Adjustable Vacuum Advance

  1. halifaxhops

    halifaxhops It's going to get stupid around here! FABO Gold Member

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    halifaxhops submitted a new Article:

    Adjustable vacuum advance

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    • Mattax

      Mattax Just the facts, ma'am FABO Gold Member

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      Great cutaway view.
      Link added from the "How to adjust" article
       
      Last edited: Dec 22, 2020
    • hotrod swinger

      hotrod swinger Active Member

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      Just checking, are the instructions backwards? When the key is turned in clockwise the spring is compressed, higher vacuum needed to start advance. When it is turned out counterclockwise, it is decompressed, lower vacuum needed to start advance.
       
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      • RustyRatRod

        RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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        No. CCW compresses the spring, CW relaxes the spring. When the spring's compressed, it's harder to compress further, causing more vacuum to be needed to compress it further and give advance. When the spring is relaxed, it allows the advance to come in earlier. That's how it's designed. I've always said the vacuum can controls "when" not "how much" as hoppy explained.
         
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        • hotrod swinger

          hotrod swinger Active Member

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          I see it now. When the screw is turned in CW the whole way the spring is at its most relaxed. A relaxed spring may lead to pinging under load, and in that event you turn the screw CCW. Looks like the best setup would be earliest vacuum possible without pinging, or as far CW as possible without pinging.
           
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          • RustyRatRod

            RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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            I'm not sure id would ever spark knock under load. That's when vacuum drops out completely. It's just something you'd have to mess around with . Every combo is different.
             
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            • Mattax

              Mattax Just the facts, ma'am FABO Gold Member

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              Correct. The tricky part is the part throttle loads. Too much vacuum advance under light load can feel like 'trailer hitching' but that's rare.
              Don't know if you went to the linked thread but this is an example of how turning the adjuster counter clockwise shifted the vacuum advance.
              upload_2021-2-9_11-57-20.png

              Because the purpose of vacuum advance is to add timing for low density fuel conditions, we can say that there should be no vacuum advance when the mixture is rich.
              So an engine that needs full enrichment at 10.5 in of vacuum would need the vacuum advance to go to zero around 10 or 11" Hg. If we know the power valve or step up opening point then we can match the vacuum advance to be in the ballpark.
               
            • Mattax

              Mattax Just the facts, ma'am FABO Gold Member

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              Should have mentioned that one of the 'secrets' of part throttle power is the fuel mixtures get leaner.
              They only need to go richer again when approaching full throttle. That's the power valve opening point (or rod step up in Carter)
               
            • vynn23

              vynn23 Active Member

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              What size "key" is used to make adjustments? I've tried a few sizes of hex wrench, and haven't been able to fit the screw in my vacuum advance.
               
            • Mattax

              Mattax Just the facts, ma'am FABO Gold Member

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            • hotrod swinger

              hotrod swinger Active Member

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              I mucked up a vacuum advance once by turning too far counterclockwise.
               
            • Bewy

              Bewy Well-Known Member

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              VA is best used with manifold vac [ MVA ] , NOT the useless, emissions related, ported VA. This especially true for these conditions: low[er] CRs, larger cams, big sluggish ports, single plane intakes. All of these require more lead time for the ign to fully combust at idle & cruise. For modified engines, an adjustable VA is ESSENTIAL. One of the reasons VA gets a bad rap is folks trying to get a non-adj VA unit to work on an engine with reduced idle vacuum, resulting in idle timing varying & being erratic. Not the fault of the VA concept, fault of the idiot doing the job.....

              Beware also that some adj VA units [ like the Crane ] have a sleeve inside the spring that limits the TOTAL VA available. Un-crimp the can & remove the sleeve or find a VA that still has full plunger movement when adjusted fully CW or CCW.
              A big cammed engine may need as much as 50* [ yes 50*!! ] at idle for best idle quality, highest vacuum, best tip in response & coolest running at idle. One of the few things Mother Mopar got wrong was NOT to use MVA.

              To set up: turn Allen Key [AK] fully CW [ softest spring setting ]. AK is usually 3/32".

              [1] Warm up engine, chock wheels, put in D if auto. Disconnect & plug VA.
              [2] Loosen dist clamp, & starting at around 10* BTDC, slowly turn dist to advance the timing. Idle speed will increase & get smoother; keep advancing timing until the highest idle rpm is achieved. Toggle dist to make sure this is the highest rpm. I call this the 'sweetspot'.
              [3] Now check what your timing is. Say it is 34*. And you are using 14* initial [ static ] timing. The VA needs to add the remaining 20*. This the TOTAL timing added by VA. Most adj units will provide up to 30*.
              [4] Make/use a stop to limit the VA to 20*.

              Vac Adv RATE:
              [1] Connect VA. Check timing, in gear, engine running. Turn AK 2 turns CCW & re-check timing. Keep doing this until timing drops or becomes unsteady. Then turn AK CW one turn at a time until the timing is steady. Then go another 2 turns CW.
              [2] Job done, VA is adjusted.

