Constant or Ported Vacuum Source for Distributors? ...AGAIN!

Electrical and Ignition

  1. cuda66273

    cuda66273 Well-Known Member

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    I found this article on ChevelleStuff.Net I don't know the author so I can't give valid credit.
    I have tried to explain this theory to 1000's of callers over the years and most get it. However, there's still a constant stream of posts all over the web that still want to argue about how the vacuum can works, why and how.

    I first saw this article article years ago in I believe it was Hotrodders.com or something close

    It explains it clearly answers all the questions and should be compulsory reading for everyone here. If everyone reads this it will end the argument.

    Quote:
    "As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance.


    TIMING AND VACUUM ADVANCE 101


    The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.


    The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.


    At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).


    When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.


    The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.


    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.


    What about the Harry high-school non-vacuum advance polished billet "whiz-bang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.


    Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; low vacuum advance cans are available (from real ignition company's) to provides the same amount of advance as the stock can (15 degrees), but are fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.


    For peak engine performance, drivability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts." end Quote.


    A few notes from over 100 years of the combined experience of Jim and myself in tuning, setting up distributors and developing our own computer controlled Mopar ignition systems:

    *The adjustable vacuum canisters only adjust the sensitivity of the vacuum can, not the stroke.
    *Limiting the mechanical advance and increasing idle timing to be compatible with today's fuel formulations are a necessity that can not be overlooked.
    *Beware of the Imported distributors that appear to have the hex shaped vac cans that should designate it as an adjustable vac can, many are not. (Spectra is the worst)
    *
    Any one who tells you that the vac can is not needed with their ignition system or distributor is playing on your lack of knowledge and probably needs to read this article so THEY understand how UN-knowledgeable their statements make them appear or maybe their just trying to increase profits at your expense? Non adjustable vac cans out of China are about $2 a good quality adjustable can is probably closer to $10-$15 to build.

    *Last comment beware of the "Snake Oil", our industry is loaded with false and rediculous performance claims, misinformation, country of origin claims, repackaging to hide source and vendors with very little knowledge of how their products really work and why. When you want reliable information go to the guy who engineered it and makes it.

    *Our products are all designed in house, manufactured and assembled in the USA and most components are made by ISO 9000 compliant vendors.

    *ISO 9000 is a quality management standard that presents guidelines intended to increase business efficiency and customer satisfaction. The goal of ISO 9000 is to embed a quality management system within an organization, increasing productivity, reducing unnecessary costs, and ensuring quality of processes and products.
     
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    • 65-440

      65-440 Well-Known Member

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      Oh boy, be ready for the comments ... I use manifold ....
       
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      • MOPAROFFICIAL

        MOPAROFFICIAL FABO Gold Member FABO Gold Member

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        Heard all that, old news dude, Chevy tech at it's finest pointing out port locations...that have been since the 60's when there was no smog requirements, idiots...
        People here, most of us, know what the hell it's for AND WHEN IT'S APPROPRIATE... As for someone saying others are idiots for not using a vac advance on the street, i say to them...you are complete and udder fucking idiot, you have not the amount of experience with built up motors to be speaking any further on the subject and should stick to tuning on pos smog low squeeze 350 chevies. Thee end.

        I have had 9.5 340s and 360s ping when stabbing the throttle because the vacuum advance does not fall off quick enough for the split second/immediate pressure increase. It will work however if you drive a smoged out low squeeze 350 Chevy that you couldn't get to ping at 45 mechanical advance...

        There are pros to using non ported as well as ported, band aiding idle issues.
        Every motor is different, unless it's a Chevy...theyre all the same pos ;)
         
      • prorac1

        prorac1 Well-Known Member

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        It does appear that manifold vacuum is the way to go. Below the throttle blades.

        I'll admit. 20 years ago I always used the ported vacuum port on the side of my Holley metering block. Lol
         
      • prorac1

        prorac1 Well-Known Member

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        I agree. This is definitely aimed at very mild to stock street engines. The author even says that in the article.

        But as we're all aware of. One persons version of a street engine, can be very different than another.
         
