I installed the Banks Monster exhaust sytem today, 3.5 inch downpipe, 4" pipes, with a 5" inch tip. I put the cat back on with the honeycomb knoced out to pass inspection. There is a very noticable difference. I also did the foil delete at the same time. With the new exhaust I notice the turbo spool much faster, but can also hear it give that beautiful whistle, I think due to the foil delete mod. The truck is much, much quicker now, with a huge difference in throttle response. I am definately hooked on diesel performance, next for me is gauges then a tuner.