Boost Referencing the P/V
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Boost Referencing the P/V

  1. #1
    Senior Member BAHNER79AUSTRALIA's Avatar
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    Default Boost Referencing the P/V

    Read most of the forums here on PB after a few searches on how to reference the P/V in a blown application. Found diagrams, some people cut new channels in the base plate, others do it via the metering block etc.... All depends what type of carb you have i guess...

    I went a different way which i just wanted to share.

    This carb is a 4150 Mighty Demon 850cfm.

    First i used a pipe cutter and cut a small piece of 3/16 steel line and flared the end in anticipation of a external vacuum line.



    I decided to drill directly from the outside of the main block, directly into the P/V vacuum chamber. I marked out my tradjectory and my planned exit point onto the P/V chamber for the dill bit!



    I decided to drill through the center of the fitting that hangs off the side of the body of the carb. Not sure what they are used for, i assume they are a generic casting and they are tapped for screwing on attachments of somesort?? Anyway, i thought it was the perfect place for my vacuum point.



    I used a 9/64 drill bit for the hole. I emerged into the P/V chamber as planned! Bit scary, didnt know what to expect while i was drilling, just did it slowly, took my time. Had to drill on a slight angle, and tilt to get into the chamber from the fitting on the outside.



    After i had drilled through, i used a larger drill bit for the 1/2" or so to allow fitament of my steel line.





    I then used a 2 part epoxy to seal the steel line into place.





    I will block the existing vacuum pickups in the bottom of the P/V chamber from the underside of the body with either some lead shot or tap the hole and install a small screw in brass plug.

    I then decided to give the carb some protection with a fuel / temp resistant metallic paint, the vacuum fitting looks almost factory due to the location i installed it. (Having second thoughts about the colour though.... Looks too blingy... Might strip it off yet...) Now i can run an external vacuum line directly down underneath the blower to the manifold and the P/V will be boost referenced. I will cap the vacuum port seen coming from the base plate. (Bowls are wrong way round for the photo... was only a mock up. Putting a kit through the carb before re-assembly)




    I know many people say running a P/V on a blown deal in a boat is a waste of time, but my Bahner will not be a race boat, just a tough blown lake cruiser. Atleast with my modification i can run it with a P/V and see how it goes. I can always remove the P/V and block it, and just cap my new port if i change my mind. Im only running a P/V in the primaries, secondary P/V location is blocked and will run a larger jet size. Now i have to do the other carb!
    Last edited by BAHNER79AUSTRALIA; 06-23-2012 at 05:43 AM.
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  3. #2
    gn7
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    Nice job mate.
    I have modified Holleys in a similar method, but with the BG that little raised boss makes it look factory.

    There is a very good chance that a PV will work in you case. Light boats with low boost are the best candidates. Like you said, if it doesn't need the PV, you can remove it.

    Now don't forget to block the original vac feed hole.



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  4. #3
    Senior Member shan's Avatar
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    Quote Originally Posted by BAHNER79AUSTRALIA View Post
    Read most of the forums here on PB after a few searches on how to reference the P/V in a blown application. Found diagrams, some people cut new channels in the base plate, others do it via the metering block etc.... All depends what type of carb you have i guess...

    I went a different way which i just wanted to share.

    This carb is a 4150 Mighty Demon 850cfm.

    First i used a pipe cutter and cut a small piece of 3/16 steel line and flared the end in anticipation of a external vacuum line.



    I decided to drill directly from the outside of the main block, directly into the P/V vacuum chamber. I marked out my tradjectory and my planned exit point onto the P/V chamber for the dill bit!



    I decided to drill through the center of the fitting that hangs off the side of the body of the carb. Not sure what they are used for, i assume they are a generic casting and they are tapped for screwing on attachments of somesort?? Anyway, i thought it was the perfect place for my vacuum point.



    I used a 9/64 drill bit for the hole. I emerged into the P/V chamber as planned! Bit scary, didnt know what to expect while i was drilling, just did it slowly, took my time. Had to drill on a slight angle, and tilt to get into the chamber from the fitting on the outside.



    After i had drilled through, i used a larger drill bit for the 1/2" or so to allow fitament of my steel line.





    I then used a 2 part epoxy to seal the steel line into place.





    I will block the existing vacuum pickups in the bottom of the P/V chamber from the underside of the body with either some lead shot or tap the hole and install a small screw in brass plug.

    I then decided to give the carb some protection with a fuel / temp resistant metallic paint, the vacuum fitting looks almost factory due to the location i installed it. (Having second thoughts about the colour though.... Looks too blingy... Might strip it off yet...) Now i can run an external vacuum line directly down underneath the blower to the manifold and the P/V will be boost referenced. I will cap the vacuum port seen coming from the base plate. (Bowls are wrong way round for the photo... was only a mock up. Putting a kit through the carb before re-assembly)




    I know many people say running a P/V on a blown deal in a boat is a waste of time, but my Bahner will not be a race boat, just a tough blown lake cruiser. Atleast with my modification i can run it with a P/V and see how it goes. I can always remove the P/V and block it, and just cap my new port if i change my mind. Im only running a P/V in the primaries, secondary P/V location is blocked and will run a larger jet size. Now i have to do the other carb!
    I did something similar on my dominators. Many will say you don't need PVs on a boat, but PVs and a main will flow more fuel than just a main. Sometimes that's important.
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  6. #4
    Or Seth, either one Budweiser's Avatar
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    Well done!

    Nice write up. Thanks for sharing your project.

    -Seth-

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