i have been monitoring ignition timing on my impreza and cruising on the highway i recorded between 32-34 degrees IGN. i have heard modern knock sensors in new cars are pretty advanced and sometimes you don't actually even need an ecu tune to advance/retard timing. So I decided to run 91 octane and see if the car will 'advance' the ignition automatically and voila. My ignition timing according to scangauge now is 45 deg at highway speeds. i do have a before and after video.
this before video is from a while back but you can kind of make out the 35 ignition timing and ~33mpg
this one is from today and you can make out a whopping 45 deg IGN and ~37mpg on same stretch of road at same speed:
the thing that amazed me was how quickly the car adjusted the ignition timing and how much difference timing advance made. just after a few minutes cruising on the highway the ignition timing went as far as it could without knocking for ~4 more MPG. i remember playing with A/F by leaning out the ECU map, but that made not much difference and it made my engine run at unsafe temperatures. So this is much better. however i don't do any highway driving so it's good old 87 octane for me. but it is obvious to me now that my car CAN take advantage of higher anti-knocking index gas. the only downside is extra cost of 91 octane and increased engine braking. anyone else experienced this?