Be careful if you live in an emissions state though. If you tune for a lean fuel trim, this will effectively reduce your CO output which is good but any reduction in CO is directly proportional (and vice versa) to increases in CO2 emissions which is a greenhouse gas. Since the LS series engines run @ 210 to burn as much of the hydrocarbons as possible. To keep combustion chamber temps down (not the same as engine operating temperature) additional fuel is required for cooling capacities. The amount of oxygen present in the exhaust as sensed by the upstream and downstream o2 sensors tell the ECM how efficient the fuel trim is. Running lean with regards to NOx levels is the same as more timing advance as far as creating more nitrogen oxides are concerned. A water mist or methonal injection can cure some of those problems though.