Exhaust - Catalytic Converter position

[benharris]

Backstory: The gasket between the turbo and the downpipe on my daily-driver Impreza (classic shape) is blowing, so needs to be replaced and looking at the work involved, it seems to make sense for me to replace the notoriously restrictive standard downpipe which is currently fitted, to free up a few more ponies, whilst I'm at it.

Currently the (only) catalytic converter which is fitted is in the OEM downpipe and this is one of the main reasons I'm led to understand that the downpipe is so restricted, as it's so close to the turbo. Now, I can buy a downpipe with a sports cat fitted in it to replace the OEM one with, but most of them are 3" where they connect to the centre exhaust section and the rest of my exhaust is a 2.5" system. The only downpipe with a sports cat I can find that end in a 2.5" is hideously expensive (twice the price of a 3" one). To add to this, I've already got a decat downpipe that ends in 2.5" waiting in the shed which I bought a while back, that will fit perfectly with the rest of the system I already have - however, this would mean I no longer have any cats fitted at all - so not MOT friendly. :-(

So... one of the options I've been considering was to buy a 200 cell sports cat and get it welded into my existing centre section. However, what concerns me is that no-one else seems to have this setup. All the information I've been able to find seems to be from people who have the cat in the downpipe. Now I realise that ideally the cat wants to be as close to the engine as possible, which I'm assuming is why it's normally in the downpipe, but I've had other cars in the past where the cat is in the middle of the exhaust, albeit they've been standard cats, not sport cats.

I guess I'm looking for general (i.e. not Impreza specific) advice as to whether the above is a sensible/feasible thing to do or not, or am I potentially going to cause myself problems come MOT time by moving the cat location? Is there a good reason why everyone who retains the cat seems to have it in the downpipe rather than centre section, or is it likely to be just that they've used off-the-shelf parts?

Any advice appreciated! :-)

[stevieturbo]

Cats need to be hot to work effectively, hence they want to be close to the engine.

Just buy the 3", it will mate up to the exhaust just fine, as almost all of them step down to 2.5" at the mating flange anyway.

[anonymous-user]

If you have the Cat anywhere but right up front, you'll need to make sure you run the car under load, or "rev the sh*t" out of it immediately before you let the MOT guy hook up the gas analyser. Otherwise at idle, the car will "go out" (drop below the temperature at which it actually converts) and your car will fail the MOT test. On my rally car, i have an MOT map (not just VW eh ;-) that runs with a massively retarded ignition angle and lots of throttle to maintain airflow at a fast idle. This is because the Cat is in the tailpipe! (only place it lives for more than about 2 mins with the ALS active.....)

[benharris]

That makes a lot of sense. Sounds like the sports cat downpipe is the way to go then. All of the decat downpipes seem to end in 2.5" but the only sports cat downpipe I've found which converts down to 2.5" is the Scoobyworld Afterburner:

http://scoobyworld.co.uk/catalog/product_info.php?...

All others seem to stay as 3" all the way through, including the mid-pipe connector. If you know of any others that I might have missed, please let me know! Alternatively, I have found an adapter to convert a 3" downpipe to a 2.5" mid-section, so that may be a suitable solution, too:

http://www.grimmspeed.com/3-downpipe-adapter-after...

[benharris]

OK - so looking at it from a slightly different (cheapskate money saving!) angle where I fix my gasket leak but keep 'an' OEM cat - I also have (in the shed) the original centre pipe with cat.

So... keeping in mind that I've already got all the bits to do this, if I were to keep just one, non-sports cat, would I seem any benefit by replacing my current OEM catted downpipe with the decat downpipe and refitting my centre section with an OEM cat? I know this should be OK from an MOT/emissions point of view, but am wondering if by moving the cat (restriction) away from the turbo I'll see any benefit to spool speed?