Scoobynodd

iTrader: (1)

Hi all, I know it's advised that when de-catting you should ideally remap.

However, I also own a twin turbo Supra. Removing the 2 cats from these is also a popular mod and to avoid/hold off remapping until further/more serious improvements are wanted a restricter ring/gaskit (metal plate with smaller diameter hole) is installed in the line of the exhaust system that limits boost to its standard 1.2 bar and prevents overboosting. I was just wondering if a similar procedure could be done with the WRX?

Shaun

Absolutely futile to fit a decat and not remap as well. You are just wasting your cash, regardless of what you do to reduce boost.

Scoobynodd

iTrader: (1)

Shaun, that isn't really answering the question.

Also, I'm not looking to 'reduce' boost, just stop it/keep it from creeping up higher than where its supposed to be which is exactly what will happen if the cats are removed!

And don't please don't assume I'm new to all this as you couldn't be further from the truth. But what I have found over the years is the impreza turbo engine seems to be one of the fussiest around and far too many people on here offer more conjecture than genuine techinical input. Not to mention that you should never under any circumstances ask on this forum which oil's best ...because that is a can of worms!

What's even more futile is remapping after doing nothing other than fit a Blitz exhaust system and that's where I'm at the moment!



(apart from coilovers, brakes, bushes and other bits)

joz8968

iTrader: (13)

I don't think the WRX suffers form probable overboost spiking, compared to if you decat an STi?

Whatever, best thing to do is to hook up a boost gauge, and after decatting, monitor for any overboosting. That way you might not need the temp. fix - and hassle - of a restrictor plate....

NEILB1

iTrader: (9)

how much is over boost and wht is normal boost ?

joz8968

iTrader: (13)

'Normal boost' is whatever Subaru chose to program the ECU to deliver - in a std car. It varies from model to model.

Anything at +/= 0.02bar over std is considered overboosting I think... And also if it regularly does it.

Don't forget that even normal target boost can vary up or down, depending on what transient info is being fed into the ECU by various sensors, etc.

Obviously, cold days can see higher boosting. But the ECU is controlling it and 'taking care of everything'... so that's when it's okay.

Scoobynodd

iTrader: (1)

Thanks Joz, that's an idea that way I could actually finitely monitor the boost myself, onhand. That way if I get concerned I can refit a cat for a while maybe.

Neil, another way of describing the effect that is 'overboosting' would be to describe it as the turbo version of your clutch slipping (when you clutch is passed it); when your clutch slips the engine races away (the revs climb) but the physical car speed doesn't increase with it - the engine sounds like it's racing away and trying to leave the car behind lol. Now when a turbo charger overboosts it's a very similar state of affairs (but depends how loud your exhaust is - you might not hear it!) but you can hear the rush of turbocharged air getting louder/increasing and yet the engine not gaining power along with it.

Joz, which boost guage would you reccommed as I've heard some being more reliable/accurate than others. Defi? Or say a digital one?

joz8968

iTrader: (13)

Dont get a digital one - hard to immediately realise the boost level.

With an animated needle, you know at what part of the boost gauge you EXPECT it to shoot to...


I have - and people will scoff at this - a Glow Shift cheap £35 one. They work perfectly - the indicated boost level is precisely what my car is supposed to be producing (plus the font used all but matches my STi 5's clocks too lol). I also had one on my previous WRX too - and that was bang on accurate on that as well.

It's this white faced one and you can press a button to choose from 7 backlit colours, to suit your mood and/or to have max. legibility and/or to match your binnacle's clocks. It remembers the last colour chosen upon start-up, so you don't have to keep readjusting every time... :-

Glow Shift White Series -1.0 to +2.0 bar boost gauge

Scoobynodd

iTrader: (1)

Can't knock it if it works and you're happy with it Joz :-)

I'm quite liking the black face/blue characters and red needle ensemble lol

joz8968

iTrader: (13)

Yeah, when ign on, it's the markings that light up and the needle lights bright red - really easy to read in all conditions...

joz8968

iTrader: (13)

Also, I like pure mechanical ones - they're so 'immediate'

The dearer, more complicated electro-mech. ones' needle is slower to respond to throttle inputs.

poodlemuffer

im in the same boat with wanting to decat a 02 sti , my background is turbo honda's and if you mentioned a cat people would think you were silly. always been the case of 3" downpipe and exhaust. i dont quite see what the problem is , can the wastegate not regulate the boost ? or is it the ecu not keeping up ?.

Shaun

You are looking to reduce boost as you already know that boost will increase, even if it's back down to what you think it is without the decat. That was what I was referring to.

What about compensating the ramp rate of the boost. The fueling and ignition will be set to account for the ramp associated to the back pressure that is currently there. You reduce the back pressure and the boost will build earlier and quicker. I have no idea what effect this will have, as I have no idea on what the OE fuelling is like lowdown (assuming yours is still on an OE map).

Less back pressure will mean that whatever wastegate duty cycles are set, will now be out of kilter with the actual boost achieved. Overboost spiking imo is only one area that could be affected, as I would not be surprised if held boost would also be altered.

This is one of the reasons why it is recommended to drive off-boost if decatting and waiting for a map to suit.

Perhaps you could replace the boost solenoid pipework with a clear bit of vac hose. This would remove the restrictor and knock the boost right down. It could be re-fitted when you get it mapped, if your bothered about driving it around.