This optional resonator is used when additional sound reduction is desired. 3" diameter inlet and outlet. It is designed to be used with the Borla(r) Cat-Back(tm) systems 140750 140750BC 140750SB 140751 140751BC 140751SB 140741SB. Adding this resonator to these systems will achieve the Touring sound level. It is NOT designed to be used with 140597 or the original equipment exhaust system. Built from premium T-304 stainless steel superior to T-400 series knockoffs (such as 409) to give you the absolute best in performance and durability. Ultra-smooth mandrel bends ensure maximum flow & power and precision computer numerical control (CNC) manufacturing ensures an accurate fit. Best of all Borla(r) stands behind every direct fit resonator with a Million-Mile Warranty. Just what you'd expect from the worlds most experienced (and most winning) exhaust brand.