lambda is defined as 14.7:1 best complete burn, no side emissions correct. however, lean is mean as 15.5:1 at wot creates a hair more power on the dyno but creates less torque and extra heat. in a lot of cars it was also found that 12:1 mixes actually increased horsepower at wot by increasing the fuel to cool the engine components down. that is why in open loop systems the maf measures air and actually increases the fuel slightly as the air velocity steepens, if the velocity at the sensor is shown less than actual, the richening effect lessens, creating a leaner mix at wot throttle and open loop times, including hard acceleration and towing. the closed loop is only used at cruising and idle. the closed loop at cruisiing can be modified to 15.5:1 to lean the mixture and save fuel as you are running constant rpm and you are already overcoming the rest of the restraints on the engine (a/c water pump power steering etc.) the tuner themself has to decide what the vehicle is used for and if the modification is necessary, the performance ppl (me!) want their units closer to the 12:1 range to protect the engine during revs and hard acceleration between 70% and 100% throttle, therefore a proper maf signal is required to maintain this ratio properly.