If we split this phrase apart and look in the dictionary we will come across:
Compression – as an adjective means to squeeze.
Ratio – as a noun means a proportion of two things.
When talking about engines we classify two kinds of compression ratios (CR): static and dynamic.
Static CR
The static compression ratio is defined as the volume of the combustion chamber when the piston is in the bottom dead center (BDC) – very bottom of it’s journey – divided by the volume of the combustion chamber in it’s top dead center (TDC) – very top of it’s travel.
Static compression ratio is one factor that influences how totally the air-fuel mixture is burned, once it has been lit by the spark. If you burn all of the air-fuel mixture you get more hp. If there is some leftover unburnt air-fuel mix after the spark has been lit, you have not gotten all the power you can make out of the mix. This fullness of burn is named thermodynamic efficiency.
Improving thermodynamic efficiency is one of the 3 main power-gaining techniques existing for engine builders. Though, the problem is that as you increase CR, you increase cylinder pressure and temperature inside the combustion chamber. As air is squeezed hard inside a closed container like a cylinder, the pressure in goes up the harder you squeeze. As pressure goes up, so does temperature. This can initiate a process named self-detonation – the air-fuel mixture ignites by itself without the spark.
Secondly, as you rise static CR more and more, the cylinder pressures grow progressively. The piston must work a lot harder to squeeze identical amount of air-fuel mixture delivered into the chamber by reason of this higher pressure. This negative work slows the piston speed momentum which influences the power you make.
So you can produce additional power by improving burn efficiency via growing the static CR up to a point. For street engines, the maximum static CR on pump gas is close to 12.5:1 CR if you know how to tune. If you don’t, the maximum is close to 11.5:1 CR. For a race engine, the spot at which cranking pressure causes negative work and affects power production is approximately 14:1 CR.
Dynamic CR
The piston is continuously moving up and down however the intake valve opens and closes during this time also.
When the piston is starting to squeeze at BDC, the intake valve is starting to close. The intake valve is not wholly shut until the piston is almost at TDC. There is a link between the cylinder combustion chamber and the intake port/intake manifold runner, when the intake valve is still partially open. As the piston is squeezing and moving toward TDC, some cylinder pressure can loose pressure up into the intake port which reduces whole cylinder pressure.
If you use your adjustable intake cam gear to close the intake valve earlier, the amount of cylinder pressure bleeding up the intake port is reduced. The cylinder pressure builds up sooner and you get a better burn.
If you let the intake valve close later, more cylinder pressure will bleed out or be decreased and the burn will be less complete.
This is why it is key to rise your static CR when you get very longer duration cams.
You desire a fast, complete burn of the air-fuel mix to make power
