I am now thinking of de-catting my S4 downpipes to get the best from a 3" Cat-back milltek install to along with my upgraded (facelift) manifolds.. I have a spare set of downpipes that I can practise on - whats involved? Having chatted with Matt82 yesterday, we came to the conclusion that: a) it must be difficult to get them out without angle-grinding the cats in half, poking them out and re-welding the downpipes? b) it must have an effect on the lambda sensors? On b), I have done a trawl and it looks like the pre-facelift S4 only has 2 lambda sensors, so am assuming it monitors the gasses on the way out of the engine, BEFORE the cats, then couldnt care less what happens? I guess its only later cars that might compare readings pre and post the first cat? Are my assumptions right, and will I DEFINATELY be ok MOT time if my main cats are still good? On a) - Has anyone done this and can they advise best / quickest method? Finally, is there any side effect I need to be aware of - I am presuming de-catting gives the turbos a chance to spool up quicker, and better flow, so with the Milltek I should get a power gain too? (My car is a standard K03'd S4 with MRC remap)?