I know that every aftermarket company wants an arm and a leg for exhaust systems. so has anyone have/or built a cat back system? what diameters did you use for the best flow rate, and was there any special equipment added for better sound and breath ability? If any one has suggestions help me, please.
Eric

 

I run/ran custom exhausts on all of my cars, much cheaper than aftermarket and you can tune the sound of the exhaust to your liking by choosing which muffler you want.
Basically the A8 has down pipes with the cats built in, they then have a connector clamp that holds the rest of the exhaust to the downpipes. So you just need to eliminate the clamps and make the exhaust from there back. If you want to have any sound out of your exhaust at all you need to take out both the resonators and mufflers. Weld in two custom mufflers that you want and buy yourself a set of tips that you want to come out of the back. Magnaflow, flowmaster, borla, remus all sound great on our cars. You would want anything from 2.5" to 3" exhaust piping. No larger than that.

 

supersprint muffler replacement in the rear, have front resonator removed and replaced with straight pipes, then replace the H-pipe with an X-pipe or balanced pipe and you've got a mean *** a8.