After reading the article on compression, we can take a look at what we can do with that compression.

The compression ratio is rather important for a engine and is not the same for all engines.

Like the word already says: it is a ratio we are talking about here.

When for instance the compressionratio is 9,2:1 like with my 4A-GE, This means that the air inside the cylinder will be compressed 9,2 times it's own volume.

This number is determined by the volume of the cylinder itself of course, but also by the lenght of the conrods, the stroke of the cranckshaft and the height of the pistons. All of this determines how much the top of the piston moves up, relative to the crackshaft to compress the air.

That means some of the performance of the engine is also determined by the compressionratio of the engine. Because the more compression it has, the more it can compress the air in the combustionchamber, which means a higher combustible mixture and a beter combustion! This means a higher power output and a more effective engine. 

Thats why tuners resort to high compression pistons and such. But milling down the cylinderhead or the cylinderblock, or even replacing the headgasket with a thinner one can already increase the compression of a engine. You actually decrease the size of the combustion chamber (Where the air is compressed in to) with these things.

It may sound as a pretty easy way to get some more hp's out of the engine. But every advantage, has it's disadvantage aswell.

You just can't keep increasing the compression of a engine and expect to get away with it.

When the air is compressed, it generates friction and friction generates heat.

The more the air is compressed, the hotter the air will get when it is fully compressed at a point just before it is ignited by the sparkplug.

When the compression is tó high, the air might get so hot that the fuelmixture will self detonate at a moment we don't want it!

This spontanious self combustion is also known as engine knock.

This can sometimes be heard, and it sounds like a metal sounding knock coming from the engine.

That sound is caused by multiple flamefronts that collide with each other. This can create multiple very hot pressurewaves that can seriously damage the internal components of the engine!

The temperatures can get so high that it can cause the pistons to melt!

And that is something you really don't want to happen with your very expensive high compression pistons!

But we can also use the compression to test the engine.

During its lifetime, there is going to be some wear and tear on the engine. A part of this wear goes into the pistons, pistonrings, cylinderface, valves and such as the continiously have to face the combustion in the cylinders. When these parts wear out, it can cause air to leak out of the cylinder. Not very good for the effectiveness of the engine.

You can check this by doing a compressiontest.

To do this, you first have to remove the sparkplugs and you place a compressiontester in the thread for the sparkplugs.

With a compressiontester, you can meassure the pressure inside the cylinder on the compressionstroke.

When this pressure is to much out of spec, it can have several causes. But you should probably dismantle the engine to take a better look at things.

This way you can at least have an idea of the state of the engine internally without having to dismantle it. But remember that this method will never give 100% certainty that nothing is wrong inside!