in greater mixture compression at TDC.

The assembly is just the beginning

Once we've selected the method to increase compression and assembled the components, the tuning process remains. Since the engine's internal operating conditions related to air and fuel have changed, it's necessary to readjust the ignition and carburetion/injection correctly.
When we've completed this step, we'll have a happier engine that will accelerate brilliantly and be more efficient.

Frank Burguera

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