[serialk11r]

Higher compression ratio is more complicated than it seems. First of all in the ideal thermodynamic cycle one assumes constant heat ratio, but the more you compress the intake air the hotter it gets and the more the heat ratio drops. This is why in theory it's useless to increase compression ratio on spark ignition engines past a certain point; the temperatures get too high and the heat ratio drops.

Now for the practical side: You have to remember that higher pressures increase friction (particularly, at the rings). Additionally, reducing combustion chamber volume can cause a decrease in burn efficiency, and the higher burn temperatures result in greater rejection of heat to the cylinder walls and piston.

On the other hand, higher compression ratio given the same piston/rod geometry means more time is spent at lower chamber volume and thus higher pressures and so more of the charge burns at higher pressure. This effectively makes the exhaust valve act as if it were opening later, since more work is extracted on the expansion stroke. This effect gets more and more precious as the revs climb, because you have less and less time for the charge to burn. The "better solution" is to just increase the expansion ratio a la Atkinson cycle but this obviously decreases specific power, and you lose mechanical efficiency as a result.

Empirical data suggests that most of the time the benefits outweigh the drawbacks, until you get to the 13-14 :1 area, where careful attention has to be paid to combustion chamber shape, interference with in cylinder flow, etc. or else you can actually lose power/efficiency.