You need a tuner with a custom tune to turn off the rear oxygen sensors. We sell SCT tuners with free tunes for life for only $379. 1994-2011 4.6L 5.0L Ford Mustang SCT XCalibrator X3 SF3 Hand Held Tuner - Free Tunes for Life - UPR Products The S197 tuners make a huge difference!! Really though, if you buy a tuner, that would be a good time to add a cold air intake as well. These parts add around 40hp. :bigthumbsupI have a 2006 Mustang GT currently with an h-pipe & no mufflers, but I want it louder. I already have the x-pipes without the cats, but what does it take to install them so that the car will run properly and not freak on me?