[mnmarcus]

I think this is a good question but has not been answered yet. I think Old Mechanics response is the best so far. I am sure I do not know the answers to this question, but would like to be enlightened if possible. Quotes are from original poster.

So from everything I have seen, the only way lean-burn seems to help anyone save fuel is by allowing the engine to operate in a more efficient BSFC range. I.e. more RPMs but same output power to cruise at 60km/hr etc.

I believe this is essentially true, ex. when "low" power requirements exist the lean burn is more efficient (BSFC) because instead of throttling back to get to that low power requirement, which would consequently increase pumping losses, less fuel is injected for the same(?!) HP at the same rpm. I believe the actually answer would be much more complex, for instance during maximum acceleration would lean burn be more efficient? I would guess not because the throttle is wide open which means minimum pumping losses. Your mpg may still go up because you are putting less fuel through the motor, but your bsfc would be worse because you are not getting as much power per unit of fuel.

Which would explain why I'm only seeing a 12% gain, when I have leaned out the Closed Loop mixtures by 20%.

Again I'd say this is essentially true. Leaning the fuel to air mixture would only coincidentally equal an MPG gain. If there was a direct one to one relationship we could just inject zero fuel to achieve unlimited mpg.

Correct, wrong or not exactly right?

I think not exactly right but close, and I am sure I am not exactly right either but perhaps someone can explain it better or more accurately?

Some additional questions I have that seem to relate;

If there are 2 identical cars traveling at the same steady speed which require 25 HP and the only difference is one car runs (say whatever%) leaner how would you describe the differences between the 2 cars engines?

The engine rpms would have to be the same for the same gear and same speed.

The compression would be identical because it never changes in an engine, does it?

The lean car is injecting less fuel per stroke so it has to use less fuel per minute.

The only difference would be the throttle is opened wider in the leaner car which means the less fuel makes less HP but less pumping losses as well so same net HP to the transmission. Right?

My next question would be why aren't all cars lean burn? I assume lean burn turns on and off or is adjusted by the engine control unit? I'd also assume there are times lean burn is not more efficient mpg wise, right?