[Goran]

Hi Mossy,

I cant comment too much about the sound, in both cases for both cars I had them done at the same time as getting a custom exhaust. The custom exhaust changes the sound much more than th cat. Although on the current S8 I did run catless on the stock exhaust system for a while, I just couldn't wait for the custom exhaust because one cat was rattling badly. From memory, it was not much louder catless, the stock exhaust is pretty well silenced. So if you just change the cats I doubt it will be much louder.
It should be a straightforward job, cut out the old cats, weld in the new. If the new ones are significantly shorter they may need a length of pipe same diameter as stock to lengthen it. Oh and yes on one side there will be a slight complication. Audi in their wisdom made one cat easily removable with a bracket joint just in front of the cat, but the other stock cat is a part of the downpipe with the pre-cat lambda sensor in it, so they may have to remove that whole pipe. Unless they can just chop it off and weld in a new bracket in situ?
The more expensive cats I posted should have the same entrance/exit diameter as the stock exhaust 61.5mm, the cheaper ones are a little bit bigger diameter at 63mm but hopefully that's not a big problem.

The 200cell cats should definitely release some performance, they are much less restrictive than the stock 400cell ceramic cats. It wont be much though, and you would have to dyno it before and after to get a sense of any increase in hp. I did dyno my car before I changed the cats and the exhaust but haven't had a chance to do another one after so I dont know if its gone up (or down)

I forgot to mention, for post-facelift cars with a second set of lambdas after the cats, the 200cell cats may occasionally trigger a post lambda oxygen reading error, and even less frequently may trigger a warning light on the dash, although it may not. There is a solution which is to weld in a couple of spark plug anti foulers in the post cat lambda mounting points and mount the rear lambdas into those. This removes them from the direct exhaust flow and will prevent the error being triggered and the CEL appearing. This is how I had it done on the previous S8, its the cheap solution. On my current S8 I had the post-cat lambda errors coded out so thy never appear. Annoyingly I couldn't remove the lambdas altogether, I think MRC (who did the coding) said there is a part of the start up routine which checks the post cat lambdas so it would't be happy if it didn't find them. Maybe if I got a whole custom remap they could get rid of them, but I stayed with the stock map for now.