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Manifold vs ported vacuum article I just read

Discussion in 'The Hokey Ass Message Board' started by Cantstop, Mar 5, 2013.

  1. Cantstop
    Joined: Jul 11, 2005
    Posts: 239

    Cantstop
    Member

    I was out in the garage working while my brother in law( an Engineer) and another Buddie had the existential argument about ported vs manifold for your distributor. Being the guy I am, I decided I would find the answer and shut them up. I came across this article on a couple of other forums ( like 10 variations) and I thought I would share it with you guys. I'll give credit to the guy who wrote it, as his name showed up about half the time.


    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. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

    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 "whizbang" 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; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.


    Courtesy John Hinckley
    Retired GM/Chrysler Engineer
     
  2. brokenspoke
    Joined: Jul 26, 2005
    Posts: 2,967

    brokenspoke
    Member

    Please not again
     
  3. WillyKJr
    Joined: Sep 5, 2009
    Posts: 152

    WillyKJr
    Member
    from Blackstone


    Heh heh :p
     
  4. You know what, even I understood that. Thanks for the lesson.
     

  5. Cantstop
    Joined: Jul 11, 2005
    Posts: 239

    Cantstop
    Member

    Hey, I'm not trying to dredge up or start a crap flinging off topic hoop-la. I'm on this site every day LEARNING. Some of the recent posters could use this info. I myself have had multiple people give BACKYARD bullshit advice, like my bother in law who swears that ported vacuum was for better performance and fuel economy. What I came away with is that the earlier distributors WITH manifold vacuum ARE better for performance. Anyone hooking a ported vacuum distributor to manifold is an idiot( read WAY too much advance).
    SO if you want to get your popcorn and flamethrower by all means go ahead, any topic around here gets beaten to death if its been done before. At least this article is relevant to some of us trying to figure out if the toothless guy telling us that we got our distributor hook up wrong indeed knows what hes talking about.
     
  6. Drew.Morris
    Joined: Mar 8, 2010
    Posts: 74

    Drew.Morris
    Member

    This is good information as I have a early vacuum advance distributor and was unsure of ported or manifold vacuum. Question answered. I will be trying it out after work tomorrow.

    Drew
     
  7. big duece
    Joined: Jul 28, 2008
    Posts: 6,826

    big duece
    Member
    from kansas

    manifold is the way to go, it will idle cooler. Ported was targeted for emissions.
     
  8. hoop98
    Joined: Jan 23, 2013
    Posts: 1,362

    hoop98
    Member
    from Texas

    I would agree as to the general principle, that said it is always hard to make one change in a system. If a carburetor was designed to work with PV, you will have to close the throttle pates to lower idle speed with MV, which could be a problem.

    And when you add timing at idle you "GENERALLY" increase vacuum which may change any vacuum dependent system.

    As noted above, this generally effects those engines trying to increase EGT with retarded or not advance timing at idle.

    Anytime I look at a carburetor, say a Quadrajet, I look at the idle crew, I know about how many threads I expect to see, more or less tells me I need to find out why.

    So I guess I am saying, that on late 60's cars up till EFI just hooking up direct vacuum advance could start a domino effect.

    Now when I first learned to time an engine we did it off piston position because the combustion chamber cares about that, not about crankshaft degrees.

    The larger the Rod length/Stroke Ratio is the more time in crank angle you will spend at or near (called dwell period) TDC. So for purposes of discussion I will say that we are talking about a generic engine with a L/R of 1.7.

    So why do we care when the ignition/combustion starts, we don't. We care when it ends. In our generic engine we want it too end (flame travel) at about 12 Degrees ATDC.

    So when we pull the trigger we want our bullet, (the flame front) to arrive then.

    Now one of the main variable in flame front speed is charge density, which can be inferred in a throttled engine by manifold vacuum.

    So when our charge density is low we need to fire sooner. How much time are we talking about, let's take a look.

    [​IMG]

    So that's the difference in time we are allowing for the flame to go thru ignition lag (say a millisecond or more) then burn itself almost out before the piston starts to accelerate down the bore.

    When we retard the timing heat is still being added to the cylinder while the gases are expanding, so they don't fully expand and are therefore hotter.

    So that helped when we were trying to afterburn in the exhaust (AIR).

    Things that effect combustion time:

    [​IMG]

    To be continued, or not :)

    rh
     
    Last edited: Mar 5, 2013
  9. Cantstop
    Joined: Jul 11, 2005
    Posts: 239

    Cantstop
    Member

    that is kind of why I thought the article was good, it explained in simple terms the whens and whys. I understand that its not perfect for every application. It explains the series of events in both ported and manifold applications. And to some extent ,why not using vacuum advance and not hooking the distributor the way it was intended can hinder your progress no matter what else you do.
     
  10. potshot
    Joined: Jul 15, 2005
    Posts: 70

    potshot
    Member
    from MT

    I have a '56 235 chevy in a '50 chevy styline. It has a Holly 2bbl on the stock intake with fenton split manifold. I have the vacuum advance hooked up to ported vacuum. So should I use manifold vacuum instead? I was always told to hook up vacuum advance to ported vacuum.
     
