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Intake Selection; Square Bore Vs Spread Bore

Discussion in 'The Hokey Ass Message Board' started by flt-blk, Jul 5, 2007.

  1. What is the difference between Square Bore and Spread Bore.

    Small Block Chevy
    Demon carburater
    I am looking at an Edelbrock performer RPM Air Gap and it is offered with both openings.

  2. Square bore = both the primary, and the secondary throttle bores are the same size.

    Spread bore = the secondaries are much larger compaired to the primaries.

    Pick the one that matches you carb. spead bores are suppose to have a better part throttle feel/economy due to the smaller throttle plates
  3. RacerRick
    Joined: May 16, 2005
    Posts: 2,753


    If you have a demon carb, you need the square bore.
  4. Lobucrod
    Joined: Mar 22, 2006
    Posts: 4,122

    Alliance Vendor
    from Texas

    spread bore is for Q-jets and the holley and motorcraft spread bore carbs. Square bore are for Carter and Edelbrock AVBS and all of the holleys with the same size primary and secondary throttle bores. Oh and the demons.

  5. Thanks,
    Any other suggestions besides the Edelbrock for a good street dual plane intake?
  6. Deuce Roadster
    Joined: Sep 8, 2002
    Posts: 9,519

    Deuce Roadster
    Member Emeritus


    Spread bore above :)


    Square bore above ... :D
  7. atch
    Joined: Sep 3, 2002
    Posts: 5,105


    ok, i've always wondered but never asked:

    are the mounting bolt patterns on all spread bore carbs/intakes the same?

    same question for square bore carbs/intakes?
  8. SinisterCustom
    Joined: Feb 18, 2004
    Posts: 8,271


    I think? Q-Jets and Thermoquads have same patterns??...
    But there are different square patterns.....a larger one for Holley Dominators...
  9. Crankhole
    Joined: Apr 7, 2005
    Posts: 2,626


    I learned something today.
  10. jalopy43
    Joined: Jan 12, 2002
    Posts: 3,085

    Member Emeritus

    You can put a square bore carb on a spread bore manifold,with a flat plate adaptor(Edelbrock Performer). You can't put a spreadbore carb. on a square bore manifold(directly) but a thick adapter may work,,, BTW the Performer manifold is a good one for the street(1500- 5500rpm)
  11. George
    Joined: Jan 1, 2005
    Posts: 7,532


    If you look at the Edelbrock carbs, you'll see that they are double drilled for the small (50s or so) & large bolt patterns.
  12. BinderRod
    Joined: Jul 9, 2006
    Posts: 1,737


    My twin brother runs an AirGap intake and a 750 double pumper and he swears by it. Keith
  13. pasadenahotrod
    Joined: Feb 13, 2007
    Posts: 11,776

    from Texas

    I've bolted Edelbrock AFBs to aftermarket Q-jet spreadbore manifolds without adapters, you just have to use the right gasket to assure the seal at the big bore end.
  14. Spreadbore secondary plates will not clear all adapter plates, they will hit about half open, don't ask me how I know this.
  15. RacerRick
    Joined: May 16, 2005
    Posts: 2,753


    There are 3 common 4 brl carb bolt patterns. There is a small one used by early small 4 brl carbs, the standad large one that all modern carbs take, and the 4500 dominator flange and bolt pattern which is huge in comparison.

    Most aftermarket carbs are double drilled so you can use either of the non dominator bolt patterns.
  16. admells
    Joined: May 13, 2012
    Posts: 1


    I already have the edelbrock performer rpm air gap #7501 but its square bore and i want to put my 69 rochester quadrajet on it, does any know what adapter i need? Thanks for any help.
  17. They are the same pattern. I had a T-Quad on a Chevy 327 many years ago.

  18. LUX BLUE
    Joined: May 23, 2005
    Posts: 4,407

    Alliance Vendor
    from AUSTIN,TX

    The intake you want is the performer r.p.m. Air Gap. If you can, top that with a wilson tapered 4 hole spacer. Then hold on.

    Sent from my DROID device using the TJJ mobile app
  19. Fellas, its been 5 years.

    But this guy might need the help , he bumped it.

  20. Deuces
    Joined: Nov 3, 2009
    Posts: 22,127

    from Michigan

    I have a Holley square bore carb that has 1-9/16" primaries and 1-11/16" vacuum secondaries...
    It's off a '70 Chrysler car...
    I think it's a 735 cfm carb that has the standard Holley bolt pattern...
    That be a cool carb to tweak for mileage and performance...:)
  21. wow this is an oldie.
    I ended up with an Edelbrock air gap and a Demon carb. Have put 20-30k on the car since I got it going.

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