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Technical Ported vs "common vacuum" advance.

Discussion in 'The Hokey Ass Message Board' started by 34Larry, May 18, 2020.

  1. 34Larry
    Joined: Apr 25, 2011
    Posts: 1,531

    34Larry
    Member

    I'm going to go out on a limb here and say that there are "tons" of members who visit the HAMB who are like me, pretty well versed in things automotive, have had great cars, have had the street machines, have the needed knowledge to keep them up to speed and running like they should, and yet still come across issues/things not understood and "fake it to make it." In changing carbs and searching for the constant high rpm miss that I finally was able to find and fix I now know about and understand there is ported vacuum on some cars and the difference between that and common vacuum, and why the need. I have always just blew through the "advance" needs, basically understanding it but not really. I had no idea that there was "ported vacuum" and when coming across the term in the Edlebrock manual I also just blew it off thinking "hey vacuum is vacuum" and went merrily on my way.
    Well over the weekend my guy Jim the REAL Deal mechanic came by to see how the car was running. I told him it needed timing I thought. When we moved the rotor two positions clockwise last week he tightened the distributor so much I could not turn it, (arthritis pain) the bolt holding it was to tight. Well long story short we got it done. He mentioned the ported vacuum thingy also, so I went to MR. Google and now have a more informed knowledge of timing and the difference between the two and that I have mine advance plugged into the wrong vacuum port coming off the rear carb.
    Now off to get it hooked up right and get on with the hobby and things to learn that I have just passed over in the past. :D:eek::confused:o_O:rolleyes:
     
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  2. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 3,103

    2OLD2FAST
    Member
    from illinois

    I find it difficult to believe the engine would run vat all with the "2 positions off" . You might consider sharing the details behind and the choice you made about your vacuum source decision .
     
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  3. Fortunateson
    Joined: Apr 30, 2012
    Posts: 3,758

    Fortunateson
    Member

    Speak oh Wise One...
     
  4. 34Larry
    Joined: Apr 25, 2011
    Posts: 1,531

    34Larry
    Member

    Explanation: In my effort to locate the high RPM miss, I went painstakingly through everything taring the himi down to leaving the crank/pistons in the block and leaving the water pump alone. I first went through all the lifters for preload/lash. I had earlier replaced all ignition sources except the electronic distributor. I went to the 55 Motor Manual and noticed that it showed #1 was (2) positions clockwise from the way mine was. In being so the plug wires were run correctly to that configuration, but to be (per the book), we pulled it, moved the drive gear to the correct positioning on the cam, and moved the wires of course also. Hope that answers your question 2OLD2FAST, (and I know about that also).
     
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  5. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 3,103

    2OLD2FAST
    Member
    from illinois

    So , it was a wire position thing , gotcha .What about the vacuum ?
     
  6. jimmy six
    Joined: Mar 21, 2006
    Posts: 8,377

    jimmy six
    Member

    One needs to remember, under wide open throttle there is little to no vacuum to advance the ignition timing. Under light cruise with high vacuum and NOT the centrifugal timing all in is totally different. Today with the OD transmissions and cruising down the highway at 1700-2000 rpm at 70-75 mph it is tough to know what your timing is as your vacuum continues to adjust. I have a timing monitor on my 2018 5.0 F-150 and have seen 43 degrees cruising. If I take my foot off the gas it goes to zero to slow the truck down. I owned an eco-boost that went to Zero towing my trailer on a grade. I continued to down shift until it was at least 10 degrees. All electronically controlled of course.
     
  7. Different strokes for different folks, err, engines. Full vacuum gives a lot of advance at idle. Port gives no vacuum advance at idle. Port is the way Buick straight eight is designed.

