Thoughts on Catalytic Converters for S3

[Imola-S3]

Hi again,

I've recently had a full stainless exhaust system fitted on to the car. My aim was to go as quiet as possible and to begin with my exhaust fitter used a 2.5" post CAT pipe with two decent sized silencer boxes, which led out to a large twin exhaust tip.
Unfortunately, the exhaust fitter didn't really consider the expansion of the pipe work as the cars engine got hotter and the original layout of the exhaust caused one of the bends to catch on the rear diff / prop shaft section - NOT GOOD. Also, he'd used an exhaust tip with a 2" inlet - meaning he was downsizing from a 2.5" pipe at the back box and taking it down to approx. 2", which when coming in from a sharp bend, plus splitting off into two exits caused some awful whistling when the turbo was on boost at higher revs (obviously the air coming through the exhaust system was sort of recompressing itself as it exited the exhaust).

Anyway... I took the car back and he said that keeping a middle box would be tricky in avoiding the prop shaft due to the bends etc - perhaps he was just being lazy - but he offered to swap out the middle box with a 100 CEL Milltek Sports CAT... as it should help to reduce the noise a little - plus it would benefit my emissions as my existing 200 CEL Sports CAT wasn't quite doing the job right.

My current layout is like this

3" Downpipe to 200 CEL CAT, 3" Pipe into 100 CEL CAT, Downsizing to 2.5" pipe through one large back silencer box and now leaves through a 2.5" letterbox style resonated exhaust TIP.


What I would like to know is.... would adding this 2nd CAT have any negative effect on the cars performance? Because to me , it seems as though the car isn't pulling quite so strongly as it did before - or could it be that because the car is now a lot quieter due to there being no screaming out the back due to the 2.5" to 2" downsizing as the exhaust TIP that it doesn't seem as fast.

If the exhaust gases have just travelled through a 200 CEL Cat I would've thought that it wouldn't be much of a hindrance having a freer flowing CAT situated immediately afterwards. Or am I just messing up the flow and doing more harm than good?

Cheers in advance for your time and responses

 

[Alex C]

I would have thought that if the 100cel cat is more restrictive than the middle box it's replaced then it's more restrictive regardless of its comparison to the front 200cel cat?

 

[superkarl]

The best are 1 CEL cats, as in, one big round pipe.

But yeh, in terms of performance your exhaust doesn't sound too great. 2 cats? 2.5 inch AND 2 inch at the back? Not great at all.
Also, don't rule out the tune and other reasons as to why you can't pass emissions

 

[Imola-S3]

I've had that 2" exit sorted out... so it's 2.5" all the way through from the CAT and 3" before the cats right up to the turbo.
It made 273bhp last month on the 2" exit pipe but it had one CAT fitted then. I've since added a 2nd CAT and had the tail pipe changed to 2.5"

I was just looking to see if this would've had any adverse, or limiting affects on power? Or should a setup like this still achieve a minimum of 300bhp?

 

[ayfreetee]

You would probably have been better off with the standard set up if you want it to be quiet.