  11. gimpyshotrods
    Joined: May 20, 2009
    Posts: 23,233

    gimpyshotrods
    ALLIANCE MEMBER

    Hook it up to manifold vacuum and plug the ported vacuum port. Your fuel economy, such as it is, should rise.
     
  12. Panel Pete
    Joined: Feb 8, 2007
    Posts: 146

    Panel Pete
    Member

    I read the article posted by the o.p. MANY years ago - in fact I printed it out at one time (and have since lost it!). Any way, as I've had personal experience having my C10 Panel set up both ways I thought that I'd post my .02 cents.

    The driveline is for the most part straight out of the donor vehicle, a 400 sbc and th350 combo from a 1975 caprice wagon. It doesn't get any better as far as a early smogger engine than this! The only upgrades are the ubiquitous performer manifold with a stock quadrajet carb, a pair of hedman full length headers with true dual 2.5" exhaust and an aftermarket air cleaner.

    With the advance hooked up as from the factory (ported vacuum) and the engine properly tuned it ran hot (borderline overheating), detonated at cruise, and got abysmal mileage. After making the change to manifold vacuum and dialing in the carb and timing to match the new configuration the engine runs cooler in all situations (no more "oh shit, am I gonna make it" moments), the detonation is gone and I now get 12+ mpg average (up from 9 mpg before). As a side bonus, the engines idle characteristics changed for the better as well - no more "hunting/searching".

    Pete
     
    Last edited: Mar 6, 2013
  13. hoop98
    Joined: Jan 23, 2013
    Posts: 1,362

    hoop98
    Member
    from Texas

    Most of my experience with ported vacuum was getting cars thru the 2-Speed idle test.

    Any car should pass the high speed test pretty easily but the idle test was trickier. On some cars I would need to go to ported vacuum to pass the test.

    One comment I would make about the original article is that very few 65 and earlier carburetors are lean (A/F > 14.7/1) at idle. Most are adjust to the rich side to compensate for A/F variations by cylinder. Some of those long log manifolds had A/F varying by 2 or more.

    By far the major reason you need more timing at idle is lack of turbulence. Turbulence is the main factor in flame speed. At idle we are at our lowest in-flow velocity which means our lowest turbulence.

    Still a tune on a pre-emission engine is a bit of an art and the skill and knowledge of all factors in the tuning af an engine will override any one size fits all global statement of what is better.

    Due to all the variables a combination of parts that were never factory engineered require working with the combination to see what makes it perform best.

    And finally the downside of no vacuum advance at cruise is significant mileage and elevated EGT, and heat transfer thru the cylinder walls to the cooling system, the downside of ported vacuum at idle is not nearly as significant.

    jm2c
    rh
     
  14. tommy
    Joined: Mar 3, 2001
    Posts: 14,757

    tommy
    Member Emeritus

    Don't just assume that a vacuum port above the throttle plates is in fact a ported port. The factory V.A. port on a Rochester is in the side of the carb well above the throttle plates and many think that makes it ported but the port takes a 90* turn down and comes out below the throttle plates....manifold vacuum. If you are running a Rochester you have no choice there ain't no ported vacuum source.
     
  15. hoop98
    Joined: Jan 23, 2013
    Posts: 1,362

    hoop98
    Member
    from Texas

    Long before emissions came along it was common to have ported vacuum to the distributor. The vacuum advance was primarily calibrated to provide optimum spark advance at cruise conditions.

    This amount of advance may have caused issues when applied at idle.

    Also advancing at idle changed the throttle valve relationship to the off idle slots.

    So, it depends on the entire combination. I don't think it was a common practice in the 50's because of emission regulations, rather it was the best compromise.

    rh
     
  16. Lou39
    Joined: Dec 7, 2007
    Posts: 128

    Lou39
    Member
    from Cedar, MI

    Hmmm...both the installation instructions for the Mallory dist. and the Holley Street Avenger say use the ported source to the can. Its always been a poor idler and a bitch to get it moving off idle when cold. Its a C4, SBF, dual plane, X303 cam.
    Some experimenting is in order
     
  17. hoop98
    Joined: Jan 23, 2013
    Posts: 1,362

    hoop98
    Member
    from Texas

    What is the initial set at (with 24 in the distributor I'd guess). If you have 24 i the distributor you need 10-14 initial.

    The thing is, hook it up to manifold vacuum, if it runs better, leave it.

    :)
     
  18. gas pumper
    Joined: Aug 13, 2007
    Posts: 2,957

    gas pumper
    Member

    yes it is . And don't give up too quickly. you may need to readjust the initial timing, change the idle speed and idle mixture until you get a happy combination. But you will be able to get a much smoother operating engine if you keep with it.

    Sometimes you just need another 100 rpm at idle to get rid of the initial stumble and lag.
     
  19. Lou39
    Joined: Dec 7, 2007
    Posts: 128

    Lou39
    Member
    from Cedar, MI

    Just got back from the Deeeeeeeeetroit Autorama. took an informal sampling of the cars where I could see the vac source and the distributor. All used the ported source regardless of the carb type.
    Great show btw. If you followed the chemical city coupe thread it was there and looked tits.
     
    indcontrols likes this.

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