let me just start by saying that i'm not positive about the results i'm just wondering based upon the characteristics as i understand them.

octane in gasoline is basically a burn rate retard.

if this is true, then with the higher compression engines (like ours) one normally runs a high octane pump gas (92/93).

the c6, as i understand it, has knock sensors that report to the computer which in turn can advance and retard spark accordingly.

as octane levels are increased (i.e. superchargers, turbos, etc) the spark advance is also increased (timing is retarded).

won't the combination (retard timing and high octane) result in an even later ignition and slower burn of the fuel? normally one decreases the compression ratio for forced induction.

if that is the case, then why can't one use a lower octane (87/89) and get a quicker burn which would result in higher compression at an earlier crank position (if it is too soon it will cause preignition and try to push the piston down on it's upstroke - bad bad) which can produce knock. would the knock sensors pick this up and adjust the timing accordingly ? would the computer retard the timing enough to get the proper ignition time for the burn rate and in turn create a higher compression ratio and more power ?

please forgive me, it's early morning and i just may have a hernia.

 

First, octane does not describe the rate of combustion, but the conditions (i.e. temp and pressure) that combustion will occur at.

The computer doesn't change timing continuously based on the knock sensors. Timing is held constant unless knock is detected, then it is retarded. The computer can't know if it can start to advance timing because it has to try it and see if there is any knock first.

"as octane levels are increased (i.e. superchargers, turbos, etc) the spark advance is also increased (timing is retarded)."
I'm not sure what you're saying here as you contradict yourself. Turbo/supercharging increases pressures and requires a higher octane. And yes, with higher octane spark can be advanced. (We are talking about spark timing only, not valve timing) The reason higher octane is used in forced induction is that you start out with higher pressure which is good because there is more air/fuel in the chamber. After compression the pressure is very high and therefore closer to preignition. With high boost applications lower compression ratios are also used to avoid preignition where 93 octane isn't enough. This lowers the pressure somewhat after the compression stroke, but there is a lot of air/fuel to be burned and that produces power.

Nothing the computer can do will affect the compression ratio. That is only dependant on the combustion chamber volume at TDC and BDC. When the computer adjusts timing, the peak pressure of combustion will occur at a different crank position, and the amount of this pressure can change. That is how power levels can be changed.

To sum up, lower compression ratios don't cause slower burning, they allow for more fuel mixture in the chamber without raising the pressure too much. Using lower octane will limit the amount of spark advance you can use. In high compression and forced induction, this usually means running a later spark than what would be optimum. In some configurations the lower octane will allow spark to be advanced to the optimim point, but those configurations do not put out large amounts of power.


I like your questions and thoughts, and hope I answered everything correctly. If not someone please chime in. I'd like to see where this discussion leads to.

 

Sorry I can't add this since you've covered it so well.

In a nutshell, the octane rating dictates how much you can compress the gasses before they will ignite from the heat of compression (bad) as opposed to the spark (good).