Evo 8 wont hold boost past 6k
Sep 8, 2012, 06:18 PM
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Evo 8 wont hold boost past 6k
Ca runs great holds 19-20 psi till 6k rpm then rapidly drops off to 12 psi sometimes lower. I have adjusted the BC which is a forge unos and does not seem to make the boost go any higher or hold longer ( did this as a elimination process) so i turned it back dwn. I thought it may be the snorkel sucking to the hood since im on stock air box but that made no difference. BLT has revealed nothing. My afr's are solid mid to low 11's.
Could it be the BC? I hear these bc have problems? Got mine used.
Could it be the BC? I hear these bc have problems? Got mine used.
Sep 8, 2012, 09:55 PM
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K&n drop in, Evo 9 Bov, Megan o2, dp, test pipe, Works exhaust, BC. Pretty much stock. After 6k its almost like the car just takes boost away. Does not feel like Timing is being pulled.
Sep 9, 2012, 09:48 PM
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Try something for me: Back your MBC's adjustment knob all the way out counterclockwise(-) to the lowest possible boost setting. Now, do a WOT run in 3rd or 4th and see how that boost reading compares to the 12 PSI reading that you are seeing at 6000 RPM with the knob turned all the way in clockwise(+). Would you do this for me please?
We shall get to the bottom of this, Igor!
We shall get to the bottom of this, Igor!
Sep 10, 2012, 04:01 AM
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I started out with the adj knob in full open setting (-). I turned it up halfway and the boost seems to still drop at the same rate but spike higher for obvious reasons. Its very perplexing. It holds its setting perfectly till 6k rpm and then it begins a rapid decline all the way to 12 psi by shift time. I have removed the top part of the air box to alleviate the snorkel cause out of the equation.
Last edited by re-rx7; Sep 10, 2012 at 04:05 AM.
Sep 10, 2012, 04:16 AM
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I need to know when you started out as you state, whether you did a WOT run with the knob set at full soft(-)! Specifically, I need to know what the peak boost level is with the MBC set to full soft. What was the highest spike with the controller set to full soft(-)? Pweeze.
Last edited by sparky; Sep 10, 2012 at 04:26 AM.
Sep 10, 2012, 04:46 AM
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Assuming that the boost leak test that you performed was definitively negative, then there are a couple things that we could try in order to determine proper MBC functionality:
First, you can sustitute a known accurate MBC in place of your current unit. This will tell us right off whether or not your Forge is funky. This is assuming all vacuum hoses are routed, installed, and connected properly and that there are in fact no boost leaks.
Alternately, you could swap your Unos onto another MBC equipped car that exhibits no taper anomalies with respect to boost control. Leaving everything else as is, if the introduction of your MBC into a perfectly functional boost control system brings on the identical taper issue, then we could conclude that your MBC is dysfunctional and is thus causing the problem.
First, you can sustitute a known accurate MBC in place of your current unit. This will tell us right off whether or not your Forge is funky. This is assuming all vacuum hoses are routed, installed, and connected properly and that there are in fact no boost leaks.
Alternately, you could swap your Unos onto another MBC equipped car that exhibits no taper anomalies with respect to boost control. Leaving everything else as is, if the introduction of your MBC into a perfectly functional boost control system brings on the identical taper issue, then we could conclude that your MBC is dysfunctional and is thus causing the problem.
Last edited by sparky; Sep 10, 2012 at 10:40 AM.
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Sep 11, 2012, 04:57 AM
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Over the entire range of adjustability with your MBC there is only a 1-2 PSI difference in peak boost level. It goes from 19# at full soft to 21# at highest setting. WTF. There is only a 1-2 PSI difference......essentially nothing when you consider the variables of testing such as ambient and barometric, coolant, intercooler and other operating temps as well as other control varianles like repeatability of road grade, clutch slipping and etc.
Sep 11, 2012, 05:20 PM
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Yeah, most likely. The boost tapering all the way down to 10-12#(base pressure w/no preload) points in that direction. Also, the fact that the boost drops 8-10# overall, from a peak of 20# to the bottom level of 10#.
The funny thing for me at least is that it boosts all the way up to 20# before dropping and that the MBC is not controlling anything at all. It is just along for the ride. But, this last detail is almost academic. He can verify MBC function by simply substituting a known "good" controller.
As you have so aptly pointed out, as a practical matter he should just disconnect the MBC and run the actuator straight off the compressor discharge nipple so as he can set boost directly via the adjustable actuator rod.
The functionality of the MBC is secondary.
Last edited by sparky; Sep 11, 2012 at 05:23 PM.