Remember this when deciding on the 4" or 5". .
With the HP levels your probably at you wouldn't have any need for the 5" system(except for looks) and may cost yourself horsepower. When you install the bigger 5" setup on a truck which has lower HP(say under 450-500hp estimated) you will not retain enough heat in the pipe to allow for proper exhaust flow. The gasses cool off to quickly inside the exhaust pipe and make the turbo push and work harder to exit the gasses. When this happens you will loose HP and elevate your torque levels. On a normal properly flowing setup the exhaust will flow easily and not cool inside the pipe which provides the proper environment to make maximum HP and normal torque levels. By going to a 5" on mine I lost 18hp at the rear wheels with a setup that had rolled 330hp prior to the change in exhaust. The larger exhaust also elevated the torque level almost 100ft lbs. I also this winter seen a drop in my fuel milage over what I had ran in previous years. When I look over my records now with winter basically over I should have sold the 5" and kept the 4". The easiest way to define the sound differences in the exhausts while straight piped is this,3" has a bark when you hit it,4" is a more of a growl than a bark and 5" has a deeper bellow. As for the "dreaded Drone" issue,well,I drive truck for a living and always keep in my mind this is a diesel and its not supposed to be quiet. If I wanted quiet I'd be driving a Dakota... ... ... . Andy