Ported vs manifold vacuum advance solved!!!

Electrical & Ignition

  1. Turboomni

    Turboomni Well-Known Member

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    When I bought my 69 Fury it was running ok but hard to start especially when warm. I dug out my tach /dwell meter and vacuum gauge and tried to refresh my memory with all this stuff I hadn't dealt with since the 1970's. Timing light showed 12 degrees btc ,and I thought fine ,dwell good ,Idle speed good vacuum good. I did notice slight surging at cruise especially going up a slight hill but it ran ok. Still hard to start sometimes cold and bad when hot which I thought was the usual heat soak of the carb etc. I saw talk about ported and manifold vacuum debate. Checked my car and the vacuum advance was on the manifold port. Plugged the port and idled without the vacuum advance and the timing light showed 3 degrees btc. Bumped up the initial to 12 and connected my vacuum advance to ported cured my start problems except if really hot. Surging disappeared too.
     
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    • Ceedawg

      Ceedawg Well-Known Member

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      I didn’t read all these post but I hope you people understand what a VACUMN advance does. It’s like that carb accelerator pump, gives the engine a boost to get moving. It’s there to help before the centrifugal advance part of the dist. starts moving. Furthermore I’ll go with what Chrysler engineers did before I’d listen to the Edelbrock ‘engineers’
       
    • Ceedawg

      Ceedawg Well-Known Member

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      Telling your age there dude
       
    • Steve Wood

      Steve Wood Member

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      Actually, the guy is correct but it can be difficult to set up manifold vacuum advance when running a lot of initial advance and many don't have the patience or understanding to do it.

      Ported advance came along when emissions first raised its head. Too much initial advance increase NOX (NO2) so the factories started removing initial advance and sometimes even went from before top dead center to after top dead center to be able to pass emissions requirements.

      Once the butterflies open a little, ported vacuum and manifold vacuum reach the same level and the effect upon timing is the same. Put a vacuum gauge on each port and start opening the throttle. The gauges will quickly read the same.

      Mechanical advance is dependent upon rpm solely whereas vacuum advance is a function of load (load increases, vacuum drops).

      The benefit of adding vacuum sourced advance comes in improved cruising mileage when the total advance between mechanical and added vacuum rises to somewhere around 45-50 degrees.

      A problem arises when one runs a lot of initial advance and the vacuum starts increasing the advance above the mechanical advance and we get light throttle detonation at cruise or the applied vacuum does not drop immediately when the throttle is cracked for acceleration and the timing does not drop back to the mechanical advance only.

      The problem is complicated by compression ratio and camshaft duration.

      It becomes a juggling act. Adjustable vacuum advances are available to trigger the activation point and one also can limit the total amount of available by limiting the total available from the vacuum can rod movement.

      If one does a lot of cruising, then one might like to get a couple of mpg by learning how to set the ignition up. If one does not do any real highway driving and just wants to play on the street, then it is probably way too much hassle.

      In the end, vacuum is vacuum whether comes through the manifold port or the ported vacuum port. The only difference is at idle and just off idle.

      Ported was invented to try to beat the emissions requirement at idle with regard to NOX.

      EFI solves most of the problem when it comes to performance.
       
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      • Nevada dan

        Nevada dan Well-Known Member

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        so your saying port vacuum was invented after emissions came out in the 70's
         
      • Ceedawg

        Ceedawg Well-Known Member

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        What I was going to say! ALL Chrysler’s WERE ported (off idle) till the 73 emission junk came about with the firewall piece and the electric aided dist in 70. good god man we were alive and working on them then.
         
      • Nevada dan

        Nevada dan Well-Known Member

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        Ported was invented to try to beat the emissions requirement at idle with regard to NOX.

        EFI solves most of the problem when it comes to performance. this made no sense to me, sorry
         
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        • 65-440

          65-440 Well-Known Member

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          Hahahahaha...was thinking the same thing!
           
