The v-band outlet of the stock turbo is smaller than 4", as is the V-band at the front of the front pipe. Why run something that is 4" in between but only 3" on each end? I have an ATS true 4" down-pipe on my truck but my S475 has a 4" V-band outlet and I had to cut the front bend off the front pipe and connect under the truck to the 4" section. The Diamond Eye and MRBP downpipes for the LLY and newer trucks are 3" IIRC and have much smoother mandrel bends and stay round instead of pinching to an oval like the stock one. The Diamond Eye piece fits better and is all around nicer IMO. I typically recommend waiting until you upgrade your turbo (if you have plans to eventually) and then getting a downpipe to fit it.