the crossover in the catalyst pipe is either shaped like an X or an H. The xpipe has a higher pitched sound with less rumble and the h pipe sounds more like an older muscle car. FYI- On the S197 cars, you aren't gonna get that "old" muscle car sound either way. I have had several setups on my '06 GT and none of them sound as good as my old 5.0 liter. Or maybe I am just too old also