HELP WITH BOOST CONTROL PLEASE!!!
07 November 2002, 04:39 PM
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Need some help!!! This is a frustrating story!!!
I fitted a ScoobySport style turbo back catless exhaust made in South Africa to my 02 STi and a MRT boost controller.
I could not get the BC to hole boost above 5000rpm. Between 3000 and 5000rpm the boost was held solid. I then connected the wastegate direct to pressure side of the turbo (by-passing the soleniod) and re-fitted the standard exhaust and checked to see if the boost would be held at 0.6bar, which is the setting of the wastegate spring. This worked 100%. In other words the wastegate opened at 0.6 bar and held this all the way to the redline. I then kept the wastegate connected straight to the pressure side of the turbo and put the catless exhaust back on. The boost stayed at 0.7bar from 3000rpm till 5000rpm then just started to rise to 1.1Bar by 6000rpm. The problem must be the exhaust!! But what is the problem??? The wastegate is not catching anywhere otherwise the boost would continue rising after 4000rpm at least!! The spring in the wastegate must be fine since with the standard exhaust the boost is held at 0.6 bar all the way to the REDLINE. There cannot be any more backpressure with a catless exhaust compared to the original exhaust forcing the wastegate closed. But why does the boost rise to 1.1 bar by 6000rpm and stay there till the redline???
I am totally confused.
What about you?
I fitted a ScoobySport style turbo back catless exhaust made in South Africa to my 02 STi and a MRT boost controller.
I could not get the BC to hole boost above 5000rpm. Between 3000 and 5000rpm the boost was held solid. I then connected the wastegate direct to pressure side of the turbo (by-passing the soleniod) and re-fitted the standard exhaust and checked to see if the boost would be held at 0.6bar, which is the setting of the wastegate spring. This worked 100%. In other words the wastegate opened at 0.6 bar and held this all the way to the redline. I then kept the wastegate connected straight to the pressure side of the turbo and put the catless exhaust back on. The boost stayed at 0.7bar from 3000rpm till 5000rpm then just started to rise to 1.1Bar by 6000rpm. The problem must be the exhaust!! But what is the problem??? The wastegate is not catching anywhere otherwise the boost would continue rising after 4000rpm at least!! The spring in the wastegate must be fine since with the standard exhaust the boost is held at 0.6 bar all the way to the REDLINE. There cannot be any more backpressure with a catless exhaust compared to the original exhaust forcing the wastegate closed. But why does the boost rise to 1.1 bar by 6000rpm and stay there till the redline???
I am totally confused.
What about you?
07 November 2002, 06:40 PM
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You're not the 1st to see this. I think there is another thread in Drivetrain from EMS about the same problem. If you do a search for Boost Control you should find it.
There was another largish thread about the problems with Boost Control with a Full decat on the MY02 STi's.
Matt
There was another largish thread about the problems with Boost Control with a Full decat on the MY02 STi's.
Matt
07 November 2002, 06:50 PM
#3
This is happening on my car (94 WRX)
I've got a turbo back HKS and now think the HKS downpipe is my problem.
Is your downpipe open neck?
Mine isn't and I'm starting to think the plate that bolts to the turbo is restricting the gas flow of the wastegate at the top of the revs. This does make sense to me now as the downpipe is in line with the compressor blades on the turbo so flow there will be better than flow at the wastegate area which is hitting the plate of the downpipe where it bolts to the turbo.
The extra flow of the performance exhaust and the restriction of the wastegate might be causing this problem of boost rising at the top of the revs.
James
I've got a turbo back HKS and now think the HKS downpipe is my problem.
Is your downpipe open neck?
Mine isn't and I'm starting to think the plate that bolts to the turbo is restricting the gas flow of the wastegate at the top of the revs. This does make sense to me now as the downpipe is in line with the compressor blades on the turbo so flow there will be better than flow at the wastegate area which is hitting the plate of the downpipe where it bolts to the turbo.
The extra flow of the performance exhaust and the restriction of the wastegate might be causing this problem of boost rising at the top of the revs.
