There's been quite a lot of talk for a while now about fitting a smaller grommet in the turbo wastegate pipe - from 1.2mm (standard MY00) down to 0.8mm (standard P1). Causes the wastegate to close a bit later, allowing a bit more boost.

Tried it on my MY00 today with strange results! It does indeed give more boost - about 1/4 throttle in third from about 40mph... boost gauge swings round temporarily to <B>24psi</B>, promptly cuts out (not violently like others have found thankfully) back to very little boost, then back up again etc, cycling about every second. A similar exercise in second nearly resulted in spinning the wheels at about 30mph!

Clearly it's cutting the boost back when it's getting too high, which is a good thing IMHO as it's protecting the engine, and sustained 24psi probably isn't a good idea http://bbs.scoobynet.co.uk/wink.gif. However, watching the boost gauge and accelerating gently shows it's actually cutting back at about 10psi now (normally get 15psi peak, 14.5psi held in this weather), so basically it's confusing the system.

This is weird, as StanS has had it fitted with great success to his '98 (?). Any ideas? I think the best thing to do is bore it out about 0.1mm at a time until it stops doing it, but for now I can drive very fast for about 0.5s, then slowly, then very fast for 0.5s, etc http://bbs.scoobynet.co.uk/smile.gif or just keep it off turbo of course.

Any of you Scoobers had any experiences with this mod?

Richard

Hi Richard,
I have heard that this "oriface" has different effects with different year cars. Mine seems fine - no cut out, but there now seems to be a slight rough/flat spot around 6K (for about 200 rpm) which may be related. Still think the oriface makes a difference - although I wonder if I had the optional "easy to fool" brain fitted at birth.
I wonder if my rising rate fuel regulator is making it run too rich, coking the plugs so when I give it some, it takes a few blasts before it clears - it seems to be fine when really hot.
Stan

I have made a full set of these restrictors going up in 0.1 mm steps and have ended up using the original .I made them when I was trying to cure the overboost problem after getting fed up taking the car to the dealers.

My car is a my98 and has the two port solenoid which works the same as yours ie making the hole smaller increases the boost.

The 0.2mm smaller(1.0mm)restrictor gave the boost cut every time.

The 0.1 mm smaller restrictor increased the boost by about 1.5 psi but I still got the cut occasionally.

Contrary to popular opinion all Imprezas come as standard with a restrictor and they are not there to restrict power.All they do is set the range over which the boost control solenoid can work.

As standard my cars ECU wants to control the boost at about 13.5 psi.With the standard restrictor it can weather its a hot day or a cold day etc.(Sorry about the pun)

When you fit a smaller restrictor the ECU tries to lower the boost to standard by reducing the 'open time' of the boost control solenoid so less air is vented and more is applied to the wastegate actuator.However the ECU only has limited control over the solenoid so the boost rises above standard.

This is good from a performance point of view but means the boost is no longer under control and will vary with the weather etc.

This means you are trying to run it closer to the boost cut without any control.

My car would not run at the higher boost level without occasionally hitting the boost cut.I got fed up of it cutting out when overtaking and put it back to standard.

Having said all that you may find it works OK on your car

PS The smaller restrictors make the wastegate open a bit later not a bit earlier

Andy

Stan - I remember you saying about someone else with a MY00 who had overboost problems too - I just didn't realise it would be to this extent!

AndyMc - thanks for the explanation, interesting... I suspect I'll end up with the original back in, but might go via a 1.0mm and 1.1mm first, as you did, just to see. I'll keep you posted...

Cheers
Richard

ps I said later, not earlier?? http://bbs.scoobynet.co.uk/confused.gif