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Thread: Blow off valve (dump valve) questions

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    Mechanic beeRS's Avatar

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    Blow off valve (dump valve) questions

    I'm planning my first turbo installation, and I need a little help with the blow off valve. Forgive me if these sound like dumb questions...

    1. Do you have to fit a blow off valve? With an internal wastegate in the turbo, is it still common practice to have a blow off valve as well?

    2. Does it matter where its located, or is anywhere between the intercooler and the throttle body OK?

    3. I'm hoping to use quad throttle bodies mounted directly to the head. This gives no take-off point for inlet manifold vacuum. If a blow off valve is supposed to take a signal from the inlet vacuum, where can I route the pipe? I'm guessing that it can simply go to atmosphere?

    Any help from you knowledgeable folk would be appreciated.

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    Re: Blow off valve (dump valve) questions

    you dont have to use a dump valve, it could go either side of the intercooler and yes it needs connecting to the engine side of the throttle bodies, that way when you shut the throttle teh vacuum in the inlet manifold will help open the dump valve and when under boost the pressure will help keep it closed

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    Mechanic beeRS's Avatar

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    Re: Blow off valve (dump valve) questions

    Quote Originally Posted by Graham View Post
    you dont have to use a dump valve, it could go either side of the intercooler and yes it needs connecting to the engine side of the throttle bodies, that way when you shut the throttle teh vacuum in the inlet manifold will help open the dump valve and when under boost the pressure will help keep it closed
    Getting a vacuum line to the engine side of the throttle body butterflies is going to be tricky, but if I don't actually need a blow off valve, that would be problem solved! Cheers

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    Re: Blow off valve (dump valve) questions

    you only really benefit when using a lot of boost if that helps, i ran my race car with and without, and to TBH couldnt tell any difference

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    Re: Blow off valve (dump valve) questions

    Quote Originally Posted by Graham View Post
    you only really benefit when using a lot of boost if that helps, i ran my race car with and without, and to TBH couldnt tell any difference
    I'm not likely to go above 1 bar / 15 psi max.

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    Re: Blow off valve (dump valve) questions

    then in my opinion at least, its not needed, and one less potential source of boost/vacuum leaks

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    Re: Blow off valve (dump valve) questions

    Quote Originally Posted by beeRS View Post
    I'm not likely to go above 1 bar / 15 psi max.
    Sorry to disagree with Graham but slam the throttles shut at high rpm and that's enough boost to stall the compressor / bend blades - see it with less boost and a U/S BOV. Also set up right it'll work off boost while vac is available as a priority valve. Surely taping a 1/8NPT nipple into an inlet tract ain't gonna be that difficult?

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    Re: Blow off valve (dump valve) questions

    Quote Originally Posted by katana View Post
    Sorry to disagree with Graham but slam the throttles shut at high rpm and that's enough boost to stall the compressor / bend blades - see it with less boost and a U/S BOV. Also set up right it'll work off boost while vac is available as a priority valve. Surely taping a 1/8NPT nipple into an inlet tract ain't gonna be that difficult?
    perhaps, but i raced turbos for years without and never had a fail

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    Re: Blow off valve (dump valve) questions

    I have thought about this but not for a BOV. I thought about making a sandwich plate which sits between the ITB's and head and tap each runner for an vacuum feed. There is a company on ebay that make flanges etc in alloy so could easily done. This is of course all theory so far , Or use a manifold and dcoe style ITB's then you can tap the manifold.

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    Re: Blow off valve (dump valve) questions

    Good ideas guys. I spoke to the guys at AT Power today




    The good news is that they have blind holes in each of the 4 runners. They are already tapped 3/8"BSP - you just have to drill them through. I was surprised because the throttle butterflies look so close to the mounting surface.

    Just to be safe, I'll fit a dump valve.

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