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Paxton/McCulloch Pressurised Air-box

Discussion in 'The Hokey Ass Message Board' started by biggles, Apr 18, 2012.

  1. biggles
    Joined: Feb 8, 2007
    Posts: 88

    biggles
    Member
    from England

    Why is it thought necessary to have a pressurised air-box surrounding the carb fed from a Paxton (or McCulloch) centrifugal blower? And how do you get the throttle linkage in?
    I don't think Studebaker Avantis had one - just a sealed tube from the blower to the carb throat.
     
  2. metalshapes
    Joined: Nov 18, 2002
    Posts: 11,130

    metalshapes
    Member

    They did it both ways.

    My '28 has a Vs57 with a bonnet instead of a box that goes around the entire carb.

    I think it looks a lot cleaner, and its easyer to get to the jets.

    Main drawback, from what I've read and from what I've been told, is some leaking of raw fuel out of the carb past the throttle shaft.
    Its probably happening a little bit, but I'm not seeing any evidence off it....

    '28 Roadster 007.jpg
     
  3. PBRmeASAP
    Joined: Aug 26, 2002
    Posts: 6,893

    PBRmeASAP
    Member

    I cant find pics right now but i have a factory one for a packard that has the carb base (2bbl) in the box and the linkage is outside. if you want i can get a few pics for ya
     
  4. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,756

    Rusty O'Toole
    Member

    Some had one setup, some the other. The box was simple and trouble free in service, but bulky. The carb bonnet took up less room, was cheaper, but you had to seal up the carb shafts and pressurize the float bowl.

    The box or enclosure had holes for the throttle and choke. Not sure if they were sealed with grommets or not. They also had an overflow vent to drain off spilled fuel, under some conditions the carbs could flood.
     
  5. Judd
    Joined: Feb 26, 2003
    Posts: 1,894

    Judd
    Member

    Some pictures of a box Paxton setup.
     

    Attached Files:

  6. Silverplate
    Joined: Mar 4, 2011
    Posts: 242

    Silverplate
    Member

    Avanti R3's had a pressurised box. The R2 did not.

    [​IMG]
     
  7. moefuzz
    Joined: Jul 16, 2005
    Posts: 4,951

    moefuzz
    Member


    The pressurized box is to help keep the carb from seeping gas out of every seam (or shaft) as the supercharger pressurizes it.
     
  8. Enbloc
    Joined: Sep 27, 2004
    Posts: 1,900

    Enbloc
    Member
    from London, UK

    If you have a blow through system you need a carb that is modified to handle the pressurised air that is being p***ed through it. A normally aspirated carb creates a vacuum within itself so wants to pull the outside atmosphere in. A pressurised carb wants to do the opposite.

    You have to the seal throttle shafts and other items otherwise you'll get fuel/air mixture being pushed out between the gaps. You also have to vent the fuel bowl into the venturi rather than the atmosphere. This balances out the two. If you didn't you would be actually pushing fuel back down the main jets and into the fuel bowl.
    There are other considerations but these are the main issues. Some factory set-ups have the open carb as you say but these will have modified carbs to handle the boost. The principle is the same for early blow through turbo carb set-ups.
    Some carbs can be modified to handle boost and others can't.

    The "in the box" trick allows you to run a standard carb with no modifications. This is because the carb is completely surrounded by pressurised air so functions as normal because everything is balanced. Throttle and chokes linkages are fed into the box but would have to sealed where they enter the box. The box is of course completely sealed otherwised you'd would lose boost.
     
  9. biggles
    Joined: Feb 8, 2007
    Posts: 88

    biggles
    Member
    from England

    Thank you, chaps.

    On a Ford Flathead which carbs would be suitable for blow-through without a box?

    Were boost-referenced fuel pumps available in the '60s?
     
  10. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,756

    Rusty O'Toole
    Member

    The first McCulloch blowers offered to the public in 1953 were for Ford V8 cars. They used the stock carburetor with an L shaped rubber pipe from the blower to the carb. It looked like a radiator hose.

    They modified the carb with bigger main jets pressurized float bowl and possibly seals on the throttle shaft (not sure if they had such seals then). They also pressurized the fuel pump.

    Do a web search and you should find some old photos. There is a site called VS 57 or something similar that has a lot of info.
     
  11. rlsteel
    Joined: Apr 10, 2005
    Posts: 523

    rlsteel
    Member

    Nice Judd ,That paxton still lives at my house. RLS
     
  12. Enbloc
    Joined: Sep 27, 2004
    Posts: 1,900

    Enbloc
    Member
    from London, UK

    I believe 97's are good because they are vented in the throat of the carb.

    I don't believe its the pump that is boost referenced but the fuel regulator.

    These are available....

    http://www.burtonpower.com/parts-by-category/fuel-system/fuel-regulators-filters-carb/fsefpr009.html

    They increase the fuel pressure as the boost increases to compensate. Top quality and period looking as well.
     
  13. biggles
    Joined: Feb 8, 2007
    Posts: 88

    biggles
    Member
    from England

    Thanks Enbloc.
    You are, as usual, a mine of technical information.
     
  14. Enbloc
    Joined: Sep 27, 2004
    Posts: 1,900

    Enbloc
    Member
    from London, UK

    What you got brewing?
     
  15. The box is so that the float bowls are at the sam atmospheric pressure as the venturies. I read something on the theory on it a while back. Off all that I read (about 20 pages as I recall) the idea is that the entire carb be under the same pressure as opposed to just the throats being pressureized. it als mentuioned that you have to reference your fuel pressure regulator to the amount of boost to maintain flow to the carb. IE if you normally want 7 psi to the needle and you are running 10 psi boost you need to have your regulator bump up to 17 psi when that boost is achieved or you will not get fuel into the carb.
     
