Well in new cars like the Edge and the Mustang have newer mafs that work with the electronic throttle body. The pcm needs to know how much to open up the TB base upon what the maf readings, too much airflow (over voltage) would cause the car to run lean. On older cars like your 5.9 do not have mafs installed, instead they use a MAP sensor to read the manifold pressure instead of the flow/volume of the intake air. When you get the tune you can up the volts going to the maf over the stock limits and allows the maf to read the new air flow. Plus when you get a tune you can add more timing and fuel to make better hp @ the wheels. New mustangs w/ the cold + tune have seen up to 15-25rwhp.
