BrianC72gt

I've got 3 years on CARB approved Walker replacement cats. They are approved for the D2 A8/S8, but are not bolt on, and require a little bit of welding. If fading memory serves...about $600 all in for both. The cats were like $139 each. I did replace all my O2 sensors before swapping the CATs, and thoroughly cleaned the PCV valve. Running rich will kill the cats, and a bum PCV will secretly suck oil into the intake manifold...and then on through.

Adjuster

I am looking at this same issue, and options to replace the cats with higher flow designs using a metal core, not ceramic. (100 or 200 cells per inch, v/s the much smaller, and lower flowing ceramic cores the OEM Audi degisn uses.)

Some are saying the 100 Cell cats might not clean the exhaust enough, and using a off set device, or moving the rear O2 sensors out of the exhaust flow slightly with the spacer/device will cure the CEL problem. (And you get the higher flow, while still retaining the cats cleaning the exhaust.)

My 2006 Yamaha FZ1 had cats like this on it, and they flowed very well, but I removed the one in the header, and took the stock muffler off, and replaced it with an Akrapovic one that flowed better, was lighter, and when it was all done, added power from idle to red line.

The turbo guys use these free flow cats to still get top power, but have a free flowing exhaust.

They are weld in only, so cutting out the old cats, and welding in the new ones is needed, and the higher flow units can be had in various sizes, so you can make it work with some simple cuts and welds.

But so far, I've not replaced them with the high flow, 100 cell units. (The guys in England talk about this much more, so some research over there is a good idea.)

Mil Spec is a supplier here in the USA that appears to have a good quality cat that will not crack, or break up, and it's rated well for cleaning up the exhaust, so you might not have CEL's if you go with 200 cell units, or just space the second O2 sensors out slightly with spark plug oil control inserts, and call it a day.

RicksS8

Thanks for the information, I have not heard of Mil-Spec but will look into them.

I have been looking into HJS for the same set up but it appears they only make converters with 200 cells/inch. I found them by looking for a German made converter but it is hard to find anyone running them.

Has anyone here had any experience with either of these companies?

123quattro

I wouldn't expect 100cpi cats to play nice for very long with the catalyst diagnostic. They aren't nearly as efficient (in conversion) as OE units.