    Last edited: Apr 21, 2012
  16. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,756

    Rusty O'Toole
    Member

    Incidentally if you want to make a pressurized carb box, an old pressure cooker should work well if you have room for it. You can pick them up for a few bucks at yard sales and thrift stores. The real old cast aluminum jobs have a fifties vibe to them.
     
  17. metalshapes
    Joined: Nov 18, 2002
    Posts: 11,130

    metalshapes
    Member

    The floatbowls are the same pressure as the throat of the carb, because thats where the F/B vents are.

    So that doesnt change with bonnet or a press box.

    And they put them there to have them under the airfilter in N/A applications.

    So any blow through carb will need at least foam filled floats, because the hollow br*** ones will collapse.


    The main differences are only, as far as I can see, that the little bit of pressure that will escape past the spindles is dry air with a box, instead of mixture with a bonnet.
    And that you can probably go a lot higher with the boost pressure with a box, because if you have pressure on the outside of the carb as well, the boost cant rip it apart.

    With a relative low boost setup like a Vs57 the bonnet works fine, and it looks much better than the ugly Paxton pressure box.
    And for what it is ( just a simple box with a linkage in it and some fittings ), it is ridiculously expensive...
     
  18. biggles
    Joined: Feb 8, 2007
    Posts: 88

    biggles
    Member
    from England

    Well, there's always some crazy thing brewing in my mind, but this is as a result of meeting another crazy old person recently. I went looking for a Lancia sliding pillar front end but the first things I saw in his shed were three Studebakers, from a 1930s doctor's coupe to an Avanti. The Avanti didn't have a blower on it, but he showed me a couple of Paxton/McCulloch units on the bench, one of which was variable speed (VS-57) while the other wasn't. There was no sign of a pressure box on the Avanti which got me wondering. Incidentally, he also had a flathead in a Lancia Lambda special so I'm trying to get him to bring it out to join us on the hill climbs.
    Everywhere you go there are crazy people and intersting things - I'm happy to say.
     
  19. tommy
    Joined: Mar 3, 2001
    Posts: 14,756

    tommy
    Member Emeritus

  20. Judd
    Joined: Feb 26, 2003
    Posts: 1,894

    Judd
    Member

    LOL! Thanks for the 351W that's in my buddies Fox Mistake now. You have the Paxton hooked up and running yet?


     
  21. Judd
    Joined: Feb 26, 2003
    Posts: 1,894

    Judd
    Member

    Biggles
    With the box set up you have top use a boost indexed fuel pump also to bring up fuel pressure as pressure in the float bowl increases.
     
  22. Normbc9
    Joined: Apr 20, 2011
    Posts: 1,121

    Normbc9
    Member

    Here are a couple of old photo's from the '57 Ford Factory Y block 312 with the Paxton-McCulloch super chargers on them. There were a few differences if you look hard.
    Normbc9
     

    Attached Files:

  23. Shaggy
    Joined: Mar 6, 2003
    Posts: 5,207

    Shaggy
    Member
    from Sultan, WA

    I'd be scarred to run it without a box on low pressure carbs such as 97's, because the fuel needs to be at a higher pressure than the incoming air and strombergs hate over 2 or 3lbs of pressure anyway
     
  24. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,756

    Rusty O'Toole
    Member

    Original McCulloch blowers only put out 4 or 5 pounds pressure. The Kaiser version was 4 pounds the aftermarket or universal model 5 pounds. Don't know about the Ford or Studebaker model but they must have been about the same.

    The McCulloch has a regulator that allows pressure to build up to a preset level then does not allow it to go higher. This is done by a movable pulley that controls the impeller speed.
     
  25. mart3406
    Joined: May 31, 2009
    Posts: 3,055

    mart3406
    Member
    from Canada

    You can - (and need to) - 'boost
    reference' any normal stock or
    aftermarket mechanical fuel pump
    to use it in a blow-through
    super-or-turbocharged application.
    On most pumps it's simple to do.
    Somewhere on the fuel pump
    housing - on the "dry" - ie -
    'non-fuel' side of the diaphragm
    housing, there will be a small vent
    hole that normally references the
    diaphragm to atmospheric pressure.
    To reference the pump to boost
    pressure instead, you need to
    dismantle the pump and drill and
    tap the atmospheric vent hole for
    a hose or line fitting. Re***emble
    the pump, install it on the engine
    and connect the new vent fitting
    you've just added to a line going
    to the pressure box, if you're
    running one, or to the carb
    pressure bonnet, if you're not -
    and you're done.

    Mart3406
    ===========
     
  26. mart3406
    Joined: May 31, 2009
    Posts: 3,055

    mart3406
    Member
    from Canada

    ---------------
    It doesn't matter whether you're
    using a complete pressure box to
    enclose carb - or just a pressure
    bonnet to seal the top of the carb -
    either way - YOU *NEED* TO
    BOOST REFERENCE THE FUEL
    PUMP ON *ANY* BLOW-THROUGH
    SYSTEM.

    Mart3406
    =========
     
  27. metalshapes
    Joined: Nov 18, 2002
    Posts: 11,130

    metalshapes
    Member

    Yep..
     
  28. rlsteel
    Joined: Apr 10, 2005
    Posts: 523

    rlsteel
    Member

    Judd Still in a box waiting for a nice mustang -falcon to come along RLS
     
  29. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,756

    Rusty O'Toole
    Member

    Correct on the boost reference pressure to the fuel pump. McCulloch also recommended adding an electric pump for more volume if necessary.
     
  30. Shaggy
    Joined: Mar 6, 2003
    Posts: 5,207

    Shaggy
    Member
    from Sultan, WA

     

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