Originally Posted by chazzz
What is ACN91?
Arizona, California and Nevada (ACN) fuels are slightly different than the rest of the country and are thought to be less knock resistant, for this reason many tuners go a little less aggressive on cars running these fuels and even some "canned tunes" have special ACN maps that are less aggressive.
And to the OP, yes as far as i know all "normal" 91 octane gasoline sold in California will be ACN91 blend. There are a few stations around with 100 octane available at the pump and E85 depending where you live, and im not sure if these fuels differ from the rest of the country or not.