I got a 4 inch downpipe. into 4 inch straight to a 7 inch dual stack kit from sbs diesel, i love the sound and the bull hauler stacks helps direct a lot of the noise away from the cab, i love it. before that i had a BTM muffler on there just srtraight piped with a turn down and it was LOUD....to he annoying point.... the first thing i did to my truck though was chop and drop the muffler.