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2x4 linkage - BBC

Discussion in 'The Hokey Ass Message Board' started by T-Roy, Sep 10, 2006.

  1. What dual 4bbl linkage would be best to run on my 396, Progressive or 1:1? I've got an Offy mid rise intake and dual Holley 1850 600 cfm carbs. Motor is a 396, Comp 280 hyd cam, 10.2:1 compression, heavy ass car and will soon have a TH400, 2800 stall converter and 3.73 gears.

    It seems to me that running progressive linkage with dual 4's would tend to make a couple cylinders run leaner than the others. This is mainly the reason why I'm asking. Does this make sense?

    -Troy
     
  2. man-a-fre
    Joined: Apr 13, 2005
    Posts: 1,311

    man-a-fre
    Member

  3. tommy
    Joined: Mar 3, 2001
    Posts: 14,757

    tommy
    Member Emeritus

    No, sorry, that doesn't make sense (no offense:D)...because the air fuel ratio gets determined in the carbs before it ever gets to the intake manifold. Unless you have a vacuum leak that will add air after it leaves the carb, the mixture will not lean out because it travels farther to one cylinder than it does to another.

    There may be exceptions but every non-cross ram factory dual quad system that I can think of used progressive linkage to kick in the second carb when needed. 283/270HP Corvettes. 409/425 Impalas Buicks, Cadillacs, 392 Hemis etc. etc. I'm not saying that progressive is the best set up for you. I'm just saying that it works on the street without hurting the engine. If that's your only concern...not to worry.
     
  4. 1:1?

    If it's not progressive, straight linkage is the common term.

    The straight linkage shown in the pic is easy to make at home.
    Couple of 1/4" female Heim joints, length of 1/4" stainless rod threaded 1/4-28 on the ends and a pair of stainless locknuts.
    Cost = $12. when I made it and still under $20. nowadays.

    Some guys like to use right and left hand threads on these.
    I've found right hand threads will work just fine.
    A half turn is only moving the center to center distance 1/56th of an inch.

    Left hand dies are available as are left hand Heim joints.
    Left hand taps are available as well.
    Search the machine supply outfits such as MSC or Enco.
    A left hand locknut will have to be home-made.
    There are left handed nuts available, but you'll have to do some searching for those.

    If you're gonna run progressive linkage, get a quality unit.
    Edelbrock has a very nice dual quad progressive linkage.

    Some of the progressive linkages for triple 2 bbls are total crap.
    I've seen a few of them that were nothing more than bent rod.

    Straight linkage works well, I had my 462" Buick idling at 600 rpm with excellent throttle response from the Carter 500's.
    Cam and diff gears were about the same as yours.

    You shouldn't have a serous fuel/air ratio problem in individual cylinders when running progressive.
    Use the rear carb as the primary - as most do.
    The primaries on that are near the center of the intake and the mixture will be fairly even.

    Even if the front cylinders run a little leaner, it's not enough to hurt anything at part throttle.
    Assuming the car is jetted right to start with.

    A stock 4 bbl right out of the box* will work just fine with the factory installed jetting.
    Run it for a while and then do the jetting that is correct for your engine and especially correct for your home altitude.

    You can get an idea of where to go on the jetting - for a single carb, but it works for dual quads - from the Edelbrock site.
    Go here: http://www.edelbrock.com/automotive/eps_intro.html
    Peruse a similar CFM size Edelbrock, note the percentage change for altitude etc. and use that same percentage change to select jets for the Holley.
    Sounds difficult, but it's easy.
    Just have to do a little extrapolating.



    *Holley, Carter and Edelbrock cars come jetted about 5% rich for the engine size that particular carb is destined for - single 4 bbl installs.
     

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  5. Paul
    Joined: Aug 29, 2002
    Posts: 15,211

    Paul
    Editor

    I noticed little difference either way with my last few dual quad big blocks,
    all Edelbrock carbs on
    Weiand tunnel, Edelbrock tunnel, Offy medium rise and Edelbrock medium rise

    but in the end went straight linkage with all because it was easier to tune and may have run just a bit better

    why not try both and see which works better with your combo?
     

  6. Excellent point Paul.

    It may be a good indicator to run linkage to the rear carb only - front carb plumbed in and tuned for idle - and see how the car runs up to about the half throttle point.
    That would simulate progressive linkage as long as full throttle wasn't used.

    A little plug reading along the way wouldn't hurt.
     
  7. man-a-fre
    Joined: Apr 13, 2005
    Posts: 1,311

    man-a-fre
    Member

    Tried both myself on 377 stroker in 65 vette dual 450,s like the "1 to One".We all new what he meant by the term.
     
  8. squirrel
    Joined: Sep 23, 2004
    Posts: 47,921

    squirrel
    Member

    do the carbs sit sideways? or in line?

    I have progressive linkage on my blown big block, two holley double pumpers on a 6-71.
     
  9. Thanks for all your responses guys... Very useful information. =)
    The motor is already broken in with about 6000 miles on it. It runs great with the 750 dbl pumper jetted 78 pri and 82 sec, but have always planned this motor to run dual 4's. It is an inline setup...

    I'll check out the Edelbrock link that C9 suggested and Tommy -
    This is something new I've never heard before. I've read other posts on different boards and nobody has ever mentioned this. Maybe not many people really understand how a carburetor works?

    -Troy
     

  10. Well . . . I didn't.
    Which don't mean much, but I bow to your superior knowledge....:p
     

  11. You're welcome.

    Now that I've mentioned how hard left hand nuts are to find . . . I went to True Value Hardware this morning for some bolts and stuff.

    Sure enough, they had a plastic slide-out box with left hand nuts....:eek:


    And . . . once upon a time I cut right hand and left hand threads on a male thread adapter for a wood lathe.
    Weird looking they were. :eek:
     

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