[lee]

Adam,

You're correct that higher octane number in simple terms means harder to ignite. However, it's not just compression ratio, but also combustion chamber shape and material present that determines the number necessary. For example, older cars/motorcycles with carbon build-up in the combustion chamber sometimes need a higher octane to keep from "knocking", or experiencing pre-ignition. Pulling the head and cleaning it up will restore the use of lower octane number - old British motorcycle tech manuals refer to this as de-coking. So all you need is sufficient octane number to stop pre-ignition. That's why use of higher than required octane number fuel does nothing for you except lighten your wallet.

I'm not a chemist, but octane is one of the hydro-carbons like methane or propane. I was once told the octane number originated as a simple measurement of how much Octane there was in the gasoline mixture along with the various other "anes".