[E4ODnut]

I did some research on "lean burn". It looks to me like a true lean burn engine is designed to run on much leaner fuel mixtures throughout the complete power band and that a major stumbling block to these designs is that of maintaining emissions standards.

Having said that, and emissions concerns aside, I see no reason why conventional gasoline engines can't get improved BSFC at lower engine loads by leaning the mix, adjusting spark timing to suit this leaner mix, and opening the throttle somewhat to maintain the required power. The larger throttle opening will reduce pumping losses or what ever you want to call it, as well as improve the thermal loss. The exact cause of the improvement doesn't really matter, what matters is that the end result should be better low load BSFC.