Timing/vacuum advance help.

Discussion in 'Troubleshooting & Diagnosis' started by RMTZ28, Nov 9, 2019.

  1. RMTZ28

    RMTZ28 Member

    Messages:
    90
    Likes Received:
    16
    Joined:
    Mar 29, 2015
    Location:
    Alb. nm
    Hey guys. I need to get some more pep out of my Z, but not knowing much about total timing here's what I know. Right now my base timing is @12* with vacuum disconnected.It jumps 20* more to 32*when I connected, so is my vacuum can giving to much? Where do I go from here to get my total timing to 38*. Altitude where I live is 5000 or so. I have a stock 350 with a mild cam. been years since its been rebuilt and never set total timing. btw hooked to manifold vacuum.
     
  2. Michaels

    Michaels New Member

    Messages:
    28
    Likes Received:
    5
    Joined:
    Aug 11, 2019
    hey man, i must say the chevy small block is one hell of a motor. i just ripped off my top end and decided to replace a bunch of parts and really work on my ignition system. it all works together.
    you need good plugs, wires, distributor, carb set up and of course timing.
    i have an offset timing light i picked up for 100 bucks. pull the vac tube from the carb and put a golf tee in it. have your buddy hold the RPMS at 3500 and point your offset light set to 36 or what ever you want and adjust the dizzy to 0 on your timing tab.
    mine sit at 12 TDC when idle

    there is deeper adjustments in the dizzy with springs and weights but you get the point.

    good luck and watch that fast turning fan
     
  3. grzewnicki

    grzewnicki Veteran Member Lifetime Gold Member

    Messages:
    3,428
    Likes Received:
    161
    Joined:
    Dec 9, 2009
    Location:
    Gordon from Jacksonville Fl
    If you got time to read this is something I downloaded a while back that explains timing, vacuum advance, centrifugal advance etc. And this was drilled into me during auto school, dwell affects timing (only need to worry about if your running a points distributor), timing affects carb so always do you adjustments in that order, dwell, timing and last carb.

    Here is my downloaded info:

    "The only reason MSD says to use ported is the same reason why ported vacuum was invented and thats for emissions

    One of the main benefits of running vacuum advance is the extra timing at idle - using the ported source totally negates that

    This was written by a guy much smarter than me[​IMG]

    "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.

    For peak engine performance, driveability, 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."

    MORE ON TIMING


    ell that depends. If you've set your distributor up to get full mechanical advance at or below 3,500 RPM, or have at least checked it to see that it’s all in by 3,500 RPM, then your answer is yes, once you hit full advance it won’t keep advancing as more RPM comes up. Once the mechanical advance weights sling-out at their given RPM, which is set by the type of weights it has, and more-so by the stiffness of the springs to give you your “mechanical advance”, they don’t (and can’t) come-out any farther. Most stock distributors are set to get full mechanical advance at about 4,000 RPM. Brand new MSD distributors come with ridiculously stiff springs in them that don’t see full advance until upwards of 4,500 – 5,000 RPM. You want all of your advance in at about 2,400 RPM. 3,000 is too high, 2,000 “might be” a bit low, so somewhere in there is where you want to be. This is why re-curving your distributor is essential. Not having full advance at or below 3,000 RPM is pretty useless considering an engine needs its advance to run at its best and to get that car moving. Stock, or un-curved, distributors that don’t see full advance until 4,000 – 4,500 RPM are a bit late in most cars and WILL cause your car to not move off the line or accelerate very well below it’s full advance RPM. But like I said, once the weights have slung-out… they’re out and won’t give it any more advance. The trick is to know how much advance you have, and WHEN it comes in. If it comes-in too late, then you need to change the springs and maybe even the weights to make it come-in sooner. If it has too much advance, then you need to limit the amount of advance inside the distributor and give it more “initial” timing on the crank to achieve the 34-36 degrees of desired total. I usually like to see about 10 – 14 degrees in the distributor and the rest on the crank in most performance engines.
     
    RMTZ28 likes this.
  4. cadillac_al

    cadillac_al Veteran Member

    Messages:
    841
    Likes Received:
    87
    Joined:
    Apr 22, 2015
    Location:
    Maine
    Your timing would be close enough for me. That's about where I set my timing for stockish 350's
     
  5. Jeep43

    Jeep43 Veteran Member

    Messages:
    1,298
    Likes Received:
    48
    Joined:
    Mar 30, 1999
    Location:
    Connecticut
    The vacuum advance only comes into play under lighter load. You need to disable the vacuum advance to check total timing. Pull the hose off of the canister and plug it. You will also probably need to increase curb idle screw to get it to idle if you were hooked to full manifold vacuum. Check total timing by increasing RPM until the timing mark doesn't continue to advance any more with more rpm. That's your total timing reading.

    Adjust your distributor to get what you want for total timing, then lock it down and hook the vacuum cansiter back up and reset idle. I like using full manifold vacuum as outlined in the article above.
     
  6. RMTZ28

    RMTZ28 Member

    Messages:
    90
    Likes Received:
    16
    Joined:
    Mar 29, 2015
    Location:
    Alb. nm
    I'll be doing that soon. Tonight I was using a piston stop to identify true tdc. Will be using manifold vacuum. Just unsure how to set adjustable vacuum can. Right now I have it set in the middle of its adjustments.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.