2006+ Honda Civic Si Header with Metal Core High FlowCat. Converter Engineered to make power, these headers feature large diameter T304 Stainless Steel mandrel bent primaries and precision CNC machined Stainless Steel flanges that are hand ported for smooth exhaust flow. They also feature a huge 2.5” diameter outlet, which is the perfect compliment to our 2.5” diameter cat-back systems, although they will work with the stock exhaust system as well