Chambers cause backpressure and thats good for NA cars, but for a turbo car, the turbo does enough. You want the least amount of restriction for the turbo, so that means a strait through muffler. But once you go to a certain diam, you reach a point of diminshing returns. 3" will be more than most will ever need. Some Evo catbacks come in 3.5" and even 4". [sm=yikesomg.gif]