[shadrach74]

Air and fuel "rubbing against each other" is one way of explaining what's happening...

As pressure increases, the molecules in the combustion charge will vibrate, increasing the temperature of the charge.

The concern about increasing temps and pressure is detonation.Detonation is caused by high peak internal cylinder pressure.

High octane fuel is less volatile and able to burn evenly at higher pressures and temps than lower octane fuel. The octane needs of almost any normally aspirated vehicle will go down as altitude increases. This is because the atmospheric pressure drops with altitude. This equates to lower intake manifold pressure which means less air going to the engine. This is why power drops with altitude.

This is the reason why the pump octane number of gas at altitudes above 5K is 2 points lower across the board.

If you only have 91 available, I have to assume you live ~5K or above.

If the ECU has retarded ignition timing to prevent knock, I believe it is logged and should be accessible with a durametric computer. If durametric shows that the ignition timing has been unaltered from the stock timing map than higher octane is a waste of money under the current conditions.