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Help! 2 Problems (Both boost related)

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20+ Year Contributor
Jan 13, 2003
Houston, Texas
Ok here's the deal... I run (now) essentially everything stock. I have stock 2g BOV, and a crappy MBC that never worked. So I backed it all the way out to run stock 14psi.

This worked fine until today when I swapped my 2g BOV for a 1g BOV. On my test launch it spiked up to around 20 something psi and I just let off, afraid to blow things. This kept happening, so I replaced the 1g with the 2g again to see what happened.

Now, with the 2g, and the MBC still assembled (but backed all the way out), I only hold 11-10 psi. When I remove the bolt and vent the line (MBC located between turbo housing and wastegate), I achieve 14 psi again!!! WTF?!

Also, when I leave that the same and swap in the 1g BOV again, I climb to upwards of 20 again (never let it go all the way to see how far). If I put the MBC bolt back in and seal that line, it stays at 10-11psi.

Is it possible the 1g BOV has just not been used for awhile and needs lubing? If so, how, and where? It has a slight metallic screech to it as if its rubbing metal on metal. Perhaps the flapper isn't opening properly, forcing my boost to climb out of control?

And what the hell is up with that MBC? That makes absolutely no sense to me! I can't understand how simply sealing the line forces it to run at LESS than stock boost???

Please help guys!!

I think I figured it out (although not the initial spike).

There must have been a kink or block or something in the MBC line initially. With the 2g BOV, I could adjust the MBC all I wanted to no avail. It would just stay at 14psi (which I thought was stock). In actuality it was limitless boost since it was blocking pressure to the wastegate actuator (if my guess is correct), but was leaking off at the BOV at 14psi.

This is why when I put on the 1g BOV, the boost held into the 20's (I'm curious how far it could have gone, but don't want to try). But when I fixed the MBC line and left it sealed (but backed out), it reverted to TRUE stock of 10-11psi. I thought it was seriously screwed up. That's why when I vented the wastegate actuator line (opened MBC), it let it climb to the limit the BOV would hold.

Just so I know for sure, the line from turbo to wastegate actuator does the following:

When the pressure builds past XX psi, it opens the wastegate actuator and vents the turbo. Otherwise it tries to hold boost. That's why an MBC blocks the flow stream, to prevent the actuator from opening as early.

I guess that also explains why a longer line introduces boost spike. As the initial pressure in front of the MBC climbs, the pressure behind is essentially zero (or at least lower than in front of the MBC). When the pressure finally is enough to move the ball (or actuating mechanism) in the MBC, it opens the stream, but the initial flood of gas has to just equalize the pressure behind the ball, before it can even open the wastegate, thus the spike, then "bleed" off.

My ghetto-rigged freebie MBC sucks. I'm going to try and design something much more precise, using the BOV style (also used in industrial pressure relief valves).... we'll see!
Try this. I just installed a Greddy type s. I teed into the line from the turbo to wastegate and BCS for the bottom nipple. With it installed, boost spike over 20psi, when I took it off and blocked the line, boost spike to 15 psi then 12-14psi (essentially stock). Doesn't make sense.
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