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Hei Vacuum Advance

Discussion in 'Early CJ5 and CJ6 Tech' started by V6CJ5, Apr 2, 2019.

  1. Apr 2, 2019
    V6CJ5

    V6CJ5 New Member

    Central Oregon
    Joined:
    Dec 11, 2004
    Messages:
    21
    We have a 69 CJ5 with the 225V6. We also have a bit of confusion over what vacuum source, ported or manifold, the HEI needs.

    What say the gurus?

    Thank you!!!
     
  2. Apr 2, 2019
    Chilly

    Chilly Active Member

    Joined:
    Mar 12, 2007
    Messages:
    1,486
    Ported and manifold are essentially the same vaccum level except when the throttle is closed. My understanding is that vac adv was originally at manifold vac, but was moved to a ported position to reduce NOX emissions when idling. At idle, ported vacuum will cause ign timing to retard, lowering firing pressure and thereby also reducing NOX emissions. Since one probably doesnt care how much power they are making at idle I'd say you could connect to ported vacuum and not experience much performance degradation. Some say off-idle throttle response will suffer because the distributor has to get off its lazy butt and advance itself before timing is back where it belongs.

    I recommend using ported, then if it acts poorly coming off idle move to manifold and see how that behaves.
     
  3. Apr 2, 2019
    Walt Couch

    Walt Couch sidehill Cordele, Ga. 2024 Sponsor 2023 Sponsor 2022 Sponsor

    cordele, Ga.
    Joined:
    Mar 29, 2007
    Messages:
    5,916
    Has been discussed a plenty on this forum and my personal opinion after doing it both ways several times I found my engine likes ported and runs the coolest.
     
  4. Apr 2, 2019
    70cj5134f

    70cj5134f Member

    East Tn
    Joined:
    Aug 18, 2018
    Messages:
    749
    Most performance engines(larger cam) like manifold. Stockish engines like ported.
     
  5. Apr 2, 2019
    V6CJ5

    V6CJ5 New Member

    Central Oregon
    Joined:
    Dec 11, 2004
    Messages:
    21
    Thank you all for the replies! I will connect it to the base of the carb for ported and be done with it.
     
  6. Apr 2, 2019
    Mike C

    Mike C Member

    Austin, TX
    Joined:
    Feb 19, 2007
    Messages:
    743
    The base of the carb is usually below the throttle blades for manifold vacuum source. The HEI was developed to run with ported vac in order to provide enough advance at heavy airflow to control emissions as mentioned above. The boards I participate where I have paid the most attention are with big block chevrolets. The manifold vacuum allows the to run significantly cooler at low throttle application.

    Aftermarket HEI setup your guess is as good as mine as what they have chosen for base parameters.

    My own experience is to set your timing around 10-12 degrees initial, 36 or so all in by 2500 or so if you have enough gear, and then add 10-12 more through manifold vacuum adjusting your idle to the appropriate speed as the last step. Hot rodded and not stock engines. Testing your combo to find the best solution.

    Crane offers a vacuum advance limiter so you can control not just the rate of advance of the vac can with the allen screw but the total amount.
     
    70cj5134f likes this.
  7. Apr 2, 2019
    Focker

    Focker That's a terrible idea...What time? Staff Member

    WA
    Joined:
    Aug 18, 2014
    Messages:
    7,540
    Here are my specs...

    Plugged vacuum line (to vacuum advance).
    Idles at 650 RPM.
    Timing at 9*.
    Under acceleration it's at 17*.

    Connected vacuum line.
    Idles at 650 RPM.
    Timing at 9*.
    Under acceleration it's at 32* max.

    For more info start here on post #57
    Post #57 of the HEI distributor cap #1 terminal
     
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