GDBSTi

After much frustration and testing to solve my problem of boost creep at high engine speeds (5000rpm +) on my Australian Spec 02 STi I have a question.

The lambda sensor on the bell-mouth downpipe I purchased sits , like the pre-New Age car, right after the turbo. Also the VF 35 wastegate opens towards the turbo whilst the TD04 opens away from the turbo.

Could the exhaust gases coming out of the wastegate above 5000rpm be restricted by turbulence caused by the lambda sensor which is directly in line with the exit of the wastegate??

GDBSTi

Nobody got any ideas!!!!!!!!!

EMS

GDBSTI,

It´s good to hear I am not the only one with boost creep on a STi7!

It has absolutely nothing to do with the oxygen sensor! We have the same problem with an STi with the sensor at the end of the downpipe. You only can solve the problem by porting the wastegate so it can flow more exhaust gasses around the turbine. Or (like we did) you increase the backpressure of the exhaust. We got a good result by fitting a 2,5" systems instead of the 3" system that was fitted before.

See: http://www.scoobynet.co.uk/bbs/thread.asp?ThreadID=124996 for my whole story.

Mark.

GDBSTi

Thanks EMS I remember now reading about your problem back then and now I know how it feels!!!!
Frustrating to say the least!!!!

So why our cars and not other STi 7's???

Anyone else out there with an STi 7 which had the same problems????

EMS

The back pressure in the exhaust system is the main reason for the problem, I am sure!

We fitted a Supersprint 2,5" single pipe downpipe in combination with a 2,5" Supersprint back box, with a 1.2 mm restrictor this worked fantastic! Hardly any peak in the boost and 1,2 bar from almost 3.000 rpm, nicely dropping to 1 bar at top end. (no ECU changments yet) The Supersprint back box is not completely 2,5" until the end. At the end there is some restriction as the hole is a little smaller. However we used these back boxes without any problems before on +300 BHP Impreza turbo´s.

Mark.

GDBSTi

Mark

Did you try an electronic boost controller(EBC)? I found that a EBC set at 1.1 bar starts bringing the boost down after 6500rpm to 1.1 bar just before 7500rpm from 1.25 (maximum at 6000rpm after increasing from 1.1bar at 5000rpm) bar in 4th gear. In 5th and 6th gear I didn't wait to see if the controller brought the boost down, for obvious reasons!!!

Did the boost continue rising from 5000rpm till 7500rpm when you disconnected the wastegate? Mine stabilises at 1.1bar from 6000rpm till 7500rpm.

Did you check whether your car had boost creep on the dyno? When I put my car on the dyno I did not see any visible boost creep on the boost gauge with 2 runs. However on the road it was obvious!!

Your problem might be better, but it is still does not explain why our cars give the problem and many others have no problem. I have a friend with an STi7 who sets his EBC to 0.6 bar when he tows a caravan. It holds 0.6 bar perfectly all the way to the redline.

Rupert