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Old 10-06-2013, 09:39 AM   #1
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Default Confusing boost creep

Some of you may remeber my last thread where I built boost badly and slowly.
Well, most of that is taken care of by changing BOV and unsticking the wg-flapper from the dp.

This is a shanghai hot parts, DIY pressure and intake, 2854 churbo, BEGI separated gasses dp with 2,5" exhaust. NB 2000. Ball and spring MBC. E-manage blue.

But now I have an even wierder issue.

At WOT, the boost quickly builds to and levels out at 9 psi @ 3300 rpm, then, @5500 is goes up to 12 psi and holds that.

Ok, so I thought, this is wierd (my wg being an internal 7psi one), but I thought the "knee" in the curve was the MBC threshold and that 12 psi was where the MBC opened fully.

So i tried tightening the MBC so I made 12 psi @ 3300, but then boost creeps to 13 psi @ 5500 rpm. And if I tighten it even more I still get that span of 1-1,5 psi gain between 3300 and 5500. If I lower the boost, same thing.

So, I can get the boost to level out at whatever I like, as long as it is 1-1,5 psi over what I set the MBC to. Why is this, what can I do about it? Figured it was the normal unported exhaust housing boost creep, but that doesn't make any sense considering I can get it to level nicely after 5500 rpm at any chosen pressure. And it is not a boost leak, so what the hell?

MBC seems like a likely culprit, but is is also quite wierd considering I can place the boost "knee" anywhere I choose.
An easy fix would be to just run 13 psi, but then I go over 80% duty on my 425's, and having a stock motor that sees track, I do not want any more power, I just want my boost to spool properly so that I don't loose out on 15-20hp between 3300 and 5500 rpm.

Long post, wierd issue, has anyone had and solved anything like this?

Last edited by Sentic; 10-06-2013 at 11:56 AM.
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Old 10-06-2013, 10:14 AM   #2
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I don't see what EMS you are running, but electronic boost control could get your boost level flat at 12psi or whatever minimum that you can attain in high RPMs on spring pressure alone. Which separated gasses DP do you have? Long separate WG tube + internal wastegate was no good when I ran one- I could not maintain a flat/constant boost level with RPM. As a short-term bandaid, I just ran EBC to raise boost in the low/mid RPM range to match what it would creep to in the upper RPMs.
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Old 10-06-2013, 12:01 PM   #3
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Updated first post with requested info.

Clarifying, I can maintain flat boost in the upper RPM-band, but I have two plateaus, one 3300-5500 rpm, and one >5500, the lower not being a real plateau, instead raising the boost at a much slower pace towards 12psi with a 3 psi raise during 2700 rpm's. When at 12 psi, it holds perfectly flat.
Between 1700 and 3300 rpm i go from 0-9 psi.

Oh, and it is also consistent. I'm thinking that the vent/anti air trap hole in the MBC might be too large? Confirm my thinking here, MBC spring/ball opens, wg opens, but not really enough, because about one psi worth of air is going out through the hole. The hole is 1mm. This could perhaps explain my findings, agree, disagree?

See added super advanced diagram to see what it looks like in the log, first knee is at 9 psi, second little bend is at 12 psi.
Attached Thumbnails
Confusing boost creep-strange-boost.jpg  

Last edited by Sentic; 10-06-2013 at 12:50 PM.
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Old 10-06-2013, 01:07 PM   #4
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After putting my thoughts in text here I'll try to reduce the hole slightly, I think 0.8mm might do it.
1mm diameter = 0.785mm2
0.8mm dia = 0.5 mm2

The result would be, boost spike if too small and consistent sweet boost if just right.
I'll report back when I've had the time to do this, it will probably be a few days.
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Old 10-07-2013, 09:26 AM   #5
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What happens to boost if you remove the mbc and run straight wastegate?
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Old 10-07-2013, 02:32 PM   #6
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7 psi flat, obviously building slower than with the MBC. Everything leads me to believe that the MBC is the cause, and the relief hole in particular. Will try fix later this week and report back.
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Old 10-08-2013, 09:10 AM   #7
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I did this and it worked.


The Turbosmart manual boost controller range has an in built gate system which is set at 3 PSI. This means that below 3 PSI, the controller will clamp the signal to the wastegate, keeping it closed. If an undesirable boost spike occurs, removing the gate system by removing the ball and spring can eliminate the spike. Note: This modification is only required in a minimal number of applications.

Gate System Removal – Manual (Gated Boost Controllers)
The gate system can be removed by the following steps.
(Perform this modification on a clean bench so that the ball and spring are not lost.)

• Allow the engine to cool down before removing the Boost Tee from your vehicle

• Identify the input nipple with the restrictor at the end. Loosen the input nipple and remove the ball and spring.

• Check that the internal air passages are clear and free from debris

• Re-install the boost controller by following the instructions for your setup

• You will need to re-adjust your boost settings after the removal of the gate.
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Old 10-14-2013, 05:37 PM   #8
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So, fix:

Reducing the size of the hole reduces the problem. 0.8mm diameter gave me 11 going to 12 psi instead of 9.5 going to 12.
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