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Technical Adjustable Vacuum Advance

Discussion in 'The Hokey Ass Message Board' started by Doublepumper, Jun 9, 2021.

  1. Doublepumper
    Joined: Jun 26, 2016
    Posts: 1,019

    Doublepumper
    Member

    Helping a friend do a tune up on a basically stock 350 SBC. He bought a new distributor with an adjustable vacuum advance. I've never dealt with an adjustable vac. advance before.
    How should they be properly adjusted?
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 48,757

    squirrel
    Member

    with vacuum disconnected, set timing for around 34 degrees at 2500 rpm or so. Get the idle happy by adjusting the carb. Then connect the vacuum advance, and see how it runs and drives. If it pings at light throttle application, you might have to dial back the vacuum advance a bit. If not, then it's probably ok as is. You can check the action of the vacuum advance with a timing light. There are specs for different engines for mechanical and vacuum advance, in the factory shop manuals for all the cars that the 350 came in originally...you'll find they vary somewhat, usually due to calibration for performance and emissions for that particular year and model.
     
  3. Here are instructions for adjustable vacuum advance, near the bottom.
     

    Attached Files:

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  4. Doublepumper
    Joined: Jun 26, 2016
    Posts: 1,019

    Doublepumper
    Member

    Perfect! Just the info I needed.
    Feel much more confident in getting it dialed in now.
    Thanks guys!
     
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  5. Follow the instructions, test drive and repeat. It may take multiple tests to got it right on. Be patient and see what your engine wants.;)
     
  6. Truck64
    Joined: Oct 18, 2015
    Posts: 4,633

    Truck64
    Member
    from Ioway

    Crane (who makes adjustable vacuum canisters) suggests starting out tuning the vacuum advance with the can fully pegged to the high side and making a test drive. Bring the 3/32" with you. It will most likely ping like crazy. Back off 2 turns at a time, till engine no longer rattles. Note that excessive vacuum advance will cause a ping under completely different conditions than excessive distributor based advance.

    The engine knock caused by excessive vacuum advance will actually go away with acceleration, and then return when the load levels off. The takeaway though is once you get the base distributor timing and centrifugal curve squared away, don't mess with it anymore, and adjust the vacuum advance last, the tendency though is to try to compensate for excessive vacuum advance by retarding the distributor timing. This is the wrong way to go about it.
     
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  7. FuelRoadster
    Joined: Nov 17, 2005
    Posts: 1,059

    FuelRoadster
    ALLIANCE MEMBER

    I would think experimenting with ported vs. direct manifold vacuum would also be something to think about.
     
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  8. Joe H
    Joined: Feb 10, 2008
    Posts: 1,059

    Joe H
    Member

    I read somewhere on this site a guy installed two vacuum gauges, one to ported, the other on manifold vacuum. He drove around and both gauges read just the same, except at idle. So if you have idle issue it can help or hurt depending on where you hook the distributor to, other wise it doesn't matter once the engine is off idle.
    Here is a pretty detailed list of vacuum advance units, http://www.crankshaftcoalition.com/wiki/images/e/e4/Vacuum_Advance_Specs.pdf , You can use this as baseline for the engine you are working on, note the starting vacuum and total degrees.
     
  9. Truck64
    Joined: Oct 18, 2015
    Posts: 4,633

    Truck64
    Member
    from Ioway

    A general rule of thumb is that street driven performance engines usually with a radical camshaft benefit from a direct manifold connection. It is difficult sometimes to achieve a steady idle, which incidentally is why a ported vacuum distributor connection was invented in the first place. For stock or stock-ish engines, and people who don't want to make a deep dive into distributors and timing curves or custom vacuum canisters are probably better off using the factory connection.
     
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  10. Blues4U
    Joined: Oct 1, 2015
    Posts: 5,575

    Blues4U
    Member
    from So Cal

    That link is a nice bit of technical information, thank you for posting it
     
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