Openclasspro already answered the question, but to contribute, you can run the pipes straight back, but its not recommended. As stated you will have better torque with the X pipe mounted as close to the original Y as possible, but a hardly noticeable louder tone without a cross over. Though without a cross over you may have unbalanced sound also, as in the passenger tip may be louder toned than the drivers side tip. Same thing with H pipes, they dont balance all that great, or atleast they dont in a truck exhaust. If you want to, listen to this video clip of my old truck and check out the tone difference between 1:20 - 1:37, and again at 2:05 - 2:15.

http://www.youtube.com/watch?v=_RL-K...eature=related