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  1. #11
    Long Live the Opti


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    Ok then, someone explain to me how they operate differently then. I've seen BOV's used instead of the race bypass valves many times with no ill effect. What internally is different and how is this affecting the operation and it's effect on the air being moved? I'm looking to learn something here.
    Quote Originally Posted by Ryan Stout View Post
    I would try to work my neck muscles but I'm not invited to the LS guy parties.

  2. #12
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    Quote Originally Posted by Fastbird View Post
    Ok then, someone explain to me how they operate differently then. I've seen BOV's used instead of the race bypass valves many times with no ill effect. What internally is different and how is this affecting the operation and it's effect on the air being moved? I'm looking to learn something here.
    Blow off Valves typically have springs in them rated for a certain PSI. Because of this, vaccum cannot pull the valve open. It will only open when pressure in the tubing exceeds the PSI rating on the spring.

    Yes, people do run BOV's instead of surge/bypass valves on supercharged applications, but they should be advised not to. Because of the excess air moving, it can cause tuning (driveability) issues (expecially with mass air cars), can cause compressor surge, and also belt problems.

    It's best to stay away from them for supercharged apps.


    Josh Hamming
    1996 Z28
    8.503 @ 155.53 at the 2016 LTx Shootout

  3. #13
    Long Live the Opti


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    Quote Originally Posted by Ponyhntr View Post
    Blow off Valves typically have springs in them rated for a certain PSI. Because of this, vaccum cannot pull the valve open. It will only open when pressure in the tubing exceeds the PSI rating on the spring.
    Now, correct me if I'm wrong here, but a lot of those aftermarket blow off valves have adjustments where you can change out the springs and also use a set screw to adjust the tension on the springs, allowing fine tuning of the blow off valve. Would this not allow it to act just like a bypass valve when properly set (I.E. to bleed air at x amount of vacuum and under).
    Quote Originally Posted by Ryan Stout View Post
    I would try to work my neck muscles but I'm not invited to the LS guy parties.

  4. #14
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    Quote Originally Posted by Fastbird View Post
    Now, correct me if I'm wrong here, but a lot of those aftermarket blow off valves have adjustments where you can change out the springs and also use a set screw to adjust the tension on the springs, allowing fine tuning of the blow off valve. Would this not allow it to act just like a bypass valve when properly set (I.E. to bleed air at x amount of vacuum and under).
    No, it wouldn't. The spring is actually holding the piston shut on a BOV. It's a much heavier spring than what is in a bypass valve. This is why vacuum cannot pull it open. I've read where a BOV company was offering -x psi springs to turn the BOV into a by-pass valve, but I have not heard of anyone using them with any success.


    Josh Hamming
    1996 Z28
    8.503 @ 155.53 at the 2016 LTx Shootout

  5. #15
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    They can be adjusted to work properly very easily. I only plan to use as a secondary valve though. For one, most are reasonably loud. I plan on using it to supplement my vortech bypass and keep it fairly tight. This way it will be availible upon really high loads when i dont think the vortech is sufficient.
    Overall i prefer the Tial 50mm BOVs because of their low price and high quality. Also they can be had with a very light spring, i think as low as 2psi. Even an engine with a HUGE cam should pull enough vacuum to open that, that is overall a very small pressure differential.
    94 T/A- forged 355 LT1, bone stock heads, Vortech YSi w/10-rib, FMIC, T56, 9"
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  6. #16
    Long Live the Opti


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    Riddle me this then? Why does my HKS SSQV BOV on my Celica, the Bosch BOV's on my Corvette, and the JGS 56mm BOV for my T/A all have vacuum ports on them if they simply operate off of the valve being overpressured by the incoming air?
    Quote Originally Posted by Ryan Stout View Post
    I would try to work my neck muscles but I'm not invited to the LS guy parties.

