I have a 98 2500 with a 5.9L gas engine. I was questioning the difference in gains that I would have using a Flowmaster exhaust (cat back) versus changing just the muffler to a Flowmaster. Thanks for your input.

 

i just put a dynomax muffler on the y pipe, and a dump...and i like how it sounds over the previous full exhaust (mopar performance cat back).

 

Measure your stock exhaust. If it is already 3" from the cat back, then all you need to do is add the muffler. Unless your stock pipes are in bad shape, buying a new cat back would be a waste of money, even if the new cat back is mandrel bent. There are not enough bends from the cat back to make any noticable difference between standard bend and mandrel bend exhaust flow. If it's 2.5" or less from the cat out, then doing a 3" single exhaust cat-back would give you a slight increase over doing just the muffler.

If your contemplating duals, then just get a catback with 2.25" pipes to maintain good low end, or 2.5" pipes for top end. Keep in mind that you will lose some performance (probably not noticable except on a dyno) with a dual exhaust (unless you do true duals). Single exit exhausts perform the best with these y-pipe designed exhausts like these truck utilize. That y-pipe is actually the most restrictive part of the exhaust system. That is why you can get away with just putting in a high flow muffler, and see basically the same results you would see with a full cat-back.