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5.0L (1979-1995) MustangTechnical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.
all they really do it up the timing and you can do that yourself.
with a custom chip they have to run it on the dyno. that way you know your a/f is correct you know how much power it makes and some have flip tunes where they can program more than one tune in it