Been running 14* on 91 octane (or 92, depends on the place) for awhile now. You gotta play it by ear. I'd say start by running a tank full of premium through, then refill and THEN bump your timing up as high as you feel safe (I'd say 16* tops), then back it off a degree at a time if you get pinging, until you find the sweet spot. It might take you all day, having to pop the hood, re-set timing, go out for a drive, go back and re-set it again, re-test-drive it, etc. until you get it where you want it. I lucked out and found that 14* is my car's happy point for the moment on the first try - anything more and it knocks, anything less and it feels sluggish.