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You can't create REAL lean burn in a combustion chamber/piston crown design that wasn't intended for it. If you read the simple Wikipedia entry on it, you'll see the mention of stratified charges. At the really lean lean A/F ratios used in lean burn engines, you can't support a flame, if those mixtures were homogeneous through the cylinder. Lean burn engines, through various methods, concentrate fuel around the spark plug, generally. You end up with an area that is say 15:1, w/ the remainder of the chamber all air. You end up with a TOTAL ratio of 24:1, for example.
If it were just as simple as the right combination of timing, throttle opening, and fuel mapping, etc, all cars would've been set up for it!
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