Modify O2 sensors

[omgwtfbyobbq]

Lean burn improves combustion efficiency and torque, with the trade off being less hydrocarbon and more NOx pollution. Pumping losses are also reduced because more air is allowed in the combustion chamber and heat losses in the cylinder are reduced too, although I'm not sure how that works, probably has something to do with the size of a lean burn a/f charge.
Edit- Found something that says controlling swirl allows for stable combustion and better lean burn. Which makes sense, because hot spots in a cylinder could lead to pre-ignition, so controlling how the intake charge combusts (even combustion/heat distribution) is critical to being able to run lean without pre-ignition. Running an a/f ratio of 25:1 compared to 14.7:1 supposedly reduces fuel consumption by ~20% over the Japanese test cycle.