              Any change to the engine will require the adjustment to be done again.
              This procedure works every time, not sometimes.
               
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              • Mattax

                Mattax Just the facts, ma'am FABO Gold Member

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                shake_head-gif.gif
                The evidence doesn't support those claims.
                All you have to do is look at various pre-emissions shop manuals and cars, especially Chrysler products.
                You could also take a look at the published research and testing in fairly well known books (well known by people who made engines for cars, trucks, planes and boats - not so well known by hotrodders).
                 
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                • Mattax

                  Mattax Just the facts, ma'am FABO Gold Member

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                  For example, from 1959 (Session 136) of the Chrysler MTSC.
                  upload_2021-2-18_11-14-21.png

                  In fact, its interesting to note that on some early Clean Air Packages (CAP) Chrysler did use a manifold vacuum connection to the vacuum advance.
                  This was to insure the leaner closed throttle mixtures were given enough advance to burn thoroughly when engine braking. The connection was made to the vacuum advance through a special valve. See 1967 Chrysler CAP System: Master Technicians Service Conference (Session 241) for more details.
                  This later was dropped for automatic equiped cars.

                  But normally Chrysler did not use vacuum advance at idle.
                  upload_2019-10-11_10-13-43-png.png
                  from Ignition System Analysis (Session 259): 1969 Master Technician's Service Conference

                  Ten years ago I too thought ported vacuum was an emissions related development. This came up in discussion now
                  archived. See "Carb Issues", scroll down to #11, 17 (me), and responses #13, and #19-22 where Shrinker et al set me straight.
                   
                • Bewy

                  Bewy Well-Known Member

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                  Smart people are using MVA. Period. GM cars used it. Not sure if Chrys did. My GTO left the showroom floor idling with 26* of timing [ 6* initial + 20* added with MVA ]. 10.75 CR, with a mild factory cam, around 200* @ 050.

                  Ideas change as more knowledge is gained. Thinner piston rings are used today comparted to yesteryear, just one example.

                  I am not going to spend all day trying to prove to non-believers what experts/engineers proved long ago.
                  So just a few examples:
                  - From D. Vizard. Author, race driver, engine builder, author of 30+ books & 3000+ technical articles. In one of his carb books [ think about why ign is discussed in a carb book...]. "The optimum idle advance is typically about 35-40* for a short cammed street engine & [ though not commonly realized ] as much as 50* for a street/strip engine."
                  - By DV, PHR magazine Nov 04. :"At idle & low speed operation, the amount of adv reqd to effectively burn the fuel & air entering the engine can be as much as 50-55*. This is handled by the vac adv: a function many hotrodders believe is not needed because their favourite drag racer does not use it. By taking the time to hook up MVA you can get a big cam to idle as it were about 20* less than it really is...."
                  - Mopar Muscle Oct/15. "..raised the init timing from 15 to 25*.....There was a noticeably better off idle & low speed throttle response & another inch of vacuum at idle. The added init timing raised the idle rpm..". [ The idle rpm increase occurred because the engine made more HP from the extra timing; the extra vac was because the engine burned the mixture more efficiently. While the timing increase was done statically, vac adv could have been used & the same result would have been achieved ]
                  - PHR 8/04. "With vac adv hooked up...carb & ign allowed a very smooth 650 rpm idle". [ 350 Chebby making 485 hp ]
                  - Super Chevy 11/16: "..you should consider hooking the vac adv to straight manifold vacuum. This is counter to what you may have been taught in high school auto shop, but it is actually a very good idea.....That simple change [ adding 10* of MVA improved idle vac a solid 2"....& the engine no longer felt like it was struggling against the c'ter."
                  - PHR 12/07: "Just to give an up front idea what this was worth [ hooking up MVA ], the engine's minimum idle speed WITHOUT the aid of MVA was a lopey 900 rpm. With MVA, it was 780 rpm with only about half the lope."

                  More on MVA, scroll down to post #6:
                  www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html
                   
                • my68barracuda

                  my68barracuda Well-Known Member

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                  I agree that vacuum advance via plugging the can into manifold vacuum can be a good thing.
                  At low engine RPM, like at idle, the air-fuel mixture can have poor mixing due to a lack of turbulence. Certainly the AF mixture is not very dense. To get the mixture in the cylinder to burn its best it can be helpful to light the fire early.
                  That will keep the spark plugs clean and help off idle acceleration.
                  The major OEM’s spend countless hours and dollars figuring out what is best for the engines that they produce. On an engine that has been modified, there is no guarantee that the OE setting are still the best.
                   
