octane

[Zsolt Kovacs <zsolt@telusplanet.net>]

There is a label on my 5k tq that says it requires min 91 octane gas.
Previously in my other non-turbo Audis (5kS) I never bothered to put
anything else but regular gasoline and performance or gas consumption
wise it didn't make any difference. In my quattro I do use premium and
I'm wondering if it does have significant advantages over lower octane
gas. How is the octane controlled in non-leaded gasoline?

I just thought some of you may have a scientific explanation or you just
learned it from experience.

Thanx,
  Zsolt