    Ben
     
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  8. 34Larry
    Joined: Apr 25, 2011
    Posts: 1,531

    34Larry
    Member

    Well according to Mr Google: The question is whether the source of vacuum should be manifold source or ported source at the carburetor. The only difference between the two, is that manifold source would give you vacuum advance at idle, ported source gives you the same manifold vacuum, but only once the throttle is cracked open. Ported vacuum source is above the throttle plates, manifold source is below the throttle plates.
    My understanding is that ported came out as requirement in the 70's for emission control requirements. Engines, that do not/ or did not have emission requirements, (per 70's) use common or manifold vacuum. As you know when setting timing on a pre emission engine the vacuum line is always disconnected to the vacuum advance. The required advance (degrees is set for BTDC at the VA, locked in and then reconnected.
    Sorry I missed this part of your question the first go around. I suggest you Google, Ported vacuum vs. manifold vacuum. Hope this answers your question.:D
     
    Last edited: May 18, 2020
  9. Truck64
    Joined: Oct 18, 2015
    Posts: 4,363

    Truck64
    Member
    from Ioway

    No, that really isn't true. MoToRs Repair, copyright 1950, (long before engine emissions were even a gleam in the eye of an EPA administrator,) states a ported distributor connection (that is, above the throttle plate) is to ensure a steady idle. And as it happens even today this is the most usual complaint with a manifold connection. Performance applications with duration camshaft driven on the street will usually need this. What happens, if the engine manifold vacuum is less than the opening point of the vacuum canister, it will start to flutter, and start a kind of feedback or variation of the ignition timing, which of course causes the manifold vacuum to vary some more and ... unsteady idle. Careful selection of the vacuum canister should prevent this.

    Also keep in mind once the idle RPM exceeds factory spec, the butterflies open up and it will start pulling in ignition advance no matter where it's hooked up. This is probably why most people goose the idle RPM up a bit anyway. It smooths out and runs cooler - because of the advance. If you put a timing light on and check, it will be obvious.

    Some makes used a manifold distributor connection, some makes used ported connection over the years well before the 70s. It is true that when federal emissions standards were first enacted in the 60s and got really wound up in 1970, a ported connection became the norm because as it turns out when an engine is tuned for the best efficiency and power, CO and HC emissions are minimized, however NOX is increased by quite a lot. The latter is what causes smog over cities, when exposed to sunlight. A solenoid was usually incorporated to return to a manifold distributor connection in the event the engine overheated, which was fairly common since they buggered everything up. My Y runs about 47° cruising down the highway on flat ground if the shop manual specs can be believed. Some on topic OHV engines want 50° or more. There's no way to achieve this using springs and weights, which is why vacuum advance was invented.

    The catalysts available at the time were apparently used to burn off all that wasted fuel now coming out of the tail pipe as a result of all the smog era detuning going on.

    Reduced compression, jiggery-pokery with the camshaft or valve timing, leaning out the AFR, etc. And right in the middle of an Oil Embargo by our friends in the Middle East. Hm.
     
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  10. There was/is a second type of ported vacuum best described as 'venturi vacuum'. The above type had the port right at the throttle blades. When the blades were closed at idle, no vacuum. Open the throttle, the port was now 'below' the blade and you essentially got manifold vacuum. The second type the port was above the blades when they were open and vacuum increased with throttle opening, just the opposite of manifold vacuum. This was done for emissions purposes, and 'uncontrolled' would cause spark knock and increased engine temps. So the factories used either dual-vacuum distributors (two vacuum cans that worked against each other) or more commonly, conventional single advance with a temperature-controlled vacuum 'switch' that either disconnected the ported vacuum or switched to manifold vacuum when engine temps rose above a certain point. Most aftermarket 'street' carbs have both types (to make them emissions legal), you need to pay attention when connecting your vacuum advance hose at the carb.

    I owned a '70 Merc with a 351C that had a seemingly random issue with spark knock when rolling into the throttle. Playing with base timing had little effect. Now in 'normal' driving, this car would show temp changes on the gauge. It didn't overheat, but the gauge would move in a range. I finally figured out it only knocked when at the top of the range, found out about 'venturi vacuum' and switched to straight manifold vacuum. The knock disappeared, and the temp gauge rarely moved after that...

