So a buddy of mine who has much more experience than I do with LS performance was telling me that you will get a bit more power and economy out of an LS vehicle with 91 octane as opposed to lower grades because the ECM will "learn" it can run more aggressive timing and fueling tables. Im wondering how much truth there is to this and how much is simply wishful thinking.

 

Wishful thinking. The PCM can't learn octane ratings, it can only determine that there isn't an issue that needs timing retardation. If 91 octane were to run better than 87, then there is an issue (either tune or timing or heat) that is causing the computer to reduce spark advance to what it considers a stable number.

The only "learning" a PCM can do is making a fuel type determination; on 2002 trucks there was a fuel quality sensor (alcohol sensor for E85), 2003-up makes a guess based "ethanol or gasoline" on the O2S readings.

 

I had a feeling it was to good to be true, but at the same time it was just legitimate sounding enough I figured it wouldn't hurt to ask.