The commodores also run lean burn at cruise. To maximise fuel economy the knock sensor does work flat out all the time during cruise and at full throttle. The ecu keeps spark timing just below the level where knocking occurs.

With normal combustion, the charge burn speed varies directly with charge density and the degree of charge turbulence. Higher compression increases charge density. Good port design and chamber shape helps determine the degree of chamber turbulence the fuel charge experiences. More charge turbulence exposes the mixture to the flame front more rapidly allowing it to burn in less time. Another factor varying burn speed is the amount of chamber squish. The squish area forces gases to be pushed through the already turbulent mixture. More squish usually means more turbulence and higher burn speed. One result of the faster burn speed is that less timing advance is required, reducing the chance of detonation under load.

For a given chamber, the Fuel burn speed remains nearly constant if there is a consistent quality fuel charge. This is in spite of engine speed variations. If the fuel charge burn takes an average of 4 milliseconds duration in the chamber- at 6000 rpm the burn duration available for 140 degrees of crank rotation in the burn cycle before the exhaust opens is only 3.888 milliseconds. This means the mixture is nearly completly burnt before the exhaust valve opens. - At 8000 rpm the burn duration available is only 2.916 millisecs, so only 2.916/4 * 100= 72.9% of the mixture is burnt by the time the exhaust valve is opened if the mixture was ignited at the same point as at 6000 rpm. This is known as burn lag. The burn lag is directly related to the burn speed. It is the relationship of the burn speed and ignition timing that determines where peak cylinder pressure occurs. It is where the peak cylinder pressure occurs in relation to the crank angle that determines how well an engine performs.

The wild card with choice of compression ratio is where the dynamic comp ratio is so high the fuel mixture is heated to a point it will spontaneously ignite. When this happens, there is a rapid combustion pressure rise before top dead centre is reached. Pressure increases further as TDC is reached. Chamber pressure becomes so high that now combustion is no longer controlled and the fuel charge explodes (detonates).

Instead of a constant push on top of the piston over a few milliseconds its like the piston being hit by a sledgehammer, with all combustion energy being released in less than a millisecond.

At higher engine speeds more heat is retained in the chamber and this makes detonation more likely with higher dynamic comp ratios.

The end result is the comp ratio you use will depend on -
1. Maintaining consistent fuel mixture quality
2. Having a good chamber design and fast burn speed through development of the squish area and chamber walls.
3. Accurate control of Ignition timing.