this is the piece that is attached to the exh manifold. The outlet of you stock manifold is just under 2 1/2". The inlet size of the stock 02 housing is the same size, but where it really gets resticted is, that it bottlenecks down to 2"!! Now the one I have in my car is 3". reason being, I am running a 3" dp and a 3" exh. For the individuals who want some extra performance, but do not want such a big exh. That is where the 2 1/2"version. You see, the inlet to the stock dp is 2 1/2". Then it gooes down to the 2 1/4 " tail section, that attaches to the rest of the exhaust. What I am planning on doing is, when I make the 2 1/2". I am going to remove the smaller tail section of the stock dp, and replace it with a 2 1/2" pipe. I will be testing this vertion on a freinds srt. And yes, I do have a bung in the factory lacation for the 02 sensor. I will be offering the option of an extra bung, for a air fuel guage.