The E.D. O2 housing replaces the 2" cast iron elbow that is located upstream of the exhaust system. To take advantage of a true full 3" system, this piece will need to be replaced. The stock o2 housing is 2" vs our 3.25" inlet piece with a 3" outlet. The more HP your vehicle makes, the more HP you will yeild from the o2 housing. We have gotten reports of 10HP with an o2 replacement. The o2 housing comes with hardware and gaskets. ********** Save $20 on this item if purchased with our full exhaust system! *********** SRT-4 o2 Housing 02-srt$140.00