blow thru carb question
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blow thru carb question

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    Default blow thru carb question

    when selecting a power valve in a blow thru set up do the same rules apply as n/a or is there a differant method of selecting which number valve to use
    also would a 650cfm blow thru carb be a good match for a 6000-6500rpm 327 "336 actual ci" starting to put together a turbo set up and looking for your knowlage,
    is it posible to use a high flow mechanical pump in this type of application im running 1/2'' hard line from the tank to the pump right now not going crazy with boost 8-10psi max, ill plumb in the return line when the time comes.
    thanks, T

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    Or Seth, either one Budweiser's Avatar
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    Your Power Valve selection should be similar to a N/A deal, 2-3" less (numerically) than idle vacuum. Here's a good site for learning about blow through carbs: Blowthrough Carb How-To

    As long as your mechanical fuel pump is rated for enough flow to support your HP goal it will work just fine, however it absolutely MUST be modified to reference boost pressure. This refers to the pump it self, not a boost referenced fuel pressure regulator.

    A search should turn up plenty of info on this modification, but if you have any questions... feel free.

    Good luck

    -Seth-
    Last edited by Budweiser; 03-11-2013 at 07:04 PM.

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    Quote Originally Posted by Budweiser View Post
    Your Power Valve selection should be similar to a N/A deal, 2-3" less (numerically) than idle vacuum. Here's a good site for learning about blow through carbs: Blowthrough Carb How-To

    As long as your mechanical fuel pump is rated for enough flow to support your HP goal it will work just fine, however it absolutely MUST be modified to reference boost pressure. This refers to the pump it self, not a boost referenced fuel pressure regulator.

    A search should turn up plenty of info on this modification, but if you have any questions... feel free.

    Good luck

    -Seth-
    Never heard of boost referanced fuel pump butni looked it up and some guys just feed a signal line into the vent/weap hole is this really all that has to be done interesting i need to pull a pump apart to see what it pushes on to make since of it thanks for the info ,T

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    Quote Originally Posted by bowtiebro View Post
    Never heard of boost referanced fuel pump butni looked it up and some guys just feed a signal line into the vent/weap hole is this really all that has to be done interesting i need to pull a pump apart to see what it pushes on to make since of it thanks for the info ,T
    I just figured out why you would boost referance the vent its set up to run on 14ish psi of pressure all the time then adding boost pressure would allow it to still work with pressure pushing back on the out going fuel

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    Quote Originally Posted by Budweiser View Post
    Your Power Valve selection should be similar to a N/A deal, 2-3" less (numerically) than idle vacuum. Here's a good site for learning about blow through carbs: Blowthrough Carb How-To

    As long as your mechanical fuel pump is rated for enough flow to support your HP goal it will work just fine, however it absolutely MUST be modified to reference boost pressure. This refers to the pump it self, not a boost referenced fuel pressure regulator.

    A search should turn up plenty of info on this modification, but if you have any questions... feel free.

    Good luck

    -Seth-
    So I'm curious, browsed thru the link.
    How do you reference the PV? Does it still open with vacuum drop, or is it boost pressure activated?



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    Last edited by gn7; 03-11-2013 at 10:53 PM.

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    Quote Originally Posted by gn7 View Post
    So I'm curious, browsed thru the link.
    How do you reference the PV? Does it still open with vacuum drop, or is it boost pressure activated?
    i belive because the carb is being pressurized the valve is opened with less vaccum than its rating, but in this case its positive pressure ie boost, the PV controls metering under no boost and during it opens and enriches the mixture same as in a roots blower set up but instead of having a boost referance line the pv is living in the pressure zone

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    As far as I know of there are only two places that make a boost referenced power valve, CSU & C&S. The power valve isn't held shut with vacuum, its held by a spring and the PV is actually opened by boost pressure. You can adjust the opening point by shimming the spring in the C&S PV, not sure how you adjust the CSU, but it is adjustable.
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    Quote Originally Posted by Hass828 View Post
    As far as I know of there are only two places that make a boost referenced power valve, CSU & C&S. The power valve isn't held shut with vacuum, its held by a spring and the PV is actually opened by boost pressure. You can adjust the opening point by shimming the spring in the C&S PV, not sure how you adjust the CSU, but it is adjustable.
    Quote Originally Posted by bowtiebro View Post
    i belive because the carb is being pressurized the valve is opened with less vaccum than its rating, but in this case its positive pressure ie boost, the PV controls metering under no boost and during it opens and enriches the mixture same as in a roots blower set up but instead of having a boost referance line the pv is living in the pressure zone
    I thought I had a pretty good grip on the function of a PV until I read this. After reading it 3 times I am more confused than ever. I am guessing that you think it remains referenced to the manifold. But thats only a guess.

    Like Hass posted, vacuum has NOTHING to do with the operation of the PV in a blow thru. A blow thru PV works NOTHING like a roots. You either reference the PV properly in a blow thru, or plug it. If you leave it as is, working off manifold vacuum, it will open long before the manifold is in boost.
    Also, C&S PV system is progressive to boost, where a normal PV is pretty much open or closed. The C&S system can meter the fuel in relation to the boost pressure.



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    Last edited by gn7; 03-12-2013 at 08:52 PM.

