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Old 03-13-2009, 10:32 AM   #1
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Default Boost Overshoot

I have a home-depot MBC, with a tiny bleeder hole drilled in the side. It's hooked up to reference just before the TB.

I get boost overshoot. I would like to be running between 12 and 13psi, but if I set it up to do that it will overshoot to 15 or 16, hitting the overboost fuel cut. I have it set up now to overshoot to about 13 and settle around 10, but I'm looking for ideas on how to get rid of the overshoot completely.

It's an internal wastegate and the actuator limits boost to about 7.5psi when left to it's own devices. I attached a log of the problem.
Attached Files
File Type: xls datalog200903121837.xls (720.9 KB, 61 views)
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Old 03-13-2009, 10:50 AM   #2
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it's possible sourcing boost after the IC would help with the spike.

You could probably use a helper spring to help keep the wastegate closed with more rpms. what's happening now is, it's not opening enough then it's staying open too far.
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Old 03-13-2009, 11:06 AM   #3
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I agree with brain, and just get a stronger wastegate, closer to your target boost. I tried running my "14psi spring" with no vac hose and wired closed...no change in spoolage.

Do you have a gt2860rs? The only way I could get mine to stop overboosting was raise target to 14.7psi. In the cold it overboosts to 17 or 18, but I straight don't give a **** because I'm baller and bitches love me. Goose bottles left and right, bro.
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Old 03-15-2009, 12:02 PM   #4
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I will look into the helper spring. Is there anywhere to get a replacement actuator can that will hold better?
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Old 03-16-2009, 09:38 AM   #5
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Quote:
Originally Posted by skidude108 View Post
I will look into the helper spring. Is there anywhere to get a replacement actuator can that will hold better?
atp or ebay.
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Old 03-27-2009, 05:06 PM   #6
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I tried the helper spring with no luck. I couldn't find anything on here about the helper spring but the subaru forums have some stuff on it so I bought a spring and put it on, and stretched it pretty tight but I still get the exact same result. How strong does this spring need to be?
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Old 03-27-2009, 05:57 PM   #7
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I bought the basic same mbc from NSX on ebay. Never had a problem with boost spiking. It is worth the $15.00.

Good Luck!
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Old 03-27-2009, 05:59 PM   #8
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There's plenty of info here on helper springs- I posted some. You do need to make a run with the gate wired shut so you have a baseline to compare a gated run too. Then you'll know if there's anything to be had with the spring. If there's not, you won't waste your time.
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Old 03-27-2009, 06:20 PM   #9
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if I wire the gate shut, it will just make more boost than I want to put into the engine. I have problems keeping the overshoot under the fuel cut as it is, with the gate wired shut the boost will climb until at least 18psi.
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Old 03-27-2009, 06:43 PM   #10
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Helper springs, a different wastegate, etc. are all bandaids that don't address the real problem and won't help. Don't spend any more $ trying to fix this issue when you can set it up right for under $10 - you are not really having 'overshoot' - you are having drop off. As in, when your MBC is set for 15-16 psi and you are dropping off to 12-13 by redline because of pressure drop across the intercooler.
Read my thread here: https://www.miataturbo.net/forum/t32479/
Go do this and you will be able to set your MBC to 13 psi and you should drop at most to 12 psi at redline.
-Ryan
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Old 03-27-2009, 09:13 PM   #11
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Quote:
Originally Posted by ThePass View Post
Read my thread here: https://www.miataturbo.net/forum/t32479/
Go do this and you will be able to set your MBC to 13 psi and you should drop at most to 12 psi at redline.
-Ryan
I did that before I started this thread, it says so in my original post. Thanks for the suggestion though.

FWIW, it does act more like boost drop than overshoot sometimes, but othertimes it acts more like overshoot than boost drop. It's a weird problem.
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Old 03-27-2009, 09:36 PM   #12
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My reference was to the helper spring. Tying the wg shut will show you what you're spools like w/o the boost overpowering your internal gate and blowing by the valve on spool. If it spools better with the gate wired shut then you know it's leaking during spool up. Then a helper spring will help on spool up- even though it's a bandaid, it works and it's why they're used on gates that have a weak spring. Mine reduced spool up by something like 600rpm and with that I managed to nail target and hold boost to redline with only a .8psi variation from target.

Regarding your current problem- as mentioned above, moving your reference signal to the IC pipe right before the tb can help reduce (or even remove) the spike you're seeing.
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Old 03-29-2009, 02:03 PM   #13
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Regarding your current problem- as mentioned above, moving your reference signal to the IC pipe right before the tb can help reduce (or even remove) the spike you're seeing.
I already did that. And I put in a helper spring. So far neither fixed it. Is there any way to know how stiff the helper spring should be?
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Old 03-29-2009, 02:32 PM   #14
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There's a thread somewhere on here that discusses the spring strength. The second part is adjusting the tension on the spring with a thread rod. Once you know what the turbo spools at with the wastegate tied shut, then you aim for the same using your spring adjustment until you overshoot the mbc target, then go back to the previous setting.
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Old 03-29-2009, 03:40 PM   #15
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Back when I was fussing around with helper springs I found a variety pack of different strengths at Home Depot. Use a threaded eye-bolt to attach it to the wastegate actuator's bracket and then you can adjust the tension of the spring properly. Then just adjust, test, adjust again until you find the sweet spot.
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Old 04-10-2009, 06:13 PM   #16
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I'm still not entirely sure what I'm adjusting to. I got a much stronger spring and a threaded eye bolt to play with, but am I really just trying to match wired-closed spool to fix my overshoot/drop problem?
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Old 04-10-2009, 06:24 PM   #17
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yea know...close the bleeder hole on the valve and see what happens.
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Old 04-10-2009, 08:56 PM   #18
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I am beginning to get very frustrated with this. I have tried everything that has been suggested and none of it seems to make any difference. I've got a giant spring on the WG and I loosened the MBC way up and it still overshoots. If I tighten the spring, it just does the same thing, 3psi higher.

Hustler, I tried covering the hole in my valve and it doesn't seem to make any difference either.
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Old 04-10-2009, 09:17 PM   #19
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How about porting the wastegate?

What DP and what exhaust are you using?
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Old 04-10-2009, 11:56 PM   #20
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Quote:
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How about porting the wastegate?

What DP and what exhaust are you using?
Porting the wastegate is something I really don't want to do, and I can't imagine that's the problem since my boost doesn't creep.

My downpipe is a custom 2.5" piece, and the exhaust is 2.5 straight through with a "high-flow" cat.
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