        • Master Gunner

          Master Gunner FBBO Gold Member FBBO Gold Member

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          This comes up every couple of years, and is always good entertainment watching folks get all upity over what works for who.
          That said, I'll stir the pot, Chrysler started using port vacuum in the 50's well before emissions.

          Mark :popcorn:
           
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          • WarrickS

            WarrickS New Member

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            Absolutely correct, it just goes to prove how many people pop up on forum sites and waffle on as if they know what they're talking about.... and with such conviction ;-))) If only someone competent would tune their engines the right way, the difference would blow their minds.
             
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            • 65-440

              65-440 Well-Known Member

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              Please, feel free to enlighten the rest of us .....
               
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              • 66chargerpat

                66chargerpat FBBO Gold Member FBBO Gold Member

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                the emissions war started in the 50s, not the 70s
                 
              • Ceedawg

                Ceedawg Well-Known Member

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                Would like to see one of those ‘emission’ thingys from the fifties. Heck nobody heard of pollution till LA started bitchn about the smog in middle 60s. If that is what you read on the internet we surly have lost
                 
                Last edited: Feb 24, 2021
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                • JimKueneman

                  JimKueneman FBBO Gold Member FBBO Gold Member

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                  Not true, the beginning of scrutiny of auto emissions in LA in the 1950's when I CalTech professor discovered the link so I will bet auto manufacturers started implementing subtle design changes because they could see the writing on the wall the govt would eventually get involved and they want to fend that off as long as possible. There were protesters for clean air in LA in the 1950s. In 1970 it became federal law and there was a road map to where they needed to be so that is when all the gizmo's really started to show up.

                  https://www.amazon.com/Smogtown-Lung-Burning-History-Pollution-Angeles/dp/1585678600
                   
                  Last edited: Feb 28, 2021
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                  • R/T Boy 67

                    R/T Boy 67 Well-Known Member

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                    Shoot me if I am wrong, but I believe the first emissions gadget was the PCV system, which came into use in California in 1961ish. Emissions started to be a problem after WW2, When the car population increased considerably.
                     
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                    • 64fury74charger

                      64fury74charger Well-Known Member

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                      WOW! Is this a spectator sport??? :thumbsup: :drama: :blah: :rofl:
                       
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                      • Ceedawg

                        Ceedawg Well-Known Member

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                        Must be nice to be 30 and the only connection to the past is the internet.
                         
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                        • 383man

                          383man Well-Known Member

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                          If you look up what is vacuum advance used for you will find that it came about to help part throttle with more timing when the eng can use it since you cant give it full mech advance just off idle. Vacuum advance was not invented for the idle it was for light part throttle when the eng needs more timing and it helps fuel mileage. Sure it can be applied at idle as long as the rest of the eng timing is set up to work with it. Some engines may not take vacuum advance at idle as it will all depend on how much the vacuum advance is giving and the eng mech timing. I also dont believe the manufactors went to ported vacuum for emissions because as far as I have seen many used ported vacuum way before they started retarding idle or off idle timing for emissions. Many use vacuum advance retards during the emissions years like Mopars OSAC which will retard vacuum advance for 17 seconds when stepping on the gas and many use trans controlled spark timing where they give no vacuum advance until the car hits 3rd gear and so on which is all for emissions. You can use the VA at idle if you set the eng up for it or not use it and set the timing up that way. Ported and manifold vacuum are the same once the throttle is open some its just called ported when the port is above the throttle blade. The only other vacuum is Venturi vacuum which is what is used to work vacuum secondary carbs but that another ball game that is not used for vacuum advance. Ron
                           
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                          • khryslerkid

                            khryslerkid FBBO Gold Member FBBO Gold Member

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                            Thank you Ron! :thumbsup:
                             
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                            • Ceedawg

                              Ceedawg Well-Known Member

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                              What I said Ron, you explained it better!
                               
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