  7. #17
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    That is where your miss understanding... If you ever watch a vacuum gauge while doing a pull, you'll see that at the very moment the throttle is closed, the amount of vacuum will very easily exceed what the motor makes at idle. Therefore it is enough to pull the valve open. But at idle, the valve will stay closed.

    It's not the pressure inside the tube opening the valve at all.

    Theoretically, you could change out the spring + back off the tension of the adjuster to solve this. But I have not seen a spring that will physically fit be loose enough. We tried this with a buddy's turbo car and could not get the valve to open at idle.

    On the opposite end of the spectrum, it is very easy to make a SC bypass valve stay closed at idle by simply tightening the adjustement screw.

    So Fastbird, you can make a tubo BOV work, if you can find a light enough spring that not only fits, but will hold the valve shut under boost. But why bother, just buy a bypass with the correct spring pressure from the get go.
    - Justin

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    Quote Originally Posted by WJ Birmingham
    Boost... Makes good motors better, and bad motors junk. http://www.yellowbullet.com/forum/im.../icon_eyes.gif

  8. #18
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    ok - bypass valve, good one that won't leak.
    where do they install? what do the hook to - manifold vacuum and air before the throttle body?
    anyone have an installed pic?

    60lb injectors will handle how much boost?
    With a slightly rich tune and 10psi now - at what boost will I need to re-tune timing etc in the PCM?

    water/meth - at what compression/boost on 93 octane is this mandatory or just better safe than sorry?

    thanks guys!
    95 Z28 FORGED 355, JE 8.5:1 PISTONS, 230/239 - .530"/.546", 114 @ 110 CAM, stock heads, Pacesetter longtubes, 10 PSI from ATI P-1SC Procharger, 60#, 4L60E, Yank3600, 9" 3.73, ET Street 28x12.5x15, 3450lb

  9. #19
    Long Live the Opti


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    Quote Originally Posted by Fire67 View Post
    That is where your miss understanding... If you ever watch a vacuum gauge while doing a pull, you'll see that at the very moment the throttle is closed, the amount of vacuum will very easily exceed what the motor makes at idle. Therefore it is enough to pull the valve open. But at idle, the valve will stay closed.

    It's not the pressure inside the tube opening the valve at all.

    Theoretically, you could change out the spring + back off the tension of the adjuster to solve this. But I have not seen a spring that will physically fit be loose enough. We tried this with a buddy's turbo car and could not get the valve to open at idle.

    On the opposite end of the spectrum, it is very easy to make a SC bypass valve stay closed at idle by simply tightening the adjustement screw.

    So Fastbird, you can make a tubo BOV work, if you can find a light enough spring that not only fits, but will hold the valve shut under boost. But why bother, just buy a bypass with the correct spring pressure from the get go.
    Ok, I see what you're saying. I'm hung up on the physical operation which is essentially the same, but the forces affecting the operation that are different is what I was misunderstanding. Thanks for helping clear that up for me guys!
    Quote Originally Posted by Ryan Stout View Post
    I would try to work my neck muscles but I'm not invited to the LS guy parties.

  10. #20
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    Quote Originally Posted by chief455 View Post
    ok - bypass valve, good one that won't leak.
    where do they install? what do the hook to - manifold vacuum and air before the throttle body?
    anyone have an installed pic?

    60lb injectors will handle how much boost?
    With a slightly rich tune and 10psi now - at what boost will I need to re-tune timing etc in the PCM?

    water/meth - at what compression/boost on 93 octane is this mandatory or just better safe than sorry?

    thanks guys!
    Here is mine:





    The vacuum line that goes to the valve needs to be manifold vacuum, and you should put the valve between the blower and the intercooler. I'm not running an intercooler, so it is before the MAF.

    60lb injectors are usually good to about 750 rwhp through a 6-speed, but there are many other variables that can vary that number.

    Anytime you change boost its recommended to have the tune updated/checked.


    Josh Hamming
    1996 Z28
    8.503 @ 155.53 at the 2016 LTx Shootout

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