Here is a lesson about octane all fluids have a vapor pressure point. Gas will ignite under compression and the lower the octane the lower the compression it will ignite. Most engines perform best with advanced timing just shy of ping (pre-ignition, of course higher octane will eliminate this ping at the advanced timing) High performance (high compression) engines need a slower burning gas as not to ignite to soon.

Ok, with the new technology in our microprocessor controlled engines if you put cheap gas lower octane in your tank the engine control system will automatically retard your timing to compensate for the lower octane to reduce ping (pre-ignition). Therefore your car will not be running at peak performance, you may not notice it in normal driving, but it will effect high rev performance and also gas mileage.