Downpipe De-catting - what's involved?

[calvin]

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)?

 

[aragorn]

It can be done without cutting the cat, but its obviously going to be more tricky and awkward. Ideally you want to mount the downpipe in a vice upside down, so any loose bits dont fall down into the main cat, then you drill/hammer/chisel/poke at the honeycomb to get rid of it. Or you slice the pre-cat off, knock it all out then reweld.

Only the later S4's have twin lambdas. I think even some early facelifts have twin too, and ye as you imagined on those cars the ECU doesnt care what happens after the sensor. On later cars with four you can either move the boss after the main cat, so it still gets its expected signal, or mess about with some "anti-foulers" which seems to fool it into working.

As far as i know the S4 ECU works to a target boost pressure, and the exhaust improvements should in theory mean the engine should flow more air for a given boost pressure, so you should get some more power.