      • AJ/FormS

        AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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        No comment :popcorn:
         
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        • rumblefish360

          rumblefish360 So close, yet so far away FABO Gold Member

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          Awwwwwwe!!!!!
          Come on now!
          I know you can have. Few good words in your version.

          I myself have used booth and tune accordingly to the need for the useage and the driver. Normally it is the ported port I use. But all things change.

          (My next Trick .... underwear! LMAO!!!!)
           
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          • RustyRatRod

            RustyRatRod Lemmie see your b00bs. FABO Gold Member

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            Use whichever one you want and tune accordingly.
             
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            • RustyRatRod

              RustyRatRod Lemmie see your b00bs. FABO Gold Member

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              Well, there's a first time for everything, I reckon.
               
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              • MOPAROFFICIAL

                MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                Rolling over here, come on.. stop it, it hurts!
                 
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                • MOPAROFFICIAL

                  MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                  Hey, it could definitely clean up a big cam dirty idling sob, again... a bandaid for people building wacked applcations, we don't care about stink at the track....thats just ambience....and if it srinks on the street, well i guess that wasnt a very street friendly cam you picked was it? Lol
                  Go for the 2 mpg increase, im still passing you!
                   
                  Last edited: Jan 24, 2018
                • MOPAROFFICIAL

                  MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                  Yes, your choice, our choice, etc...

                  I didnt wanna pay for the vac can version msd, i bought the "race parts" version..lol...gawd, idiot writer, i fkn built it to go fast...not save the planet or hang back in the dust with "chevy techs".
                  I love the part where chvy says.."ported vac retards timing from its optimal value" lol, yeah because at idle you want 48 degrees, right? Why does my idle speed not adjust lower anymore..
                  I guess all those 60's holley and carter carbs are a fairy tale. If i ever build another "next to stock" motor for myself, ill use the ported vac advance!
                   
                  Last edited: Jan 24, 2018
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                  • racerhog

                    racerhog RacerHog likes his Mopars

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                    I Looked for a factory TSB but didnt find one?
                    This has got to be a Hokes....
                     
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                    • yellow rose

                      yellow rose Overnight Sensation FABO Gold Member

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                      Exactly 100% FACT. If the build has any balls you will find the vacuum can can't keep up. Then, as you say it will rattle it's brains out every time you touch the throttle under load.

                      I guess I'm stupid because I can run 11:1 on pump gas and not have it rattle. And I don't have vacuum advance.
                       
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                      • AJ/FormS

                        AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                        Ok, In 100 words or less, and one more time:
                        Everybody wants a rumpidy rump 360. Then somebody will tell him to crank the timing up to 15/20 and hook 12/18 more in, on the manifold vacuum.... And then your 230-FTH idles like a stocker. That's whack !
                        I say give her 14*, no Vcan, speed the timing up,kink it at the top to run low-grade, fast-burn gas, and then clean up the lowspeed circuit, in your 750DP. Show the world you got some tuning skilz. Then go out and ripchit.
                         
                      • AJ/FormS

                        AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                        I get you man. In some/many/most combos,that V-can just cannot decay fast enough when hooked direct.
                        Maybe the GMs do,lol. NOT
                         
                        Last edited: Jan 24, 2018
                      • MOPAROFFICIAL

                        MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                        Two things... i cant keep my foot out of it long enough for a vac adv to make a diff anyhow... and the other is already get about 15 mpg highway. I mean...Its efficient enough for what it is and what's inside it. Cranked 188 last check, got a 104 int . It'll get it on.
                        I could run 87 octane in mine, if i never go over stomp it or wind it high... but it's nice to be able to drive 4 miles away for 100 octane at the pump and bump the timing up for street or track brutality. Theres always a lil more for later.
                         
                        Last edited: Jan 24, 2018
                      • MOPAROFFICIAL

                        MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                        It's wack in that case because it drops off to slugville when you tip it..till it gets into some rpms and the mechanical advance kicks into range.
                         
                        Last edited: Jan 24, 2018
                      • yellow rose

                        yellow rose Overnight Sensation FABO Gold Member

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                        Not counting the fist line...84 words...an AJ FABO world record for most said in the least amount of words. I'll mark it on my calendar.
                         
