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Technical 8BA vacuum question

Discussion in 'Traditional Hot Rods' started by 36tudordeluxe, Jan 19, 2014.

  1. 36tudordeluxe
    Joined: Oct 2, 2008
    Posts: 496

    36tudordeluxe
    Member

    Have a 276 8BA with L-100 cam and am only getting 12.5-13" of vacuum, is this due to the cam? Re-torqued carb. to intake and intake to block, all vacuum ports are blocked off.
     
  2. ronnieroadster
    Joined: Sep 9, 2004
    Posts: 1,081

    ronnieroadster
    Member

    I gather the reading is at idel RPM if so thats pretty much the readings you will have with the L-100 or most any other performance grind cam ina flathead V-8.
     
  3. 36tudordeluxe
    Joined: Oct 2, 2008
    Posts: 496

    36tudordeluxe
    Member

    Ronnie, thanks for responding. Actually, at idle am showing zero".
     
  4. Sounds like you've got the gauge connected to off-idle or ported vacuum, probably somewhere on the carburetor. You need to find source of full manifold vacuum, like a port in the intake itself.

    There may be a manifold vacuum port on the carb but you need to make sure it's reading vacuum somwhere below the throttle plates.
     

  5. 36tudordeluxe
    Joined: Oct 2, 2008
    Posts: 496

    36tudordeluxe
    Member

    Clay, you're correct, I believe one port is for the distrib. and one for the auto. trans, apparently neither is suitable for testing. I guess I would have to drill and tap the Offy intake for a vacuum source?
     
  6. dashman
    Joined: Apr 15, 2009
    Posts: 774

    dashman
    Member
    from Viroqua WI

    Maybe we have a vacuum riser that would work rather than modifying the intake?

    Dashman's Hot Rod & Speed Parts www.dashman.net
     
  7. If you haven't already, try the port that's for the automatic transmission. Odds are that it's full manifold vacuum. The fitting on the outside of the carb might be above or below the throttle plate. Even if it's above the throttle plates, the internal porting in the bowl and/or throttle body may still go to a source of full manifold vacuum below the throttle plates.
     
  8. 36tudordeluxe
    Joined: Oct 2, 2008
    Posts: 496

    36tudordeluxe
    Member

    Okay, went to the other port and am now getting 13"'s at 850 RPM idle where before I was getting 0". increasing RPM has no affect on the vacuum reading except when I release the throttle vacuum jumps to 17" briefly. Is this the kind of readings you might expect with a hotter cam?
     
  9. ROADSTER1927
    Joined: Feb 14, 2009
    Posts: 3,157

    ROADSTER1927
    Member

    That sounds pretty normal to me.
     
  10. ronnieroadster
    Joined: Sep 9, 2004
    Posts: 1,081

    ronnieroadster
    Member

    Yes that's exactly what you will see on your gauge. When you drive as you put the engine under load you will see the reading go down usually under 10 but as the RPM increses threw the gears it will increase again.
    Once your in high gear on level road the reading will increase. The higher the vacuum reading in high gear the better mileage you get. If the engine labors in high gear due to the rear ratio being to high the vacuum reading will be a lot lower. Using the vacuum readings is an excellent way to fine tune the engine.
     
  11. 36tudordeluxe
    Joined: Oct 2, 2008
    Posts: 496

    36tudordeluxe
    Member

    Thank you, feel better now!
     
  12. Couldn't have said it better! :D

    Here's a link that will give you an idea of how to interpret what the vacuum gauge is trying to tell you. Scroll down the page a bit for some animated examples.

    http://www.secondchancegarage.com/public/186.cfm
     

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