Hi I have just aquired a standard 2004 wrx U.K.
I was thinking of putting a prodrive back box on it and replacing the 1st cat in the exhaust with a decat downpipe.
What knid of power gains should i get ??
Will I have any issues with warning lights and do I need to remap the ecu thanks in advance .

 

You'll get around 10 extra horses.

And you won't have to do anything with the ecu just for that.

 

If you can see the search mate please use it for questions like this in future, as it's already been asked a billion trillion times before!!!

Ignore the above post too - if you're taking cats out of the equation, you shouldn't "just leave it".
If you're going to have a remap, the best thing you can do is either:
1) Replace the up pipe and centre section with decat versions (the STI up pipe will do), and a Prodrive or other back box, and then a sports cat if you are worried about MOT. OR
2) Go full decat, and replace the lot for more power.

I'd be tempted to go for a full system rather than buying bits - that way you can be sure it'll all slot together nicely. There are plenty of them floating about on here, ebay, Jap Performance Parts etc.

By the way - the up pipe is the first cat in the exhaust, and not the down pipe as you stated (on a WRX that's the second one).
See here

You will need a remap after doing this because you could potentially damage the engine if you run it like that for long without having it remapped.
Best thing to do - get yourself an aftermarket panel filter (such as Green, K&N etc) or an STI filter and bung that in at the same time.

If you go for the lot with the sports cat I reckon you'll see about 270bhp, maybe 280bhp.
With a decat, possibly more.

Mine made 248bhp with a Green panel filter, and the resonator and backbox replaced.

 

Double Post

 

Righ i am confused at this, I HAVE A PRODRIVE rear box afterburner centre ok

Can i remove either the uppipe (pre cat) or the front cat converter with out getting it remapped ?

 

Possibly on a bugeye wrx but a remap will make sure it is safe and optimised (and get more power)

 

hi i just bought a bug on a x plate it has all cats removed so fails the mot,i dont know if its been remapped..
question is if i was to put a standard down pipe cat on it would it pass the emmisions test with the downpipe one in place ,

also my second lambda sensor is on the down pipe at the top behind turbo like a classic , arnt they normally on the third cat

 

it would probably pass the mot with the downpipe cat put back on.

They have probably fitted the second lambda there if there wasnt a boss for it lower down. It would throw up a cel code unless remapped or fitted with a resistor to fool it into not coming on.