    I'll also note that if using ported vacuum for your advance, if your car tends to get warm during extended idling switching to straight manifold vacuum will usually cure that.
     
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  11. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 335

    Neb Hillbilly
    Member

    Either can be used and can make a huge difference in how the car idles and runs. Ported vac never gets to the same "/Hg that manifold does and falls off quicker as the throttle is opened. I have spent a huge amount of time tuning vac advance on my carb equiped dailies esp when getting the carbs to run in the 15.5 to 16+ to 1 AF cruising. It isn't uncommon to have 52 deg+ of total timing at cruising to fire the leaner mixture.
     
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  12. jaracer
    Joined: Oct 4, 2008
    Posts: 768

    jaracer
    Member

    Back in the late 40's until about 1957, Ford used venturi vacuum to control "centrifugal" advance, that is the normal advance that is required as engine rpm increases. Everyone else used a set of weights for this. So the load-o-matic distributors had no advance weights.
     
    Driver50x likes this.
  13. I use fully mechanical advance distributors on all my "performance" engines.....no need to be concerned about manifold vs. venturi vacuum.....vacuum advance only on low performance engines in my collection.
     
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  14. Truck64
    Joined: Oct 18, 2015
    Posts: 4,363

    Truck64
    Member
    from Ioway

    52° in the garage in neutral, or?? Something I've pondered about, sounds like you'd be the guy to ask. Is it possible that under cruise conditions and very lean AFR mixtures that there could be part throttle engine knock because the ignition timing isn't advanced enough?
     
  15. 34Larry
    Joined: Apr 25, 2011
    Posts: 1,531

    34Larry
    Member

    Just want to thank you have responded, now I wont have to, "fake it to make it", when this topic comes up in conversation. I figure there is much more to know about timing also.:):D:)
     
  16. jaracer
    Joined: Oct 4, 2008
    Posts: 768

    jaracer
    Member

    Also, you should realize that vacuum advance is really only there for part throttle economy. It provides "extra" advance when engine load is light. It really doesn't matter if it is ported or manifold vacuum. However, it does matter when you set your timing and/or idle speed.

    My race car had fixed timing, there was no type of advance in the magneto. We set it at about 42 - 44 degrees, but we didn't have to worry about starting ease (used push trucks
     
  17. Warpspeed
    Joined: Nov 4, 2008
    Posts: 532

    Warpspeed
    Member

    Crazy Steve has summed it up very well.
    An engine with a warmer cam often requires a higher idle speed setting, which can bring the ported vacuum into action at idle, which can cause the ignition timing to fluctuate.

    Back in the old days, the cure for this was to drill small holes through the primary throttle plates. This allows a bit more idling airflow (through the holes) and the throttle plate can then be closed back to where it should be to a position that shuts off the ported vacuum to the distributor advance can at idle.
     
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  18. Deuces
    Joined: Nov 3, 2009
    Posts: 19,606

    Deuces
    Member
    from Michigan

    Some original L-79 Holley carbs were like that.....
     
  19. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 335

    Neb Hillbilly
    Member

    52 going 65 down the highway at 2500 and 65-70 at 15-20 in/hg. You have to full map your ignition curve and know at what depression you vac pulls it's advance in. Then you make a change and keep accurate mpg records for a couple tanks. Super lean at low load cruising doesn't ping. It surges because the flame goes out. More advance will fire the lean mixture. This is how carbs worked back before emissions.

