Scooby Singh

Hi

I was talking to an evo owner at the weekend, and he suggested that I change my dump valve and remove the catalytic converter for better performance, (i think thats what he said, im a scoobie newbie and im not too hot on the lingo yet). Ive got a MY02 and havent had it for long so obviously im a bit unsure. This will probably effect the emissions so the catalytic converter will have to be put back on when going in for a service. Is this sound advice???

DaveR

I'm not 100 % sure on the dump valve - I'm sure someone else on here will know. However as regards the catalytic converter, yes, removing this will deffo improve performance (and low-down torque as well I understand), due to allowing the turbo to breathe easier.

The new WRXs have three catalytic converters in the exhaust, one in the backbox, 1 in the centre section and the third in either the up- or down-pipe (not sure which). I was told at TSL that you can remove both the back and centre cats, and still pass MOT emissions tests. However you could experience a problem if you remove the downpipe/uppipe cat (removing this one also provides the greatest increase in performance).

Hope this helps (I'm clearly not an expert )

DaveR

image doctor

Dump valves are gay, don't do it.

scooby-si

changing dump valve will not make ne differance to bhp

but do a full de-cat m8

Michael Robinson

yeah, removing the cats makes a big difference in performance and in noise. For full effect, put an induction kit on it too HKS Super Power Flow is good and listen to it burble !

Scooby Singh

Thanks for the replies people, but excuse my lack of knowledge what do yo mean by a full de cat. Ive heard the term a lot? Is it expensive?

Michael Robinson

de-cat means to remove the catalysts. Basically if you bought a new exchaust system (HKS HIPER SYSTEM) this is an exhaust backbox and mid section that has no catalysts on it, therefore giving power increasment and louder noise.

TARManiAC

Can anyone confirm this?

Is there a catalyst in the back box? (I don´t think so)
Is the up-pipe a catalyst?

Claus

Scooby Singh

My MY02 WRX is booked in for its first 1000 mile service can I get all three cats removed, or shall I go for the the one in the mid section and the up pipe, if so will it affect my warranty? I do want the car to sound better, however I will be changing the exhaust system but not just yet.

Cheerz

Michael Robinson

best advise it to not do anything until you buy a proper exhaust system, then get that fitted and if you can strech the budget to a downpipe too, the noise is amazing and power gains are fairly high.

Gav

if your doing it one at a time then do the downpipe first. you should get an increase of 20-25 brake, and the noise your scoob will make after is awsome...


gav...

chrisp

And it will stuff your warranty

HELLOM8

Yep, doing a de-cat will mean bye bye warranty. If you need extra performance then the Prodrive Performance Pack can be fitted and keep your warranty in tact, this is alot more expensive of course.

dosenöffner

Blow Off / Dump Valves

When you get to your shifting point you're most likely also on full boost. However, when you change gears and your foot comes of the throttle, the throttle butterfly closes and the turbo is suddenly forcing air into a closed chamber. The instant back-pressure against the compressor wheel slows it down so when you get back onto the throttle, you no longer have full boost available until the turbo spools up again. In other words, you get turbo lag between gears. A blow off valve quite simply vents the excess pressure in this situation, so the compressor is not slowed down so dramatically.

The valve is activated when the pressure in the plenum suddenly turns to vacuum as the throttle snaps shut. On an adjustable blow off valve, you can adjust the amount of boost that can be held in by the valve, so in effect it also acts as a secondary waste gate, bleeding of excess pressure. Of course, if the most pressure it can hold is less than the amount of boost you want to run, you have a problem. That's why competition blow off valves can be adjusted to handle higher boost.

When it comes to choosing a blow off valve for your application, there are certain key differences to consider. First is whether or not it is adjustable. If you vary your boost regularly it will probably be worthwhile to get an adjustable blow off valve. How far it can be adjusted is obviously another point to be considered.

Next is the question of how much air the valve can flow in the short time that the throttle is closed. If the valve is too restrictive, it won't bleed enough pressure and the compressor will still be slowed down. The higher the boost and the bigger the turbo you run, the more important this becomes. The actual construction of the blow off valve will also affect its reliability.

Early model Subaru's have a rather small capacity, while later models were increased substantially.
As a rule of thumb you should never fit an externally venting blow off valve to a Subaru. As opposed to a fully plumbed valve, an externally vented valve confuses the A/F meter. The ECU does not realise the air has been vented and tries to compensate for this by adding more fuel, resulting in a very rich mixture. The end result is misfiring and flat spots while accelerating. This is why the standard one is positioned after the A/F meter.

You can fit an externally venting blow of valve to a Subaru if you are running an aftermarket ECU, such as a PossumLink. This uses an oxygen sensor for autotune functions as opposed to the A/F meter. This negates the problems listed above.

Dream Weaver

Yep, Glen Seaton, Bathurst early 90's, red Sierra RS500. Had an almighty dump valve on it

dosenöffner

As did the later Texaco Sierras. And the Skylines when they ran there.

chrisp

Heard of a current professional race car with a vent to atmosphere dump valve ?

Michael Robinson

yes, i have one fitted to my car a HKS Sequential dump (cylinder shape) and when it dumps on gear change i get flame bursting through the exhaust (looks cool, but the police don't think so).

Spoon

Having just sold a Cossie which had an ex Steve Soper(Texaco)touring car unit fitted I must tell you it had 2 VTA dump valves.

Dream Weaver

You never see race cars with induction kits though - thats one thing I noticed

Michael Robinson

most "race cars" have air boxes like ARC do.

MRK_IV

"if your doing it one at a time then do the downpipe first. you should get an increase of 20-25 brake, and the noise your scoob will make after is awsome..."

How will the noise increase? I thought all silencing was in the mid section resonator and back box?

chrisp

Changing the downpipe does increase the noise but seems to give you more wastegate and turbo spool up noise. The really boxer flat 4 sound is really enhanced by the back box and adding the centre after this increases this further. Combine all three and it can potentially become very loud (depending on which systems you run of course)

johnfelstead

blow off valves are used to protect the turbo thrust bearings, they do not improve performance. You get a higher performance, faster throttle response from an engine with no dump valve.

The only reason engines such as the RS500 race engines used dump valves was because the presures where so high once the throttle buterfly closed, they used to machine away the thrust bearing as the backpresure pushed against the turbine. Turbos used to blow all the time mid race until they sused this out.

Road cars have dump valves as standard to allow them to use low spec thrust bearings and yet still get a good service life, they are not a performance assister, quite the reverse.

aftermarket dump valves can help in reducing the amount of leakage on partial throttle, but they wont give you any more performance.

chrisp

UK MY01/MY02 WRX has three cats

One in the downpipe after the turbo
One in the up pipe
One in the centre section

(none in the back box the last cat is in the centre section). The up pipe cat it use to give clean emissions from cold start.

There is a difference between de-cat and full de-cat.

Full de-cat removes all cats, you can also do a partial de-cat which as the name suggest removes one or two of the cats.

Pre MY01 Uk cars had two cats (didnt have the one in the up pipe). As do the UK STIs.

A lot of the grey import only had one cat - the downpipe cat.

Hope this helps

ChrisP



[Edited by chrisp - 1/8/2003 8:40:55 PM]

chrisp

Oh yeah leave the dump valve alone unless the OE one is broke/leaking in which case it may be better to replace is with a good quality recirculation type. The VTA dump valve are all noise and for scaring bus queues. Ever heard a professional race car with a vent to atmosphere dump valve . Can potential screw up the fueling as well as it will cause mixture to richen and you may lose bhp .

[Edited by chrisp - 1/8/2003 8:46:30 PM]