9PHX 2017-2023 L5P Duramax 6.6L Downpipe Exhaust 3.5" for GMC Chevy. Features:. 3.5" Stainless Steel Piping. 20% better flow than the factory pipe. The heat wrap is included to keep the engine bay temperatures down. It retains more heat and won't affect emissions systems. It has maximum flow for competition applications and comes with a 4-bolt flange and two installed studs. It completely eliminates the DOC.. Fitment:. 2017,2018,2019,2020,2021,2022,2023 Chevrolet Silverado & GMC Sierra 2500 & 3500HD 6.6L Duramax pickup trucks. Will not fit Cab & Chassis trucks.