yea in the EPROM days, you had to change the actual chip, 300zx's were that way. but now the stock ECU can be flashed and retuned. the way i always explain it is that when nissan tunes the stock car, they have to tune it so that it will run in the middle of a desert, at the top of mt everest, and at the north pole all on the same tune while running off gasoline with quality so poor, you could relabel it coca-cola and last at least 60k miles without destroying itself. running the fuel you usually get in your area and tuning it on a dyno allows them to open up the constraints to get more power out of it and get it running more efficient for the fuel you use. typically you get more power and your fuel mileage either stays the same or in some cases goes up slightly... after you get done with your 40-100mph pulls the first week that is
