From our blog post on the 718 GT4 Street Headers: This is a complete replacement from the cylinder head to the factory muffler and is comprised of three pieces per side: the exhaust manifold (header), the over-axle pipes (which contain the catalyst), and the muffler adapter. The reason this is only offered as a complete kit (and not individual components) is that our system was optimized for producing horsepower (using stock mounting locations limited power gains). All components are made from mandrel-bent, TIG-welded 321 stainless steel. These connect using gasketed v-bands for easy install and a complete seal. https://blog.dundonmotorsports.com/718-gt4-street-headers Hear it for yourself: 

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Check out the help site for a full installation guide of our 718 GT4 Headers