V3 Sti Boost cut - factory ECU & boost controller

[tixelix]

Hi team, I have a V3 STi I use as an autocross car.  Engine is mostly stock, running the factory ECU and just a catless downpipe fitted.  It goes great!  I have an issue though that ever since I replaced the factory downpipe once she's warmed up and I give it the beans it hits boost cut every time.  It has the factory boost gauge which goes up to 1.5cm3 and it boosts off the gauge by the time i'm at the top of 2nd gear.  As a cheap solution I was thinking to just scale down the boost so it peaks just below the boost cut (around 23psi on a v3 i believe?). 

 

I have 2 questions:

 

1. Is this a common problem or do I perhaps have a fault somewhere?

 

2. Is a manual boost controller a good way to solve the problem?  Other advice I had was to drill out the restrictor pill by a few microns at a time and then check again... I've drilled it out twice without any noticeable improvement, and I'm a bit reluctant to keep going as it doesn't seem to help.

 

3. For the first maybe 5-10 minutes of the engine running from cold, the ECU/boost controller seem to restrict the boost down to around the 1.2cm3 mark.  It doesn't seem to be related to engine temperature.  Can anyone tell me what this functionality is actually doing?  I want to be able to avoid this on the track.

 

I guess the best solution would be to replace the ECU and controller, but I don't have the dollars to do that at the moment.

 

Any advice would be much appreciated!

 

 

Edited January 5, 2019 by tixelix

[pl0x]

I assume you are on factory boost control and there isn't a boost tap somewhere? On factory boost control you shouldn't be close to hitting boost cut, so i'd assume a fault somewhere, possibly a leak in the vac lines between turbo/boost controller/waste gate.

 

As a test, try run waste gate pressure (Vac line direct from turbo to waste gate).

 

Manual boost control/boost taps generally cause spikes but could work as a cheap fix

[RaKid]

AFAIK, the Gen 1 Impreza MAP sensor taps out at 1.27 Bar (18.42psi) - so anything above that would mean you're starting to lean out if it wasn't for short term fuel trim. 

 

I'm no expert, not even close, but my understanding is that once you've breached the upper limit the ECU only thinks your boost is the upper limit i.e. 1.27 Bar - how or if your O2 sensor can override based on a now lower than expected AFR, I simply don't know.

 

I'd be interested in getting a tuner or someone that knows definitely on this thread to explain the factory behaviour of this - I've always been curious.

 

To answer your questions:

1. I think your down-pipe appears to be over spooling the turbo and the IWG can't flow enough gas through to stop you overshooting
2. I'm not convinced a manual boost controller would do anything but it's a cheap test and then you can correct me and elaborate on the impact to the behaviour you're experiencing
3. I don't know

What I can tell you a test that I've personally carried out.

 

From a complete factory set-up I:

1. Removed IWG actuator and replaced with TurboSmart one
2. Patched compressor outlet vacuum directly to IWG vacuum inlet completely by-passing the factory BCS
3. Tested and adjusted my actuator so my boost target was 1.2 bar out of a maximum possible 1.27 bar

Why did I do this? So that I could raise the boost without using an EBC or boost tap

 

What did I experience? Rock solid boost with factory spool characteristics with a complete 3" turbo-back exhaust