De-cat advice for my WRX Please

Mr Windscreen · Post by **Mr Windscreen** » Wed Jun 16, 2010 7:32 pm

Just had my MOT this morning, needed emission testing twice to get through!, the tester was surprised i still had both cats fitted and advised i could get rid of the centre cat. Now i,ve looked on the various subaru forums and it seems the best way to do this is just to buy a centre section for the exhaust. Do i just bolt it straight on or would i possibly need a remap? I,ve also looked at sports cats for the front down pipe as i,m thinking of more power in the future. Have any subaru owners gone down this route as i,m not sure whats the best way to go about things.

fezza · Post by **fezza** » Wed Jun 16, 2010 9:17 pm

What model do you have?

The second cat is not in the centre section, its in a second pipe after the donwpipe, like the one supplied in this kit...

http://www.scoobyworld.co.uk/catalog/pr ... cts_id=388

If i were you i'd go for a sports cat full downpipe (replacing second cat too) and a decent backbox. That'll sort you out.

Bear in mind though, the more power the less likely that will pass the MOT with a sports cat.

Which ever option you chose i'd still get it mapped for it, it will also increase the power dramatically.

Mr Windscreen · Post by **Mr Windscreen** » Thu Jun 17, 2010 8:27 am

It,s a 2003 WRX with Green panel filter and prodrive backbox. Just missed a sports cat on e-bay by a pound!!

fezza · Post by **fezza** » Thu Jun 17, 2010 8:38 am

I've got an 04 WRX, K&N panel filter and full decat and it is running 286bhp - 61bhp up from standard and the torque is sooo much better.

In all honesty though, most of that power is from the remap, you could achieve 260bhp or so with keeping the cats.

Mr Windscreen · Post by **Mr Windscreen** » Thu Jun 17, 2010 12:26 pm

Did you fit an uprated fuel pump or have you kept the standard one?

fezza · Post by **fezza** » Thu Jun 17, 2010 12:31 pm

I have the standard one but if i were to go for any more power i would definitely need a 255lph on there. My standard one seems ok at this level though, its been mapped for 18 months like this with no problems.

Mr Windscreen · Post by **Mr Windscreen** » Thu Jun 17, 2010 12:51 pm

Ok cheers for the info. I,ve been offered a prodrive sports cat cheap so will probably fit this so that i don,t have to worry about MOT and then decat the second one. I was going to get dynamix to remap my car.

fezza · Post by **fezza** » Thu Jun 17, 2010 12:53 pm

Sounds good on the sports cat. That is who mapped mine for me.

You won't believe how much of a difference it makes!

ScoobieWRX · Post by **ScoobieWRX** » Thu Jun 17, 2010 1:53 pm

You have 3 cats in a newage WRX. A precat in the uppipe, main cat at the end of the downpipe and a scrubber in the midpipe.

Primary on your list should be replacing your std uppipe with a decat uppipe. That allows your turbo to spool early and quickly. Second in your list should be the scrubber in middle pipe, so a straight through or resonated midpipe should do the job. That will help again with spoolup but will allow your turbo to boost more freely and allow exhaust gasses to escape more quickly because you've removed a restriction.

If you have the extra dosh a nice backbox will enhance the burble and up volume a little bit but not too much. Won't really affect power though.

Leave the std downpipe and 600cell cat on it to get through emissions testing for the MOT or if you have a friendly MOT center go decat, always best.

You should stick with a 2.5/8" standard diameter tube midpipe with flanges that match up to the std downpipe fitting. That will help maximise torque. Too big a diameter of pipe and you will lose torque. Your TD04 Turbo isn't built for Bigpower ( see what i did there

) but it should spool up very nicely from around 2500-2600rpm and make really decent torque low down which is exactly what you want with a newage WRX to make best use of that lovely smooth nicely ratio'd 5spd box.

Add to the exhaust mods a high flowing flat panel air filter and Snorkus delete modification (airbox resonator delete) and you should be seeing between 270 and 280 bhp with around 300-310ft/lb torque based on the std injectors and TMIC.

I went for my MOT today. A sports 100 or 200cell catted downpipe will not get you through emissions testing unless it is very very hot. It's not far off but not close enough to get you through so if emissions is a big prob for you stick with the standard downpipe and 600cell cat.

I remapped a modded WRX last week just before my recent operation and it was fully decatted. It made a very healthy 275bhp with 307ft/lb torque. It's not finished yet because the actuator spring on his turbo is very weak and not holding boost plus he's going to put a 3-port electronic boost controller on it to help it acheive the numbers we want. I couldn't get more than 13psi out of it and I was looking for another 4psi for some extra torque low down-midrange, and a little bit more power top end. Your TD04 should safely make 16/17psi boost no probs. I've never ever had to push one over that to make decent grunt.

Power delivery was very smooth and linear, and the very prominent flat spot it had around 4500-5000rpm was cured.

Hope this helps

Mr Windscreen · Post by **Mr Windscreen** » Thu Jun 17, 2010 2:23 pm

Thanks for all the info. The prodrive sports cat is very cheap so will try it and see, can always sell it if i have problems. The middle pipe i have seen second hand for around