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car runs closed loop at part throttle/low load. uses narrow band 02 sensors to keep things at stoichiometric. once you go wot or are under high load (climbing a hill) the ecu goes open loop and just delivers fuel based on measured airflow. in these conditions it also richens the mix to control heating, etc.
the tuners can't touch the part throttle tune, but they can fiddle with things when the car goes open loop by leaning the mix a bit - saves gas, improves performance by running closer to stoichiometric.
oem wot afr isn't 10:1, however. insite measured it on his car in another thread and found 12:1 or something close to that. if you are getting 10:1 then something is up - car isn't metering airflow properly; ie, thinking that it is getting more air than it is. some quick math shows that the difference between a 74 mm and a 76 mm maf housing is 5%, enough to change afr from 12:1 to 11.4:1 ...
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