I wouldn't go over 11.1 with the fuel in my area. Rated at the pump at 93 the rating is from when it left the refinery. A lot of the octane is achieved from the ethanol so as it ages I'm sure the octane drops. I run 10.4 with aluminum heads. I still mix in some race fuel when I race it. Unless you are trying to make max horsepower per cubic inch there are other ways to make horsepower than raising the compression.