James
08 November 2002, 05:40 AM
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The exhaust was originally a twin dump design with a seperator plate between the the two pipes. It is originally designed for the pre New Age model Impreza. Apparently the wastegate of the pre New Age Impreza turbo opens away from the main exhaust down pipe towards the smaller wastegate downpipe. Therefore the twin downpipe system makes sense for the pre New Age Impreza. The New Age MY02 Australian Spec STi, which we have in South Africa, waste gate opens towards the main exhaust downpipe. It therefore opens in such a way that the exhaust gases from the wastegate blow directly into the seperator plate between the two downpipes. With this pipe the boost would start rising from 5000rpm to reach a maximum of between 1.2bar and 1.4bar (depending which gear, higher gears higher boost) at 6000rpm then stay the same or decrease by the redline at 7500rpm.
We first thought that since the original catted exhaust does not have a wastegate downpipe or a seperator plate we should machine the plate away which we tried. This made no difference to the problem.We examined the wastegate downpipe with torches and mirrors and decided that where it entered the exhaust downpipe it was too restricted due to poor welding and therefore could be causing backpressure from 5000rpm forcing the wastegate closed. The wastegate downpipe was directly opposite the wastegate as well and therefore the benefits of twin dump systems for the MY02 STi are questionable. So we turned the twin dump system into a open mouth collector.
Testing then revealed that the boost would be held at 0.7 Bar from 3000rpm till 5000rpm then rise to maximum 1.1bar by 6000rpm and stay there till 7500rpm (In all gears). So the problem is less but not gone!
The only thing left to check that could be causing backpressure is removing the silencer and trying again. Although I doubt very much by now that the straight through silencer could cause more backpressure that the original silencer.
I am starting to think that the spring in the wastegate starts ocillating open and closed at higher revs too quickly due to excessively low backpressure in a catless exhaust causing boost instability and needs a certain amount of backpressure to stay open without oscilating. This could explain the 0.7 bar instead of the 0.6bar held boost with the catless system compared to the catted original system.
The question is why do other MY02 STi's not experince the same problem?
I can only think that they where fitted with catless systems early in there carreers compared to mine at 30 000km and their wastegate pre-tension has adapted to the extra flow when the wastegate spring had less exercise (for want of a better word!!) behind it.
We first thought that since the original catted exhaust does not have a wastegate downpipe or a seperator plate we should machine the plate away which we tried. This made no difference to the problem.We examined the wastegate downpipe with torches and mirrors and decided that where it entered the exhaust downpipe it was too restricted due to poor welding and therefore could be causing backpressure from 5000rpm forcing the wastegate closed. The wastegate downpipe was directly opposite the wastegate as well and therefore the benefits of twin dump systems for the MY02 STi are questionable. So we turned the twin dump system into a open mouth collector.
Testing then revealed that the boost would be held at 0.7 Bar from 3000rpm till 5000rpm then rise to maximum 1.1bar by 6000rpm and stay there till 7500rpm (In all gears). So the problem is less but not gone!
The only thing left to check that could be causing backpressure is removing the silencer and trying again. Although I doubt very much by now that the straight through silencer could cause more backpressure that the original silencer.
I am starting to think that the spring in the wastegate starts ocillating open and closed at higher revs too quickly due to excessively low backpressure in a catless exhaust causing boost instability and needs a certain amount of backpressure to stay open without oscilating. This could explain the 0.7 bar instead of the 0.6bar held boost with the catless system compared to the catted original system.
The question is why do other MY02 STi's not experince the same problem?
I can only think that they where fitted with catless systems early in there carreers compared to mine at 30 000km and their wastegate pre-tension has adapted to the extra flow when the wastegate spring had less exercise (for want of a better word!!) behind it.
08 November 2002, 05:55 AM
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Where do you live?
Give Darren a call. Or take your car to Subaru Hatfield to have one of their downpipes fitted. I have one of those on my STi7, and all is perfect...they could also swop out your wastegate with another to see if it has any effect.
btw, there is a very long thread here somewhere that describes exactly what you are seeing...something like "STi7 Restrictive Wastegate"...
Give Darren a call. Or take your car to Subaru Hatfield to have one of their downpipes fitted. I have one of those on my STi7, and all is perfect...they could also swop out your wastegate with another to see if it has any effect.
btw, there is a very long thread here somewhere that describes exactly what you are seeing...something like "STi7 Restrictive Wastegate"...
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