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                        • KitCarlson

                          KitCarlson Well-Known Member

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                          EFI systems with electronic advance ignition use manifold pressure (MAP). They also have 3D timing maps, where timing is controlled in a table of RPM, vs MAP. The 3D advance table, typically has low advance values when RPM is low, and MAP is low. That is similar to when ported vacuum is off when throttle closed, with significantly more ability to adjust... how much, and where in the table. With engine temperature related timing control, and 3D advance, an engine can be tuned with precision for all operating dynamics. Getting such a tune with mechanical distributors and lack of ported vacuum ...no way.

                          ign.png
                           
                        • 65-440

                          65-440 Well-Known Member

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                          Always a great debate ..... ported guys always seem to swear manifold vacuum advance will cause poor off idle performance yet I don't experience any of that in both my cars, both smooth as can be ... one 383 auto and the other 340 4spd ..... both mild cams .... I wish I had a ported only dude living near me to sway me to their side ! I'm open and willing to learn new things !
                           
                        • Mattax

                          Mattax Just the facts, ma'am FABO Gold Member

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                          SOOooooooo, here we go again.
                          Same thing was posted two weeks ago.
                          Same nonsense.
                          and I have the same responses.
                          Those looking for plug and chug answers will be disappointed.

                          Use of "ported" predates emmissions, unless the author thinks cars from the 1940s were 'smogged'. LOL. see
                          It also was used on both CAP and non-CAP Chryslers in the 60s.

                          Which approach should be used on a 'performance' setup depends to some degree on the limitations of what is available. If one is stuck with a distributor that has a long advance curve on an engine that wants 12-20* at idle, then using manifold vacuum at idle may be a way attain that.

                          Given no such restrictions, the distributor's vacuum advance ought to work with the fuel curve in the rest of the operating range. Then decide what is needed at idle.
                          Ideally, vacuum advance should go away when the fuel curve needs enrichment. The purpose of vacuum advance is to match the slower burning leaner mixes - that much of the article is true. Enrichment may be anywhere from 60% to 90% max power depending on the engine and drivetrain.

                          blob.jpg
                          In this example from Larew (details here), enrichment is need around 60% load, which might equate to vacuum of 9 to 12 "Hg. So, lets say this is the case, and we find a PV or step up around 10.5 works best. So we would want the vacuum advance to go away as vacuum drops near 10.5"

                          And sticking with this example, lets say the engine idles best (strongest in Drive) around 12.5 AFR and timing of 16* BTDC. Finally we note that the idle vacuum in D is 10" Hg.
                          (Graph shows Fuel Air Ratio. Inverse equals AFR. eg 1/.08 =12.5)
                          Therefore, in this case, using manifold vacuum at idle really wouldn't work at all. If it was used, it might bring a little advance when the transmission was in Neutral, and then go away when in Drive. That just makes a unstable setup.

                          The right setup here is to set the initial timing at 16* and have the mechanical advance curve shape and top limit set by the springs, or the springs and a limiter.
                           
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                          • 65-440

                            65-440 Well-Known Member

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                            After our last discussion on this I went out to my coronet to play around with the advance, needless to say I found the diaphragm bad. I noticed my idle speed was a bit higher and fuel millage dropped off some a while back, but I never thought of looking at the advance can. Hopefully my new can comes in today, I plan on hooking it up to ported and playing around a bit to see if I notice any difference...
                            Would be nice if Don from FBO chimed in on this, he is a manifold only guy. Might make an interesting discussion ....
                             
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                            • mderoy340

                              mderoy340 Well-Known Member

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                              This topic (commercial) pops up every view months. As others have said tune to what the motor wants and how it's used. Street car and track tune are different in my case.
                               
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                              • MOPAROFFICIAL

                                MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                                If you can use it, great, ported vac is my recommendation. You can gain some mpg.
                                As for the adjustable aspect, you can limit the amount of vac advance a particular canister is rated at. The spring inside is compressed when you turn it clockwise with an Allen wrench, 11/32 I think. I've cut one open, have a pile in a box somewhere with a bunch of distributors.
                                 
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