Apparently, if you go over a certain amount of timing, you'll start to lose power actually end up losing mileage because you have to use more pedal.
Lean burn can be done by running open loop and commanding a lean AFR at low loads and engine speed. Downside is you have zero correction (fuel trims) in open loop.
The long term effects of a lean burn on your pistons and exhaust valves, I don't know. That's something I never looked into. Lean burn only really became a term I was aware of once direct injection became a thing.
