Could someone explain compression ratios?

[RunCelica]

Could somewhat put it here simply what the benefeits of high and low compression ratios are.... and what each part of the compression ratio is ex: x : x : x or 8:5:1 what each number stands for on the parts of compression. Or a link would do cuz I've looked for and explanation through searches and couldnt fine one.

[teknophreak]

Compression ration is the ratio of cylinder volume between top dead center and bottom dead center. So for the GTS for example, the CR is 11.5:1, which means that there is 11.5 times as much volume in the cylinder at BDC as at TDC. Generally, the more you compress the fuel-air mixture in the cylinder, the more power you will get out of it when it ignites. This results in more HP but also higher forces exerted on the cylinder walls, pistons, rods, head gasket, etc. Which is why it is so difficult to run boost on a high-CR engine because each additional psi you pump in is mutiplied by the CR, which can result in large loads on the internals very quickly.

Hope that answered your question somewhat...

[RunCelica]

Thanks that helped a lot.

[monkeywrench]

The biggest problem is that compressing air increases its temperature. Hotter air makes the engine more likely to knock, and higher compression makes the air hotter. A turbo also tends to feed the engine hotter air, even with an intercooler, so knock gets to be a bigger problem.