De-cat advice for my WRX Please

ScoobieWRX · Post by **ScoobieWRX** » Thu Jun 17, 2010 4:00 pm

If you look under my bonnet you see a v.8 Sti lump making a very modest 350/360bhp at the moment. It still looks pretty damn std though as i'm still using the std TMIC and airbox.

In my honest opinion there's nothing better than a car that goes like the preverbial but still looks pretty much bog std and retains as much std kit as possible.

I'm sure once Duncan has sprinkled some fairy dust on your ECU it will drive very nicely. Good luck

stockcar · Post by **stockcar** » Thu Jun 17, 2010 4:21 pm

be careful how you describe the 2nd cat location as its not in the "mid-pipe" but as the secod part of the 2pce downpipe that new-age cars have (unless JDM spec)

you then have a "centre" pipe and back box bolted on the back of it

ScoobieWRX · Post by **ScoobieWRX** » Thu Jun 17, 2010 4:53 pm

Std newage UK WRX exhaust systems after the turbo come in 4 pieces. A short downpipe with a built in cat, and another cat bolted to the end of that that forms the front part of the of the std midpipe, and another pipe with a built in resonator bolted to that which forms the rest of the midpipe, and then the backbox.

Normally if you fit an aftermarket system it replaces the 4 piece with 3 pieces so you're left with the downpipe, midpipe and backbox.

If you fit a prodrive sports cat the short downpipe it's built in to is shorter that a normal aftermarket downpipe so the new 2nd hand aftermarket midpipe you buy will have to be as long as the last cat and rest of the std midpipe together so if you look at it as one piece overall it will be like a 3 piece system. Whichever way you look at it you still only want one cat in the whole system.

Just don't forget the decat uppipe.

BTW....The last std cat that's bolted to the end of the std downpipe is known by Subaru, and look through a workshop manual just to confirm that if you like, as the front centre pipe. Just to clear that one up.

Mr Windscreen · Post by **Mr Windscreen** » Thu Jun 17, 2010 5:41 pm

So just to get this right, i fit the prodrive sports cat, a middle pipe and my backbox? Just noticed the sports cat i,m buying comes with another short pipe that it bolts to. Is this the decat uppipe?

ScoobieWRX · Post by **ScoobieWRX** » Thu Jun 17, 2010 5:57 pm

The thing with a Prodrive cat is that the part itself is quite short so you would have to bolt your std final cat to it, then the aftermarket midpipe, and then the backbox so it's still a 4piece exhaust.

I don't know if you can get an extra long midpipe to fit between the Prodrive cat and the backbox. Alyn will be able to tell you if that's possible.

The only other ways i know of is to either manually decat the catted pipe that bolts on to the end of the downpipe or have a custom extra long midpipe made up. That is a lot of faffing about though so it looks like you will have to have two cats in the way.

The usual way is to buy a 3 piece aftermarket exhaust system that has a sports cat in the downpipe, but IMHO i think that's a waste of time as they don't really work untill they are very hot so you may as well go decat and find a Subaru friendly MOT place which is much easier

BTW....I don't know how dense or how good the Prodrive cat is. Is it s 200/300/400cell cat??? Mine is 100cell and although my car will pass on it's lambda and hydrocarbon reading it won't pass the CO test. Mine reads around 0.4% -0.6%. Fully decat will read between 1.5% and 2%. A perfect pass will be 0% and the just passed will be below 0.2%.

Perhaps a 200cell cat will get you a pass...i don't know

Mr Windscreen · Post by **Mr Windscreen** » Thu Jun 17, 2010 6:07 pm

The sports cat i,m buying comes with an extra pipe to bolt it too. I,m hoping this is long enough to fit it to the middle pipe.

Jon · Post by **Jon** » Thu Jun 17, 2010 6:22 pm

For what it's worth, I look after & test a lot of scoobys and i've never had one pass with a sports cat; Whether it be a Prodrive, Milltek etc etc. Even getting the sports cat glowing red hot they still fail the CO test. Total waste of money imo, Far better to go full de-cat and refit the OE item at MOT time or as Francis has said and find a friendly...

Tonynos · Post by **Tonynos** » Thu Jun 17, 2010 7:32 pm

Micared wrote:For what it's worth, I look after & test a lot of scoobys and i've never had one pass with a sports cat; Whether it be a Prodrive, Milltek etc etc. Even getting the sports cat glowing red hot they still fail the CO test. Total waste of money imo, Far better to go full de-cat and refit the OE item at MOT time or as Francis has said and find a friendly...

Is the CO2 level for a pass on a newage lower than a classic? I had a Ninja fast flow cat on my classic and it never once failed the MOT on emissions.

ScoobieWRX · Post by **ScoobieWRX** » Thu Jun 17, 2010 9:06 pm

Mr Windscreen wrote:The sports cat i,m buying comes with an extra pipe to bolt it too. I,m hoping this is long enough to fit it to the middle pipe.

In which case, problem solved

stockcar · Post by **stockcar** » Fri Jun 18, 2010 7:38 am

yep thats exactly what you require as it mimics the original 2pcs down-pipe set-up of the O.E car........(as i tried to explain in my post above)

the Prodrive cats will pass an MOT but they can be hard work to sort..........