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Old 12-13-2006, 09:11 PM   #1
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Default MBC overshoot

Today I fiddled with the MBC for the first time. No matter what I do, with or without the MBC in the loop, I overshoot.

I did a lot of troubleshooting this evening and found that I can set the MBC to reach whatever max amount of boost (and have very good control over it... a 1/4 turn is about .5psi) but then it drops about 3psi as the revs rise.
In 3rd and 4th gear, I'm seeing max boost at about 3500rpm's... and then a steady decrease until aout 4500rpms where it will hold until redline.
12psi max drops to 9psi
10psi max drops to 7psi
8psi max drops to 6psi
Running wastegate only w/no MBC, 8psi max drops to 6psi.

The wastegate I'm running is anywhere from 7-10psi depending on who you talk to in what DSM forum.

So, what's the best way to fix this. I know, I know... EBC... any other way.
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Old 12-13-2006, 09:31 PM   #2
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helper spring move the mbc away from the turbo abit, what boost level are you shootin for? You could also shim the WG mounts to make it abit tighter, also get a bicycle pump and cut of the tire inflating fitting and tee in a vac boost gauge and you can see what boost pressure the WG is set for. This doesnt work very well for a mbc though just on the base boost setting your problem is the backpressure on the WG is helping to open it to early.
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Old 12-13-2006, 09:55 PM   #3
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EBC isn't a cure all. I have tried 3 different MBCs (ultra cheap to ultra expensive) and an EBC and I still get overshoot (2 psi). I spent weeks trying to sort it out and then just gave up. I don't think there will ever be anything I can do to get rid of the overshoot so screw it!
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Old 12-13-2006, 10:30 PM   #4
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I never messed with my old DIY MBC that much, but I never saw it do that. How big is your exhaust?
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Old 12-13-2006, 10:30 PM   #5
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Hmmm. Have you tried a unit with closed loop boost control?
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Old 12-13-2006, 10:38 PM   #6
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I had a boost spike when using my homemade mbc (like in Brianeack's post). Since going to the FM mbc, boost rises and holds steady. I've had it as high as 13 or so psi, and it absolutely holds. I'd kill someone to get full boost at 3.5k.

An rx7 guy was telling me how he theorized that the onset of boost is a lot like a failure in a dam where you get an inital "rush" followed by a constant flow. I slept through fluid dynamics, so I have no idea if it's a valid point or not.
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Old 12-13-2006, 11:02 PM   #7
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I believe one way to get rid of this overshoot phenomenon would be to use a massless ball and massless spring. That'd fix it right up
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Old 12-13-2006, 11:15 PM   #8
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Sam, does your MBC have a bleeder hole drilled into it?
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Old 12-13-2006, 11:28 PM   #9
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So do I take it that OVERSHOOT is pretty common? I'd like a steady 10psi. I've got Borla Duals 2.25", 2.5" test pipe, and Begi's new divorced gas DP... plenty of flow for an efficient 10psi.

I don't think I can shim the WG mounts. The idea is to move the can away from the flapper to "pre-load" the WG spring, right?

The vaccum lines I've got from the MBC are only about 3-4" long each. Would running a longer line help? What about tapping off a different source for those? I'm using the nipple on the compressor housing. Should I tap off the manifold?

I think an option might be to "straighten" my WG rod and drill a new hole on the rod closer to the can... there's some extra room on the flat part next to the current hole... that way I could "preload" it. But would that help with overshoot?

What about a new spring in the MBC?

There is a bleeder hole, it's on the bottom, not visible, and unobstructed.

In this pic, I've bypassed the MBC and am running straight off the compressor housing.

Last edited by samnavy; 08-01-2007 at 10:35 PM.
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Old 12-13-2006, 11:47 PM   #10
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The bleeder hole usually prevents spikes.

So might want to try a few things:

The helper spring method; Puts more preload on the wastegate and it's harder for a surge of boost to push the rod too hard.

Drill out the bleeder hole a tiny bit...I'm talking going from .020" to .030"

You could try putting a restrictor on the line before the MBC
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Old 12-13-2006, 11:53 PM   #11
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What does a restrictor look like. Could I just zip-tie the vaccum line narrower?

