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Air intake upgrade (plus a few other bits)

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Hi all,


Am new to the forum and have had my 2003 JDM WRX STI just under a week now and already want to start playing around with a few bits here and there.


First of all can anyone recommend an upgrade for the air intake? I personally don't think much of K&N and am looking to replace everything right back to the inlet. Preferably sticking with something japanese so AEM, Apex-i etc. Links will be much appreciated!  :)


Secondly, I get a slight "dumping" noise when i let off the accelerator and was wondering if this would be heard more from the upgraded air intake? Replacing the OEM recirculating valve has crossed my mind but again not sure what subaru's respond better to, dump valves or wastegates. I do know that wastegates dont do the turbo much of a favour on any car that doesnt have a roller-bearing turbo though. 


Last of all, how many cat's are there in a standard STI exhaust system? I have been told there are two and that one has already been removed on my car but the noise doesnt seem to suggest so. even if it has, can i remove both and replace them with straight pipes? The car isnt loud, just has a nice burble and would quite like to keep it that way!





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Wotcha and welcome Owen,


removing the box and panel and replacing the lot with a cone and larger tube will give you a little more induction noise, do you have any sound proofing on the underside of the bonnet - if so you could remove that and get a slight increase in overall volume from the engine.


You have named some top recognised aftermarket parts suppliers so I would say it would be more down to personal taste for looks as much as anything else


I believe they do have two cats in place as standard a straight through pipe will make it louder but with the sound different exhausts will vary the tone you (again down to personal tatse) tube will give you an idea and we have had some baffle in baffle out vids posted on the forum but be aware some of the members describe rattling neigbours windows and light fittings so the potential is there should you want it.


I would recommend getting a map done to complement any changes you make to teh exhaust and induction side of things.


it has been discussed on here about recirc vs dump valve and the dump valve will give you a great sound (again we have threads with vid links to different valve sounds or you tube search) but will cause the car to run rich momentarily resulting in some flamage in the exhaust :P  a recirc will do the same job but without the fuss so again down to personal taste


One thing I would say before you spend loads on aftermarket goodies check you have a good foundation, good recently changed oil, filter and plugs as you could be bolting things on and not achieving the results you would like

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Welcome along.

Didn't the later cars have 3 cats? Down pipe, centre and a small cat on the up pipe?

As jay said, the cone filter will give you much more of a sucking noise it's a kind of constant gurgling noise that increases as you put your foot down. You can really only hear it with the windows open.

The recirc doesn't really give any noise off in my experience, however something like an HKS bov will give you a great loud whoosh every time you come off boost and if you are running a full decat exhaust this will be accompanied by lots of pops and farts from the exhaust on over run.

I had an HKS green mushroom induction kit, HKS bov, and a full stainless steel decat with a Scoobyworld Afterburner Vortex exhaust. More than enough to create all the gurgling, whooshing and popping you'll need. I drowned out a Mustang V8 at a car show once with that set up :D

I was good for at least 108db.

Also check out if there are any local meets, well worth popping to one to meet other like minded people, although I'm not sure if you'd want to like mined with Gambit, he's a bit of a nutter tbh :p

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Cheers for the advice guys,


one other quick question - would i need to install a variable boost controller? mechanic mate of mine said theres a chance with the dumpvalve that there wont be enough boost to give a decent noise and therefore it may be needed. He also mentioned possibly having to fit an aftermarket actuator i.e. Forge


Is this a problem anyone has encountered before?

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I've not heard that before or had to do that with either of mine just to fit a dump valve  :wacko: And can't see why the impreza wouldn't be the same as your's.

The only thing I have heard is very few people complain that after fitting the car didn't run very well, and had to remove it and go back to OEM. And nobody seems to know why this happens, just get's put down to the car not liking it. 

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If I remember correctly the standard boost you would be running is 1 bar (around 14psi) that is enough to give you a nice sound when discharging, my old nissan ran 9 psi as standard and with a DV fitted you could hear it no problem.


The aftermarket actuator will only change things if it has a different length rod or a different spring weight to the standard item, if not it will be a shiny version of the original, Forge will supply an actuator and change the spring according to your requirements.


If you are going to start playing with boost I would advise getting a decent guage fitted so you have a good idea of what load the engine is under when you are tweaking.


Make any changes to boost levels in line with an appropriate fuel map (dont just bolt it on and wind it up) as you dont want it to run lean under higher boost, you may also find your fuel pump or injectors are unable to supply enough fuel under higher boost conditions

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