The downpipe is a 2 piece 4" OD double wall similar to the 6.4 design. Right now there is really no need yet for a down, all though we will be biulding a downpipe for these trucks as we did with the 6.4. Just don't want to rush it as other manufactures did on the 6.4, and all they ended up with is a downpipe that rubs the cab. When completed, it will be built to fit aftermarket and stock exhaust just like our 6.4 stuff. We also have a downpipe back kit that is 5" and runs a 5" Magnaflow kit with a full size muffler and 5" piping(not like the 5" kit DPF back sold for these trucks). Many options coming each week as time goes on, give me a call for more details.