octane levels

Octane is not "just a number". It relates to the flash point (temperature at which the fuel will spontaneously ignite) of gasoline. The higher the flash point, the more heat required to ignite. Low octane gas in a high compression engine knocks because it is igniting before the engine gets sufficiently into the "power" stroke.

http://www.howstuffworks.com/gasoline3.htm

Since heat is the determing factor, it makes sense that an aluminum head can utilize a higher compression ratio on a given octane fuel. Conversely, engines with aluminum heads will be less prone to knock at a given compression ratio and fuel octane.