I'm very limited to what changes I can make to my Expedition with my programmer. My question right now is whether advancing the ignition timing will help or hurt MPG. If it's advanced then power will go up, but then again if I'm driving as easily on the accelerator as possible then does it really help? And if the spark occurs closer to TDC then although the compression in the chamber will be less, the fuel will be burned for a longer period of time after TDC. I'm thinking that if I'm driving extremely lightly like I have been than retarding the timing may help MPG? Or is the ultimate goal to make the absolute most pressure in the combustion chamber without detonation (advanced timing)?
Advancing timing as much as possible without pinging will give the best mileage. You might just advance it 1-2 degrees at a time because it is possible that if pinging occurs, the knock sensor will detect it before you hear it and the ECU would then retard the timing and richen the mix. In this event you might be worse off than if you would have just left the timing alone.
So, IMO you should advance it 1-2 degrees at a time and run it for a few tanks and compare the mpg to what you were getting at the factory recommended timing setting.
Might change but I doubt it'll be measurable. I've done it on several cars with adjustable distributors. Never tried retarding timing but advancing up to the point of knock hasn't shown any quantifiable gains. what I mean by that is while there *seemed* to be a slight inprovement in FE before/after, it was less than the deviation between tanks.
power change was indeed noticeable though. not a lot but definitely there.
1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
mechanic, carpenter, stagehand, rigger, and know-it-all smartass
"You don't get to judge me for how I fix what you break"