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Old 04-17-2011, 06:34 PM   #1
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Default Boost sag despite post-IC signal

I'm taking the waste gate signal after the IC, just befoe the TB.
Boost sags from 14.8 PSI @ 4270 RPM to 10.3 PSI @ 6040 RPM (see attached graph). I'm using a 10PSI wastegate actuator can and a MBC.
Turbo is a 2554R, exhaust is 2.5" FM.

Could the "bleed off" whole in the MBC be bleeding off too much air
or is the turbo just too small to keep up?

If it's the bleed hole, would a 5-6PSI waste gate actuator help because
the pressure on that side of the MBC would be lower and this less air bleeding off?
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Boost sag despite post-IC signal-boostsag.jpg  
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Old 04-17-2011, 07:48 PM   #2
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I wonder if it's more of a spike than a taper off? What kind of mbc is it?
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Old 04-17-2011, 10:35 PM   #3
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Cheap, ebay ball-and-spring MBC.
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Old 04-17-2011, 10:54 PM   #4
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I was getting that exact same thing with my last turbo setup. A few things contributed to it. Turbo was too small, and my exhaust was too restrictive. I hit 18psi at 3000rpm and then it dropped down to 12.
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Old 04-17-2011, 11:00 PM   #5
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try adding a little pre load to the WG if you can
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Old 04-18-2011, 12:52 AM   #6
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The WG can be adjusted from 8.5 to 11 PSI, it's currently at 10.
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Old 04-18-2011, 01:03 AM   #7
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What is your exhaust? It held boost way better up high when I went 3 inch.
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Old 04-18-2011, 01:16 AM   #8
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2.5" with high-flow cat (FM).

There's got to be others with a 2554R and 2.5" exhaust around here, what's you boost ay 6000 RPM?
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Old 04-18-2011, 01:25 AM   #9
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guy I know locally here has 2.5 exhaust wtih 2554, he added a little preload to the WG and worked wonders for him, which is why I suggested it, but sounds like you already did that. Only difference is, he is catless.

Might be worth trying another mbc...just as a troubleshooting measure.
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Old 04-18-2011, 09:36 AM   #10
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looks like your hitting around 10psi at redline?

would this happen without the MBC?
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Old 04-18-2011, 01:49 PM   #11
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Yup, without the MBC, it's at 10PSI from 2800 to redline.
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Old 04-18-2011, 01:57 PM   #12
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then I'd assume it's your MBC.
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Old 10-04-2011, 01:25 PM   #13
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Found the problem, a boost leak...
The little rubber hose that goes from the intake elbow to the idle control valve was busted, small tear, only visible from below.
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