oscillating boost help - Subaru WRX Forum
 
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post #1 of 9 Old 09-29-2007, 04:12 AM Thread Starter
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oscillating boost help

How bad is it for the turbo, or engine, for boost to quickly oscillate between 2 psi below and 2 psi above target boost level at WOT?

Reason I ask is because when I adjust my wastegate arm to the point where the boost stays steady at WOT in all gears, I notice my turbo builds boost alot slower. But when I shorten my wastegate arm (about 1 to 2 turns) the turbo builds boost alot faster but oscillates as described above in 4th and 5th gear at WOT (more severe in 5th). Even shorting the arm even 1/2 turn and I see slight oscillation in 5th at WOT.

I'm trying to find the "sweet spot" for the wastegate arm, I like the "power" of the car when I have the wastegate arm shorter but don't want to damage anything with the boost oscillating. Please shed some light on what i can do. Thanks in advance

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post #2 of 9 Old 09-29-2007, 09:33 AM
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Originally Posted by HUMANPIE View Post
How bad is it for the turbo, or engine, for boost to quickly oscillate between 2 psi below and 2 psi above target boost level at WOT?

Reason I ask is because when I adjust my wastegate arm to the point where the boost stays steady at WOT in all gears, I notice my turbo builds boost alot slower. But when I shorten my wastegate arm (about 1 to 2 turns) the turbo builds boost alot faster but oscillates as described above in 4th and 5th gear at WOT (more severe in 5th). Even shorting the arm even 1/2 turn and I see slight oscillation in 5th at WOT.

I'm trying to find the "sweet spot" for the wastegate arm, I like the "power" of the car when I have the wastegate arm shorter but don't want to damage anything with the boost oscillating. Please shed some light on what i can do. Thanks in advance

Current mods:
Silencer delete
K&n drop in filter
Turbo XS TMIC
Port, polished and coated exhaust manifolds and crossover
U/P
TBE
GM BCS
XPT stage 2 w/GM BCS map
You can't just adjust the arm and not your boost mapping.You will get quicker spool by shortening it,but if your getting ocillations you need to adjust your primary wastgate duty cycle.And vice versa.When i first got the XPT map i had boost ocillations and no laptop or Tactrix cable.I just adjusted the arm to reduce the ocillations.

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post #3 of 9 Old 09-29-2007, 12:43 PM Thread Starter
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Thanks for the info Dumdum, I'll have to start doing research on wastegate duty cycles and tuning that aspect, as im not experienced with that.

Just out of curiosity though, what damage does it do to the turbo to have that oscillation? besides maybe not being efficient?
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post #4 of 9 Old 09-29-2007, 01:16 PM
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Is it over boosting or oscillating up to target boost?If it's overboosting you should change the arm back the way it was untill you get around to tunning it.Once it gets colder it will become more of a problem depending on what your max injector duty cycle is now.If you get a nasty boost spike you might not have the fuel to match the PSI and you could be pretty lean durring the spike.Also the XPT maps are not scaled to a very high load.There are other things you can do to get better spooling tuning wise besides just turning the wastegate arm since you have the GM BCS.Lot of info on Enginuity about tuning the GM BCS.If you are just oscillating up to target boost it won't hurt anything but it's just annoying.

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post #5 of 9 Old 09-30-2007, 11:21 AM
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This is boost creep, is it not?

I had the same issue before I got my Accesport. The AP fixed 95% of the oscillation.
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post #6 of 9 Old 09-30-2007, 12:34 PM
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Not boost creep.Boost creep is a mechanical/design problem that cannot be fixed by tuning.It can only be fixed by physiclly altering the wastegate size or design.The STi's with the VF39 and the 06+ 2.5L with the TD04 suffer from this,not the 02-05 2.0L WRX's.

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post #7 of 9 Old 10-01-2007, 09:34 PM Thread Starter
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Originally Posted by dumdum View Post
Is it over boosting or oscillating up to target boost?If it's overboosting you should change the arm back the way it was untill you get around to tunning it.Once it gets colder it will become more of a problem depending on what your max injector duty cycle is now.If you get a nasty boost spike you might not have the fuel to match the PSI and you could be pretty lean durring the spike.Also the XPT maps are not scaled to a very high load.There are other things you can do to get better spooling tuning wise besides just turning the wastegate arm since you have the GM BCS. Lot of info on Enginuity about tuning the GM BCS.If you are just oscillating up to target boost it won't hurt anything but it's just annoying.
Thanks again for the information. It was overboosting, so I have the arm adjusted where there is no longer oscillation. I'll check out enginuity and read up.
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post #8 of 9 Old 01-31-2012, 08:23 PM
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i am having a problem with the xpt stage 2 gm bcs map on my 04 wrx. i am overboosting in 4th gear, about 20-22 psi, but 1-3 are at or under target, i have adjusted the wastegate arm without much for results, was wondering if i need some touch ups on the tuning side
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post #9 of 9 Old 02-01-2012, 01:15 AM
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i am having a problem with the xpt stage 2 gm bcs map on my 04 wrx. i am overboosting in 4th gear, about 20-22 psi, but 1-3 are at or under target, i have adjusted the wastegate arm without much for results, was wondering if i need some touch ups on the tuning side
Did you talk to XPT? I am sure that he would be able to help you out and possibly look at some logs and tweak your map if you need it.

My name is Ken and I am a HP Junkie.
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