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Thread: very low 3rd gear boost mystery??

  1. #1
    turbo addict
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    very low 3rd gear boost mystery?? FIXED!!

    if I launch race-style trying to go as fast as possible, 3rd gear builds boost VERY slowly..maybe 8 psi after 5 seconds..never getting up to 22psi..

    but if I then let off the gas, let the rpms drop a bit, and then WOT in 3rd, goes up to 22 psi no problem

    I'm thinking perhaps the BOV deserves a look. Maybe its somehow sticking open when I shift from 2nd to 3rd for some reason. Is that possible? I'm trying to think of a way to prove it.

    I have a very long line from vac block to BOV..maybe 3 to 4 feet. I know that seems unpleasant, but I'd like to prove thats an issue. My BOV is actually that far away so I need a long line unless I relocate the BOV, which I'd prefer not to do.

    So lets say this happens..2nd gear gets up to 18psi say..

    Time to shift to 3rd..

    I let off the gas.

    Plenum goes to VAC.

    Long line to BOV drains into plenum.

    Intercooler piping is still at 18psi.

    BOV opens at some point (when pressure against its valve and its line create enough of a differential to open against its spring)

    So now its open and draining the intercooler piping.

    I have shifted into third and have just gone WOT again.

    Plenum goes to 0psi.

    At this point..some theories:

    Intercooler piping has enough pressure in it to keep BOV open, which makes building boost very slow, which keeps plenum pressure low, which does not help the situation. Vicious circle..

    Is that a possibility? In other words, I shift and re-WOT before the intercooler piping has fully drained, and things get into an intermediate state where BOV cant close because boost cant be generated because BOV cant close because boost cant be generated etc..etc..

    ??

    How can I prove the BOV is staying open when it shouldnt? Things are pretty loud and I dont hear anything obvious as far as it leaking.

    UPDATE..FIXED!!

    I did two things and the problem went away. And I'm 99% sure that one of the things was not the solution.

    I took apart my greddy type RS BOV and cleaned it. I found nothing of interest inside. It wasn't really dirty.

    I noticed the actuator side hose on my grainger/MBC may have been covering up the bleed hole on the MBC. So that could have been keeping the wastegate open longer than it should have and caused this problem. I did not actually SEE the hose covering it up, but while I was inspecting the MBC and after I had taken its hoses on and put them back on, I noticed the little hole, which I hadn't before.

    Anyways I did several full power runs 1st, 2nd, 3rd, with no boost lag.

    YAY
    Last edited by acannell; 11-24-2014 at 11:30 PM.

  2. #2
    turbo addict
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    Re: very low 3rd gear boost mystery??

    Asa,
    I've experienced this before with some MBC's. That is why I drill the hole in mine before the barb and thumbwheel, I just use a pin vice and slowly drill it by hand, since it's brass it goes pretty quick. Glad to see you found your issue.
    Todd
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  3. #3
    turbo addict
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    Re: very low 3rd gear boost mystery??

    Quote Originally Posted by 4 l-bodies View Post
    Asa,
    I've experienced this before with some MBC's. That is why I drill the hole in mine before the barb and thumbwheel, I just use a pin vice and slowly drill it by hand, since it's brass it goes pretty quick. Glad to see you found your issue.
    Todd
    that looks like a good solution, thanks. I suppose I should do that change.

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