The stock flex section is 2 1/4". If you're upgrading your exhaust to a bigger size, or if you're installing a set of our headers, adding this flex to your car will help get the most out of your new parts. We have options for inlet and outlet sizes, as well as options for a catalytic converter. Outlet size on our headers is 3", so keep that in mind when ordering. These flex pipes are made with high quality materials such as lasercut stainless steel flanges, stainless tubing, and a stainless interlocking flex section. Catted versions also have two O2 bungs located after the cat in case you are installing headers and eliminating the factory cats and secondary O2 bungs. Please note you will need a pair of our o2 extensions to located the O2 sensors in this flex pipe.