since fitting the downpipe I think I've got a little overboost. just wondering if it can be cured? if so how? and how easy is it to do? could a complete fool like me do it?

 

What type of car?

What other work have you done to the car?

Only the twin turbo models would suffer from overboost by making just that change.

Dan

 

Car has full decat exhaust and HKS 'green mushroom' induction kit. It's a MY94 WRX Wagon

 

I get what I think is overboost in 1st on my 95WRX with de-cat and uprated panel filter. It seemed to appear when the weather got colder and damper. Hopefully it will go when the weather improves. (Or I get it chipped)

 

ZAZ,
guess I've got what you've got, if that makes sense.

It'd certainly liveable with, but ironing it out would be good, especially as the RR day is coming up and this will effect my results, but maybe PE might help out to fix it, dunno?

 

Zaz, I don't think your problem is overboost. Colder/damper air would tend to cure overboost/detination

Overboost is a missing at full throttle, can be quite severe inside the car. Most Subarus have a boost cut at 14.4 PSI which is controlled by the ECU, when you hit that YOU WILL KNOW.

 

Overboosting in *more* likely as the weather gets colder - not the other way round......

The two usual fixes for this are playing around with the restrictors in the Turbo Nipple pipe or getting something like a Dawes and setting the Boost level to one below the fuel cut....

Or of course you could get an EBC or even a whole new ECU (if you want to spend a lot more)

Matt

 

oooh, dawes. mmm nice! like that idea.

 

To cure overboost you simply need to fit a slightly larger restrictor - it is in the pipe between the boost solenoid and the wastegate actuator - the larger the boost the slower the boost rises. Increasing the dimensions of the boost hose also has this effect.

I am not sure a DAWES would address an overboost problem - it simply messes with the target boost through a bleed off mechanism, rather than control the rate of rise.

Trout

 

I bow to Mr Trout as rumour has it he knows what he's talking about ( )

However, with the Dawes you can set the amount of boost, and there is (apparenly) almost no overshoot so wouldn't this cure the problem (I did also mention the restrictors). Does the speed of spool up have anything to do with this? (other than perhaps the increased speed causes a bit of overshoot?)

Matt

 

Aren't there differences between Euro and Jap cars though? I *think*, if you've got a 3-port solenoid, then you need to make the hole smaller?

Regardless, best (apparently - I'm still on the restrictors!) is to use an adjustable needle valve if you want to retain the boost solenoid functionality - at least then you can change it easily as the weather changes. Talk to John, Markus - he used this at one point to control boost with the solenoid still in place.

The Dawes is best suited if you want to do away with the boost solenoid altogether.

Richard

 

I belive overboost is common after changing the downpipe for the simple reason that the turbo spools up quicker and can overshoot. Our cars do not use closed loop boost control during spool up (although the MY01 very well may do) but on MY99/00 there is closed loop control of held boost, but it is of narrow bandwidth and cannot cope with breathing mods if your boost was near the point where it fluctated before (ie optimal). The solenoid and restrictor can also be sticky with oil and can be cleaned - a good first step before changing anything.

You need to know it is actually overboost though by watching a gauge - you cannot just assume this is what it is.

The Dawes on my car allows me to select between 9 and 20+ PSI, so could cure overboost. It is not a bleed valve any more than I am Miss Piggy.

http://www.scoobynet.co.uk/bbs/threa...threadid=56487

See here for the description of using a valve on a MY99/00 - and note your car will be different - you MAY need a smaller restriction in a different pipe to cure overboost.

The Dawes drastically increase the rate of spool up and lowers the point where it occurs and hold the boost longer up top. It seems to be able to do this on a large turbo with little overshoot, and is an ideal cure for overboost - it was on my MY00 PPP anyway - I am now holding 1-2 PSI more than I could before with much quicker and earlier spool up and fewer peaks. It makes as much difference as the PPP again (not difficult some would say )

[Edited by john banks - 2/15/2002 11:41:46 AM]

 

John, I bow to your knowledge on this subject, have read your threads about the things you've constructed (AFR readout??) and know you know what you're talking about.

I do have boost gauge, and what I see when I plant the ol right foot is it goes up to about 0.9 on the gauge, then drops to about 0.6/0.7 then goes back up to 0.9 then drops once again, then seems to be ok.

this has only occured since the DP went on, and yes the turbo spools up quicker, so this is probably why it's happening.

I think, as the car is an import, it has 3 port soleniod, what difference this makes I'm not sure.

How do I go about cleaning things? pictures and nice text description anyone? I'm a complete novice to all things mechanical, so don't want to blow anything up!

 

Classic description of underdamped boost control there. Don't know about the early models, but the late models do this about 1 PSI before "the big one" fuel cut. You can be at WOT in a MY99/00 and oscillating the boost between 12 and 17 PSI with the wrong restrictor size for your breathing mods. Milder forms are overshoot undershoot stabilise, and then short oscillations and then continuous oscillations in the worst case as above. Very unsatisfying to drive - feels great during spool up and then the solenoid pulling back the boost is REALLY disappointing. It is the mark of a closed loop system with excessive gain - cleaning things or changing restrictors will sort it.

 

glad you understood me!

so, pics and description of cleaning (soleniod?). I vaugley recall someone mentioning cleaning it using brake cleaner? this correct?

 

On a late model you can plug the diagnostic connectors under the dash together and put the ignition to on but engine off and the solenoid will cycle and you can then squirt brake cleaner into the solenoid and let it evaporate. Can't advise you where the restrictors are exactly in your model. Maybe someone will oblige. But is does seem most likely that cleaning won't work if the downpipe caused it as it is a system change not oil contamination from the sounds of it.