    Sent from my S48c using The H.A.M.B. mobile app
     
  20. Another " no consensus" thread, I believe. I fixed mine my way. Just can not tell you how.:p:p

    Ben
     
  21. tjet
    Joined: Mar 16, 2009
    Posts: 1,272

    tjet
    Member
    1. Early Hemi Tech

    I have my old delco points distributor hooked up to a full-time manifold vacuum source, so it advances at idle. This really helps if you have a big cam. The key is to install the right vac advance can which pulls completely at idle. The can I installed is from a 409 chev, part #B28. Mine is all the way advanced at 8" What you don't want is a vac can rated higher than your cam's idle vacuum. Most guys should be able to get by on a B-20 can

    Here's some specs on the AC Delco cans

    http://www.metroli.org/pdf/WHITE PAPER_Vacuum Advance Operation.pdf
     
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  22. GearheadsQCE
    Joined: Mar 23, 2011
    Posts: 2,711

    GearheadsQCE
    Member

    Now if you want to get all technical (1970-80s technology) find a guy with a Snap-on MT455 or MT1480 timing meter. This will allow you to read actual ignition timing going down the road.
    They consist of a tachometer and timing meter.
    The function is pretty straight forward. The firing of number 1 spark plug is sensed by an inductive pickup (same as your timing light). This starts a timer which is turned off by the notch in the harmonic balancer passing by a magnetic pickup. The mag pickup is installed in a tube that is part of the timing tab.
    The magic inside the meter(s) compares rpm to the time lapse between ignition and turn off and displays it as degrees of advance.
    The big 3 all had probe holders on their engines.
    GM standardized the offset as 9 1/2* ATDC
    Chrysler was 10*
    Ford had different offsets for different engines.
    You can fab up your own holder. The probe diameter is 5/16".
    If anyone actually wants to do this, start a conversation and I'll help you thru it.
     
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  23. So I'm not supposed to just suck on a rubber hose from the distributer each time I pull away from a stop??
    This thread could change my life. :rolleyes: :p
     
  24. seb fontana
    Joined: Sep 1, 2005
    Posts: 6,824

    seb fontana
    Member
    from ct

    I can't understand why bother with vacuum advance on a Hemi with dual 4 barrels.. Just because the distributor has it doesn't mean you have to use it.
     
  25. Truck64
    Joined: Oct 18, 2015
    Posts: 4,363

    Truck64
    Member
    from Ioway

    Excessive vacuum advance at steady part throttle cruise will rattle a bit, know what I mean? It actually goes away with acceleration, just the opposite as is ordinarily the case with ignition advance. It is different than a regular engine knock.

    My Y runs great with 52° maybe 54° indicated on the balancer in neutral all spooled up. It is probably actually a bit less on the highway, or right about where the engineers designed it.
     
  26. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 335

    Neb Hillbilly
    Member

    I have felt the roughness or "fish biting" or "Trailer hitching" try backing off the timing and seeing if is over advanced.
     
  27. Truck64
    Joined: Oct 18, 2015
    Posts: 4,363

    Truck64
    Member
    from Ioway

    If anything it might need a little more. It's an old school vacuum advance that uses springs, shims and fiber block so a bit of a pain to adjust.
     
  28. Neb Hillbilly
    Joined: Dec 20, 2019
    Posts: 335

    Neb Hillbilly
    Member

  29. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 1,689

    Elcohaulic
    Member

    Running full constant timing with a points distributor, I always just removed the springs to the weights, that way when you shut the engine off the weight would snap back into low timing, this made the starting much easier, then once the rpm went up the weights would kick out and the timing would go to 36-38 or what ever I had it set to... This also worked well with a CD box triggered by the points distributor..
    To me the points system offers all the advantages. I also like to use the old dual points plates, I connect a three way switch to each set of points and the common to the coil. Now I can lower or raise the timing a few degrees to remove that pesky pinging from old/bad gasoline..
    When you connect a CD box up to the points distributor and use the points just to trigger the box you remove most all there limitations and have one hell of a powerful ignition..
     
    Last edited: Aug 21, 2020
  30. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 1,827

    ekimneirbo
    Member
    from Brooks Ky

    Boy, computers sure makes all this so much easier................:D:p;) Just sayin!
     
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