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Old 09-14-2013, 07:57 PM   #1
streetking
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Default 06 sti with VF-48 stock boost surge to 20psi

Guys,

So I have an 06 STi with a brand new oem short block and heads that went through a multiangle valve job. I did all the installation on my own and I figure I might as well change to a vf-48 while I was at it. I've been breaking in my motor.

I've been monitoring boost and AFR through my aem wideband and Defi gauges with control unit and I today on the highway I noticed how my boost surge to 20psi(!) around 3-4.5k rpm range went pedal to the metal in 4th gear. It then tapered down to 15psi throughout the rest of the rpm range. When I had my vf39 on my old stock motor I the gauge would just read a 15-16ish psi top so I doubt my gauge is reading high.

Is that boost creep that I experienced? What would be causing that? I am on stock intake and exhaust with stock tune. Should I just swap back to the vf39? There was no fuel cut when I hit 20psi for the brief moment. My wideband showed 10.8 ish AFR when I was hitting full boost. Any opinion?
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Last edited by streetking; 09-14-2013 at 08:03 PM.
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Old 09-14-2013, 09:41 PM   #2
HinshawWRX
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The biggest question is: Are you tuned?
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Old 09-14-2013, 10:15 PM   #3
streetking
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Stock tune....
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Old 09-14-2013, 10:17 PM   #4
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So it's not tune. I thought vf48 flows identical to vf39 and has the same dimension so it's safe to bolt on without tune?
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Old 09-14-2013, 10:26 PM   #5
Mishal Jackson
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Quote:
Originally Posted by streetking View Post
So it's not tune. I thought vf48 flows identical to vf39 and has the same dimension so it's safe to bolt on without tune?
Well if that Is true then why run that turbo over the vf39 anyway. But yeah that's a case of the eepy creepies
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Old 09-14-2013, 10:42 PM   #6
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Lol, I just figure if I had everything taken apart, why not just replace the old stuff when I was at it. I dun understand how a vf48 would surge with stock intake and exhaust? Seems odd to me
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Old 09-15-2013, 07:32 AM   #7
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Bump for more input
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Old 09-15-2013, 10:30 AM   #8
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the wastegate actuator may be stiffer on the 48 than it was on the 39.
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Old 09-15-2013, 12:47 PM   #9
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Anytime you change the turbo, you need a tune specifically for that turbo. Not this "close enough" game.
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Old 09-16-2013, 04:05 PM   #10
Mishal Jackson
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I mean I guess you don't haaave to swap back since your afr's arnt anywhere dangerous but a tune and or and ewg would eliminate the issue for sure. A pro tune. Not a Cobb access port tune haha just for better results aftermarket boost control would help too

Last edited by Mishal Jackson; 09-16-2013 at 04:14 PM.
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Old 09-16-2013, 06:04 PM   #11
streetking
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Yeah from what I experienced it seems more like a boost spike than a boost creep. I have 850 miles on my motor break in so it's at the end of the initial breaking in. I am gonna try to stay out of boost and get my parts and tune at IAG in a month or so...
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Old 09-16-2013, 06:21 PM   #12
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the wastegate actuator may be stiffer on the 48 than it was on the 39.
And the restrictor pill used with the VF48 might be different that the one used with the VF39. Not sure whether or not OP reused the vacuum/boost line from the VF39 containing the pill for the VF39.
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Old 09-16-2013, 06:47 PM   #13
streetking
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And the restrictor pill used with the VF48 might be different that the one used with the VF39. Not sure whether or not OP reused the vacuum/boost line from the VF39 containing the pill for the VF39.
I am just using the one came with the VF48. Do you think that I should swap to the one on the vf39?
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Old 09-16-2013, 06:53 PM   #14
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I am just using the one came with the VF48. Do you think that I should swap to the one on the vf39?
Yes, you should try using the pill from the VF39. I think you should be able to just swap the piece of vacuum/boost line containing the pill (no need to actually take the pill out of the line). That may not fix the issue but it's worth a try.
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Old 09-17-2013, 06:47 PM   #15
streetking
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Yes, you should try using the pill from the VF39. I think you should be able to just swap the piece of vacuum/boost line containing the pill (no need to actually take the pill out of the line). That may not fix the issue but it's worth a try.
Thank you for the suggestion. I just swapped the line today and viola! No more boost spike! It builds boost and peaks at 16psi. Right on the money.

hopefully I didn't do any damage when I had the boost spike. It wasn't like I was hitting 19 psi all the time for the last 800 or so break in miles.
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Old 09-18-2013, 01:10 PM   #16
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Quote:
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Thank you for the suggestion. I just swapped the line today and viola! No more boost spike! It builds boost and peaks at 16psi. Right on the money.

hopefully I didn't do any damage when I had the boost spike. It wasn't like I was hitting 19 psi all the time for the last 800 or so break in miles.
Glad it worked out. I don't think hitting 19~20 psi in the midrange for a short while is that big of a deal if fueling (10.8:1 AFR is plenty enough) and timing are OK. But, just like you, I've tuned for 16.5 psi peak boost (tapering to 15.5 psi towards redline). It's easier on the tranny and diffs (good ol' 5 M/T and all).
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