I have a 350 block that I am building and I was wondering what is the highest compression I can run with the block.
I wanted 11 or 12 to 1 but someone told me that I could not fill up at a gas station because I would need higher octane.

 

Your friend is right 12 to 1 equals race gas. The general rule is 9.5 for iron heads and slightly higher with aluminum heads. I am running 9.5 with a mild timing curve ( bout 32 totoal timing) and i can get away with 89 on a good day but it really likes 93.

 

With adequate head cooling and enough EGR at the appropriate times, I can run 10.8:1 on 93 octane (gasohol) without a problem or severe retard from the PCM. This is with 54cc iron heads, BTW.