                • Mattax

                  Mattax Just the facts, ma'am FABO Gold Member

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                  Fuel distribution at idle is due to the change in air pressure, which is quite high. However the velocity is low.
                  AFR at idle should be relatively dense.
                  Ironicly, it was the need to clean up the HC at idle that resulted in the somewhat leaner idle mixtures which too many hot rodders believe is best. It is best for emissions reduction but not best for power or economy.
                  Again we can look at the published materials at the time to see this is true.
                  From page 3 of 1967 1967 Imperial & Chrysler Engine Combustion - Session 240
                  upload_2021-2-19_8-57-27.png

                  When tuning for maximum power and mechanical efficiency at idle (hot) mixtures in the 12 to 13:1 range are the norm.
                  Here are examples from varipous studies published by Larew, Obert, etc.
                  Desired Carburetor AFR Characteristics At Different % Load
                  (if you get a pop up asking you to join TT, just close the popup and read the thread).

                  Unfortunately even some of the best magazine writers were ignorant about this and other aspects of fuel and timing needs.
                  A glaring example of this was (and is) the glaring example where they ignore the advice of Grumpy Jenkins, Direct Connection, and others about how important it was to use a heavy secondary spring to control the top of the advance curve - especially when using electronic ignition.
                   
                • my68barracuda

                  my68barracuda Well-Known Member

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                  AFR at idle should be relatively dense.
                  since the combustion chamber and bore size is constant, if the AFR was relatively dense at idle, the motor should make good power with the throttle plate close and only the idle circuits functioning. Sorry but that is not the way it works.

                  From page 3 of 1967
                  you have some good ideas, but on this one, things have changed from 1967,, fuels, induction systems, ignition systems,,,
                  I will say it again,, if you want 1967 performance and drivability, set your motor up like it was in 1967.
                  If you have made changes to the motor, what the factory recommended in 1967 may no longer be the best choice.
                  And notice that I say 'may'. It cracks me up to hear folks come up with ultimatums that apply across the board regardless of engine mods.
                   
                  Last edited: Feb 19, 2021
                • Bewy

                  Bewy Well-Known Member

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                  Post #17. MVA is not dealing with the top of the advance curve [ higher rpm ]. It deals with lower rpm.
                  Larew wrote his book in 1967. Obert I believe is even earlier. As I stated in post #15, & gave an example, many things have changed over the years as more knowledge & research have shown that some original ideas were wrong....
                  Chrys used MVA in the 1970s.
                  From my 1972-78 MM, p. 2-457. 'When coolant temp at idle reaches 225*F, the valve opens and applies MANIFOLD VACUUM directly to the dist....This increases engine idle speed & provides additional engine cooling.'
                  The engine could care less what the A/F is at idle, it is trying to cool down.
                  It should be veeery obvious what is happening here: with the extra timing at idle, the engine is making more hp, which is reflected in the increased idle rpm, since at idle the engine is powering itself, & not moving the vehicle.
                   
                • Mattax

                  Mattax Just the facts, ma'am FABO Gold Member

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                  Now your purposely taking that out of context. Yes. Absolutely many manufacturers used manifold vacuum as a way to overide the retarded initial timing that was being used for emissions control. The purpose of the late timing was to put more heat into the cylinder walls. The CTO overide was there to add back timing at idle to prevent overheating in certain traffic and climate situations. My '85 AMC v-8 also has a nonlinear valve that mixes manifold vacuum and timed vacuum for certain warm up situations.

                  The point is that the use of ported vacuum is not something that was developed for emissions controls. It was used back as far the 1940s. There are many ways to shape the advance curve. Using mechanical advance for the rpm related, and the leaving the vacuum advance for the density related lets each of them do what they do best. Using manifold vacuum to increase timing at idle sometimes works out. It works out best when the manifold vacuum isn't much affected by changes in idle rpm and load.

                  If you are familiar with the books, then you'll know that they are based on a lot of emperical testing and development. If you are familiar with Shrinker's work you know he was testing with a 5 gas analyzer on dyno along with a scope and of course, at the track.
                   
                • Bewy

                  Bewy Well-Known Member

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                  I did not take ANYTHING out of context. I simply stated, & gave an example, of Chry using MVA.
                  Your 'facts' are simply wrong.
                  GM cars used MVA up to around 67/68 when they switched to the useless PVA. [ [ Maybe other brands, not sure]. Emissions tightening reqd less HC in the exhaust, & one way of accomplishing this was to reduce idle timing [ great for emissions, lousy for idle quality & economy ] & switch to PVA. While PVA does some of the things that MVA does, it doesn't do all of them, which makes PVA a loser.
                  MVA is still with us in electronic form. LS engines with their compact, efficient combustion chamber, require about 27/28* @ WOT. They idle with 22* .....

                  Hot rodders used to run locked dists, maybe some still do. Does the same thing as MVA: provides more timing at idle, engine runs cooler. With today's crap fuel, you run the risk of detonation with a locked dist. With MVA, the timing drops away under load, hence the beauty of it.
                   
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                  • Mattax

                    Mattax Just the facts, ma'am FABO Gold Member

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                    :rofl:
                     
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