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Vacuum hose routing?

citymunky

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Joined
Sep 22, 2010
Messages
1,761
Location
Chesapeake, VA
So I'm the new owner of 923/2000, and I wanted to know if my vacuum hose were routed correctly. I would post this in the "Newbies" section, but I don't think this a noob question. I searched the forum and found some good threads with good info, but nothing to 100% answer my question. What makes my setup different than the ones I read about is that my Stock BOV has 2 ports like this --> BOV Hi-Boost mod.. Here a diagram on how the vacuum hoses are routed in my engine bay. The two ports just throw me off, along with the Tee between the BOV and Wastegate. So what do guys think?



The reason I'm asking is because I don't think my Joe P MBC is working. No matter how much I adjust it, I always hit 1.5 (21.7 PSI) to 1.8 BAR (26.1) of boost. I was told I have a 1 BAR (14.5 PSI) wastegate. I removed the MBC and adjusted to the point were I could blow air from the bottom port out of the side port and Readjusted to just past the point were I couldn't blow anymore air out the side and reinstalled it. My GVR4 is the first car I have with a MBC, on my G35 I have a HKS EVC VI controlling 5.5 PSI springs. I'm thinking about installing a EVC IV or Profec B II very soon on the GVR4.
 

broxma

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Nov 16, 2009
Messages
911
Location
San Antonio Tx
"The upper part of the B.O.V. MUST AT ALL TIMES be connected to a vacuum/boost source behind the throttle body that has full engine vacuum when the engine is at idle. The new port you just added MUST be hooked to the turbo's outlet barb or somewhere in the intercooling tubing for the BOV to work correctly in fast release mode."

According to your diagram, these are backwards. In addition, the boost source you are using for the MBC is essentially being bled off orminimized by sharing the line with the BOV.

/brox
 

citymunky

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Joined
Sep 22, 2010
Messages
1,761
Location
Chesapeake, VA
Quoting broxma:
"The upper part of the B.O.V. MUST AT ALL TIMES be connected to a vacuum/boost source behind the throttle body that has full engine vacuum when the engine is at idle. The new port you just added MUST be hooked to the turbo's outlet barb or somewhere in the intercooling tubing for the BOV to work correctly in fast release mode."

According to your diagram, these are backwards. In addition, the boost source you are using for the MBC is essentially being bled off orminimized by sharing the line with the BOV.

/brox



Sorry the diagram is backwards. The upper BOV port is connected to the intake, and the lower BOV port is connected to the TEE. I'll fix the diagram when I get back home tonight.

Now back to the MBC. I didn't think it should be sharing a line with anything, but at the same time the lower BOV ports needs to be connected to the turbo. I added a check valve between the lowed BOV port and the MBC, and the car didn't like that at all. So I'm lost to the reason why the MBC is not working.
 

citymunky

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Joined
Sep 22, 2010
Messages
1,761
Location
Chesapeake, VA
Quoting 14u2nV:
ClIckY



This is one of the threads I was talking in my 1st post. Good info, but it doesn't talking about BOV's with 2 ports.
 

broxma

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Joined
Nov 16, 2009
Messages
911
Location
San Antonio Tx
I'll express my concern with the diagrams posted. Most importantly, the idea that the MBC should be on a vacuum referenced line. Why? It never will need that vacuum to close the wastegate. It only uses pressure to open it, so in my opinion, the MBC should always come from a boost only pre-throttle plate source. It doesn't need vacuum, why send it?

The BOV, opposite obviously. But the idea of the two port BOV is essentially the same as a two port External gate, just in reverse. You need vacuum at the top port to open the BOV. But the bottom port, let's look at that. It appears the factory hole in the bottom plate, which is being filled in and relocated to a reference line, has an airway to the top of the BOV. Seems to me this port is meant to equalize the pressure in the pipe with the pressure above the plunger, and with the addition of the pressure coming from the top port, it should be able to hold the plunger down. My contention is that this line doesn't need to be referenced to vacuum, but that the hole needs to be enlarged. Since it is essentially a matter of fluid piston dynamics here, it makes sense to me that increasing the bottom pressure connection size at the pipe would yield a decrease differential ratio between the two "pistons". I use the word piston only as reference to the size of the hole/surface, and how they would react to each other if they had equal forces opposing them.

Fundamentally, the surface area of the piston at the bottom of the BOV must be smaller than the surface area of the top for it to work at all, or the pressure coming from the bottom would always win a battle at equal pressure. BOV's are built with this in mind clearly, as are external gates. What the bottom reference port is doing is equalizing that pressure top to bottom, with an additional reference port specifically for the vacuum. I contend, that if it were possible to eliminate boost from the top reference signal, as long as the signal was atmospheric then switching to vacuum with the plate closed, the function would change very little. This is because the bottom reference signal is still there. The bottom signal essentially pressurizes the same space the top signal does, it just doesn't provide vacuum at any time, or it shouldn't at least.

So the top signal is really only there for the vacuum, to open the valve. The bottom signal equalizes the pressure differential and the additional spring helps keep it shut, but what really keeps it shut is the surface area differential between the top and the bottom of the valve. My point is, adding the vacuum reference line is redundant. I think a better idea would be to drill out the factory bottom passage the entire way, creating a larger opening, therefore less differential, and leave the single vacuum referenced line as it is, connected to the manifold, post TB.


Just my thoughts on the matter.

/brox
 
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