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which vacume port for vac advance?

Discussion in 'The Garage' started by weisel, Jan 24, 2003.

  1. weisel

    weisel 1/2 ton status

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    Do I need to hook my hei distributor up to a timed vacume port or one of manifold vacume? It is an 78 with an eldebrok squarebore carborator and I don't have the little vacume switch that plumbs into your cooling system. I just want to hook it from the carb strait to the distributor.
     
  2. Topdown

    Topdown 1/2 ton status

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    which edelbrock have you got? On mine there are two ports. the one on the right, as you face it is the non-EGR port, the left is the EGR port.

    -Ryan /forums/images/graemlins/k5.gif
     
  3. weisel

    weisel 1/2 ton status

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    mine has two ports on it also, as you face it one on the right and one on the left. which one do I use? the non egr port? I don't have egr.
     
  4. conoverbandit

    conoverbandit 1/2 ton status

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    I always heard the one one the right. That's where mine is and I have'nt had any trouble. Just my $.02 /forums/images/graemlins/grin.gif
     
  5. wheelin_willy

    wheelin_willy Registered Member

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    you have to see what port is ported vacumm(vacuum during acceleration only) thats what you want. use a vacuum gauge the only way to be sure /forums/images/graemlins/usaflag.gif
     
  6. uglychevyZZ4

    uglychevyZZ4 3/4 ton status

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    Yes the one on the passenger side is the timed port, which is IN-CORRECT for HEI, that one is for points crap, ,
    For HEI you need the one on the driver side which is open manifold.
     
  7. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    Nah, its not incorrect. GM actually used both on the same engine. It was switched via TVS valve according to engine temp. Most ported setups do not increase with more throttle. Manifold vac ports are located just below the throttle blade. Timed ports are located just barely above the throttle blade. When you crack the throttle, the port becomes exposed to manifold vacuum and things are pretty much the same story as full manifold vacuum once you get rolling.

    I'm not sure on the Edelbrock/AFB design carb. Holley's and Q-Jets have the port feed from in the throttle body. But say you had the port in the venturi, then yes, you would end up having more advance with WOT. Its all a bunch of boring technical crap. A venturi is a essentially a restriction in the carb. Look in the primaries/secondaries where it narrows in. In order for air to pass through the restriction, it must speed up. This creates low pressure and vacuum. Thats why a carb can meter even at almost no manifold vacuum. /forums/images/graemlins/thumb.gif
     
  8. dubl_t

    dubl_t 1/2 ton status

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    BB, so which is correct, ported or full manifold vacumm?
     
  9. uglychevyZZ4

    uglychevyZZ4 3/4 ton status

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    well, that depends on your preference i guess, we kinda have a mexicvan standoff on correctness, but i dont wanna bicker /forums/images/graemlins/rotfl.gif /forums/images/graemlins/rotfl.gif

    do it how you prefer i guess
    /forums/images/graemlins/thumb.gif

    .
     
  10. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    I'd go ported. I like to run alot of initial advance and things would be funky at idle if I didn't. Up to a point, the more timing at idle, the faster the idle will be. I have 14 initial and 14 vac advance. That would make for 28 degrees just at idle if I was running full manifold. My Holley 4160 barely has the idle screw turned in on it. Below 700 rpm, my idle screw does nothing. Now imagine if I were running full manifold on the vac advance and getting another 14 thrown on. I would have one fast idle /forums/images/graemlins/eek.gif.

    You can make a curve too if you get bored, its very easy. What year is your rig? The stock springs and weights in the distributor are actually about right on the money. My '82 and '83 have those big fat stock springs on the centrifigual advance. They DO NOT come in at redline like alot of people say. Can't speak for the cars, but the trucks should start at just about idle and be in full by 2,800. Thats about perfect. I'd still check with a timing light to be sure and inspect the weights for wear.

    SBC like about 36 degrees of total mechanical timing. That would be your initial timing setting plus your centrifigual (stock HEIs give 20). So that means you'd need around 16 degrees intial. That seems high, but its not too bad. Can't say though if you run cheapskate gas. At least try to buy the middle grade, its barely costs anymore. Oh, if you've got Vortecs or a fastburn heads, try 14 initial.

    Now for some mileage, enter vac advance. Run that sucker on ported and add as much you can get away with. I have an adjustable vac advance, so it was easy. You can hit the junkyard for different amount canisters, buy an adjustable one, or make limiter plate. To determine how much, all I did was get things warmed up, hit the highway and used a vacuum gauge (manifold vacuum). Observe the vac at a steady cruise. Keep adding more vacuum advance until your vac gauge quits reading higher or falls off.

    Confused yet? /forums/images/graemlins/rotfl.gif

    So I have 14 intial + 20 mechanical + 14 vacuum for a total of 48.

    Generic GM 350 was 8 initial + 20 mechanical + 20 vacuum for a total of 48.

    The General knows whats up, just they've designed things to run with bad gas and emmissions..
     

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