I would like to install headers, cat less h-pipe, bigger throttle body and plenum on my '04 GT. After all of these mods, I would imagine that I will need to tune the computer to get the most out of the bolt on mods. I hear you guys talking about tuners all the time but am a little unclear on what they exactly are. Are you talking about bringing your car to a hi performance shop and having someone tune the car for you? Or do you mean buying a handheld computer tuner that you can do yourself? I would like to do this myself but want to know which handheld tuner will adjust my air/fuel ratio, shut off my rear o2 sensors and the other typical things that can be done. Has anyone done this themselves and with which tool. Thanks in advance.