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    I guess i didnt realize that boost was what opened the pv in this kind of system i was still thinking it operated the same as it would in an n/a aplication and the increase of presure from vaccum to positive pressure or vaccum to atmospheric this is why im asking im trying to learn i need to pull the pv out of my carb that has been converted to see whats differant about it, is the spring on the reverse side and boost pressure has to over come the spring tension? Or if you had a conventional pv you would be in the rich side of the power valve at any thing other than idle?
    Last edited by bowtiebro; 03-12-2013 at 10:30 PM.

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    Quote Originally Posted by bowtiebro View Post
    I guess i didnt realize that boost was what opened the pv in this kind of system i was still thinking it operated the same as it would in an n/a aplication and the increase of presure from vaccum to positive pressure or vaccum to atmospheric this is why im asking im trying to learn i need to pull the pv out of my carb that has been converted to see whats differant about it, is the spring on the reverse side and boost pressure has to over come the spring tension? Or if you had a conventional pv you would be in the rich side of the power valve at any thing other than idle?
    Thats why I asked the question when I saw this was a thread on PV specifically. Because the method of opening the PV is totally diffferent on a blow thru. C&S references theirs form the bonnet pressure thru a hollow air cleaner hold down bolt, and is even fine tunable to some degree delay wise by turning the bolt into or away from the incoming air flow. I imagine you could have a PV referenced from the normal place reading manifold pressure, but still need pressure to open it as opposed to a simoly loose of vacuum opening it. You can see how a normal set up would open way earlyier if it was controlled by vaccum as opposed to pressure building to opening it.

    Yes, a normal power valve has a spring that holds the PV open and a C&S has a spring that holds it shut.



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    Quote Originally Posted by gn7 View Post
    Thats why I asked the question when I saw this was a thread on PV specifically. Because the method of opening the PV is totally diffferent on a blow thru. C&S references theirs form the bonnet pressure thru a hollow air cleaner hold down bolt, and is even fine tunable to some degree delay wise by turning the bolt into or away from the incoming air flow. I imagine you could have a PV referenced from the normal place reading manifold pressure, but still need pressure to open it as opposed to a simoly loose of vacuum opening it. You can see how a normal set up would open way earlyier if it was controlled by vaccum as opposed to pressure building to opening it.

    Yes, a normal power valve has a spring that holds the PV open and a C&S has a spring that holds it shut.
    Some of the cheap "blow through" carbs still use a vacuum operated pv (like quick fuel). But I don't consider them a real blow through carb. They are OK~ for a low boost deal, but if going with anything serious you had better steer away from those.
    Heres the problem If you use a vacuum operated pv. At a high cruise speed,with a reasonable load on the engine you could very well be into vacuum under the carb and pv closed, while if referenced from the hat, the pv might actually be begining to see boost and adding some needed fuel.
    Those two companies are very good at building a carb with a nice flat fuel curve.
    Last edited by Hass828; 03-13-2013 at 05:49 AM.
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    Quote Originally Posted by Hass828 View Post
    Some of the cheap "blow through" carbs still use a vacuum operated pv (like quick fuel). But I don't consider them a real blow through carb. They are OK~ for a low boost deal, but if going with anything serious you had better steer away from those.
    Heres the problem If you use a vacuum operated pv. At a high cruise speed,with a reasonable load on the engine you could very well be into vacuum under the carb and pv closed, while if referenced from the hat, the pv might actually be begining to see boost and adding some needed fuel.
    Those two companies are very good at building a carb with a nice flat fuel curve.
    this is my first go at putting a turbo on anything its goin into my street truck, its pretty much just off idle cruise or wot during driving, the carb i just got was modified by braswell and is not a real blowthru carb like your describing its a modified 450 mechanical secondary carb, im shooting for 8-10psi of boost max right now and maybe going more once i can get ahold of a real carb

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    Quote Originally Posted by bowtiebro View Post
    this is my first go at putting a turbo on anything its goin into my street truck, its pretty much just off idle cruise or wot during driving, the carb i just got was modified by braswell and is not a real blowthru carb like your describing its a modified 450 mechanical secondary carb, im shooting for 8-10psi of boost max right now and maybe going more once i can get ahold of a real carb
    Get some form of an 02 sensor on their so you can watch the A/F ratio. Its money well spent.
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    Quote Originally Posted by gn7 View Post
    So I'm curious, browsed thru the link.
    How do you reference the PV? Does it still open with vacuum drop, or is it boost pressure activated?
    Jeez, it's been so long since I've thought about all the intricacies and my mind is more or less consumed with figuring out another project at the moment, but off the top of my head... that particular page doesn't cover modifications needed to boost reference the power valve, so it would operate on pressure differential between above and below carb pressures, getting it's reference from the standard port in the base plate.

    There's lot's of tricky stuff that can be done once the power valve is modified for "boost reference", even activated electronically via hobbs switch and a solenoid utilizing exhaust pressure.

    To further enrichment under boost, a rubber tube/hose can be affixed to the bowl vent and extended up into the ducting, artificially increasing bowl pressure as high velocity air enters the end with a pitot tube effect.

    Lot's of stuff that particular article doesn't cover, but the modifications it does cover should suite the OP's intended use very well.

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