For a helper spring, do I want it helping hold the flapper door open or closed?
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Old 12-14-2006, 09:28 AM   #12
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Quote:
Originally Posted by bripab007 View Post
I never messed with my old DIY MBC that much, but I never saw it do that. How big is your exhaust?
2.5 inch that dumps behind the drivers seat. No restrictions at all.


Hmmm. Have you tried a unit with closed loop boost control?

Yes...
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Old 12-14-2006, 09:35 AM   #13
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Robert, I've been running an beta version of one of these for several months: http://wiki.2g-eng.com/tiki-index.ph...TwoGeeProducts

A buddy of mine built it, and he and his brother are planning to sell them. Works quite well. The MAP lookup table must be slightly off or my turbo's natural wastegate opening point screws with it a little, though, because if you want ~8psi, you have to dial in ~7psi on the controller. But, man, it just locks into to 8psi at that point. This is at 45% duty cycle controlling a GM solenoid. It seems to just lock in at about 8psi that rises to around 8.3psi by redline.

They're going to revise it to not even have the LED display and rotary ****; it'll be programmable via USB/PC and will be cheaper to produce.
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Old 12-14-2006, 09:40 AM   #14
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Quote:
Originally Posted by samnavy View Post
What does a restrictor look like. Could I just zip-tie the vaccum line narrower?
you could try it.

Quote:
For a helper spring, do I want it helping hold the flapper door open or closed?
closed. It would attach from the end of the arm back to the bracket.
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Old 12-14-2006, 09:51 AM   #15
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Quote:
Originally Posted by bripab007 View Post
Robert, I've been running an beta version of one of these for several months: http://wiki.2g-eng.com/tiki-index.ph...TwoGeeProducts

A buddy of mine built it, and he and his brother are planning to sell them. Works quite well. The MAP lookup table must be slightly off or my turbo's natural wastegate opening point screws with it a little, though, because if you want ~8psi, you have to dial in ~7psi on the controller. But, man, it just locks into to 8psi at that point. This is at 45% duty cycle controlling a GM solenoid. It seems to just lock in at about 8psi that rises to around 8.3psi by redline.

They're going to revise it to not even have the LED display and rotary ****; it'll be programmable via USB/PC and will be cheaper to produce.
When you send me one to test for you (since I have completely out of control boost) I will do a good writeup and serve as a fine example of how good the controller is. After 3 MBC and an EBC running open and closed loop I won't buy anything else until it is proven to actually work for me. Every time someone said "I KNOW this will work" it has failed. Jeremy and Bill at FM spent about 2 weeks with me before they finally gave up.
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Old 12-14-2006, 11:20 AM   #16
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On the way to work today, I dialed it in to a perfect level for now. I get to about 11.5psi and then it bleeds down to around 9psi. Still taking it easy and not hammering the gas for want of sticking to the clutch break-in.

I'm considering EBC right now, but have some questions. The Axon and the Tullos seem to offer the exact same specs. The Axon isn't out yet however.
WOT, does the Tullos for $250 include a solenoid?

The solenoid just runs inline where the MBC would go, right? I still use my stock wastegate actuator, right?

WOT, Does the Tullos also act as a boost gauge? Can I read out boost on the display?

Bripab, what's the revision timeline for the Axon... cheaper is better... but time is important. I like the DIY aspect of the Axon even if the Tullos is sexier.

First things first though... engineer a helper spring and investigate the zip-tie fix.
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Old 12-14-2006, 11:36 AM   #17
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the internal wastegate is going to do that, no matter what.
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Old 12-14-2006, 11:37 AM   #18
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Sam, running the MBC source line off the intake manifold (i.e. post-throttlebody) instead of the compressor housing will ensure it holds 11.5psi to redline. That's where I have my MAP sensor, which is my "source line" for the GM boost solenoid.
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Old 12-14-2006, 11:40 AM   #19
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I have no spike and I'm sourced off the turbo. Holds to redline at 12psi.
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Old 12-14-2006, 11:59 AM   #20
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Well, there ya go, it can be done. Running the source from the intake manifold just ensures that the turbo will continue trying to make target boost throughout the rev range, accounting for the pressure drop across the intercooler, etc.
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