Register now to get rid of these ads!

Vacuum: Ported vs. Non-ported

Discussion in 'The Hokey Ass Message Board' started by bigalturk1, Jul 9, 2011.

  1. bigalturk1
    Joined: Sep 23, 2010
    Posts: 367


    Throughout the years I've seen cars and trucks with their Distributors running both Ported and Non-Ported Distributer vacuum advance. I usually run ported in cars and unported on trucks that are either heavy or stop pinging.
    Any comments on ported vs. Non-ported?
  2. Nerner
    Joined: Jul 2, 2005
    Posts: 73

    from New Jersey

  3. rustyangels
    Joined: Sep 28, 2010
    Posts: 178


    Full vacuum for my sbc hei dizzy
  4. As quoted from the above article link-

    "Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

    If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more."

    'nuff said

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

    Member Emeritus

    Just because the vacuum advance tubing goes to the side of the carb, it does not mean it is ported. Lots of guy assume if it is above the base plate it must be ported...wrong.


    You cant tell from looking at where it is piped. These are the vac advance connections on Rochester 2 bbl. They in fact are manifold vacuum even though it doesn't look like it.

    I love it when someone says to always hook up a vacuum advance to ported vacuum. Exactly where are you supposed to get ported vacuum? There is no place to get ported vacuum if you are running 3 deuces with Rochesters and there is a good chance that it will want to heat up in bumper to bumper traffic because the timing is too retarded at idle.

    Don't get confused by just looking at where it is connected. These ports go straight down inside the carb body and come out below the throttle plates...manifold vacuum.
  6. T.L.
    Joined: May 24, 2011
    Posts: 206

    from Colorado

    Interesting read.

    The factory setup in my early '70s Ford was to have the vacuum advance connected to ported vacuum, so that is how I've always run it. The only changes after the rebuild have been a bigger carb, mild performance cam, and a bump in compression (from 8:1 to 9.5:1). I never knew that Chevys like full manifold vacuum to the vacuum advance.
    'Learn somethin' new every day...
  7. tommy
    Joined: Mar 3, 2001
    Posts: 14,757

    Member Emeritus

    I don't know Fords but I am wondering if you ever put a vacuum gauge on the port that you are using while it was idling to get a reading?

    I should have said did you put a gauge on your original carb to get a reading. I know all the universal replacement carbs have both ports and I think that contributes to the confusion. I had never heard the term ported vacuum until the smog motors started to show up. (Excepting the 49 -56 Ford crap)
    Last edited: Jul 9, 2011

Share This Page

Register now to get rid of these ads!


Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.