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-   -   Vacuum advance (http://squarebirds.org/vbulletin/showthread.php?t=21186)

Frango100 12-27-2016 06:47 PM

Vacuum advance
Adjusted the ignition timing today, and found that there is no difference in idle with the vacuum hose connected to the carburator or not. When accelerating, the ignition advances, which is probably due to the flyweights.
When is the vacuum advance part suppost to do something? Is there any way to test the vacuum advance bellow without having a vacuum source?

jopizz 12-27-2016 07:26 PM


Originally Posted by Frango100 (Post 105663)
Is there any way to test the vacuum advance bellow without having a vacuum source?

You don't need a pump. Use your own suction and you should see it move with the cap off. Hold your tongue over the hose and it should hold. When you release it it should snap closed.


pbf777 12-27-2016 07:46 PM

This posting may be a little long, but it does answer your question, and also provides good and accurate additional information which may assist other questions to follow.

I am not the author, Scott.

This was written by a former GM engineer as a response to a similar question on a Corvette board:

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.


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.

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.

Frango100 12-27-2016 08:14 PM

Thanks John, i will check that out tomorrow.
And thanks Scott for that very informative explanation about vacuum advance. I´m quite new to the techniques used on carburated engines, but am very eager to learn and understand the workings. Understanding the workings is the first step in being able to trouble shoot problems. Thanks again.

simplyconnected 12-28-2016 01:59 AM

I agree with using manifold vacuum.
There are a few points I do not agree with. He mentions nothing about high compression and how much harder it is to fire regardless of mixture. He mentions nothing about the engine having no vacuum advance at starting speed or the fact that we adjust idle mixture with needle valves for optimum performance.

Anyone who cranked a Model T by hand knows to retard timing or a backfire will take your arm off. Once running, advance timing for more power, even at idle speed. For Squarebirds, at starting speed there is no vacuum so six degrees BTDC is ok. Once running, vacuum should automatically advance spark timing. The vacuum diaphragm should dither at each light as gas is applied (and vacuum drops then regains).

He mentions EFI systems but doesn't realize that electronics on modern cars don't monitor vacuum. They look at throttle position as your foot releases more AIR into the engine. Then the computer controls injectors (gas flow). When warm, the computer optimizes efficiency by monitoring exhaust gasses.

If the engine is sitting in my garage and I'm looking with a timing light, the engine is under NO load. I don't need to cruise down the street for that. I can see timing marks move with more RPM which gives centrifugal advance. With a vacuum hose attached, the timing marks should steadily advance to ~36° at 2,500-RPM and hold there with no more increase as RPMs raise further. I want total advance to drop off as the load becomes greater, like when climbing a mountain or pulling a trailer. Do NOT run out and buy that vacuum advance he mentioned for an OEM engine.

Here's my distributor setup page: CLICK HERE

Frango100 12-28-2016 06:41 AM


Originally Posted by simplyconnected (Post 105668)
He mentions EFI systems but doesn't realize that electronics on modern cars don't monitor vacuum. They look at throttle position as your foot releases more AIR into the engine. Then the computer controls injectors (gas flow).

Thanks for that Dave. But my 2004 Jeep is using the manifold air pressure (MAP) as a prime input to the PCM for injection pulse calculations. Modern cars use MAP or MAF as a primary input.
But i will have a look at the vacuum advance to see if its doing anything while applying light vacuum and if not, order a new one from Rockauto.

pbf777 12-28-2016 01:21 PM

I wish to comment on some of the responses, to my previous unknown authored (to me anyway) post. The purpose is discuss the subject, not step on any toes.

Higher compression values generally lead to faster and more efficient combustion process (easier to fire). That's why you generally retard timing as the compression is increased (for ease of starting/cranking and to avoid detonation, all other values remaining constant), but, it IS more difficult to create the spark due to the increased resistance/load on the secondary ignition delivery system, and due to increased mechanical compression sometimes slower cranking speeds are encountered.

Although some E.F.I. systems may not monitor manifold vacuum directly, they establish "load" values thru other processes. And I think one must limit the content of ones' posting to remain relevant, and although mentioned, it was not the topic at hand, and perhaps not indicating limitations in knowledge.

If you wish to have your distributors' mechanical/centrifugal function advance the timing to a value of "X", but then expect "total advance to drop off" (retard) under increased load values (not reduced R.P.M.), without vacuum advance, how (remember: the subject is carbureted application distributors of common design)?

Although there have been true vacuum advance only function distributors (no mechanical/centrifugal mechanism), I think this discussion pertains to distributors which contain mechanical advance with vacuum advance enhancement option; and I think the values, as previously described are valid.


pbf777 12-28-2016 07:31 PM

Just some other notes on this topic of vacuum advance in distributor timing.

One needs to be mindful of the added timing value during these increased vacuum (actually not a true vacuum, but rather only decreased manifold pressure) scenarios, created generally due to the closure/restriction of the carburetors' throttle blades, thereby limiting the density of the atmosphere within the cylinders' volume. This may be of particularly concern if one is more aggressive with the mechanical values in search of maximum performance under load (e.g. hard acceleration); as at this point the "total value" (mechanical plus the vacuum advance values) may be to great. The result is often realized as a "lean serge" at low throttle angle cruising, and/or a "tip-in" throttle detonation ("pinging") on the initial transitional throttle application (more frequently experienced with manual transmission installations).

At one time, vacuum dashpots/cans were available for different vehicle applications with different timing change sums, frequently stamped on the actuating arm (sometimes). And the aftermarket provided units which were adjustable with a set screw jacking the spring seat plate, which allowed changing the springs' installed height and thereby its' pressure against the diaphragm.

Proper/ideal timing values are only established thru testing (trial & error), on your vehicle, in your environment, and best with you operating it. Also, there are many considerations in the balance between the induction system (fuel & air delivery) and elevated timing values in the quest for "best" performance.

The old adage: for best performance, increase the ignition timing until it pings, then back-off a little, actually, isn't far from the truth; though I strongly recommend using a timing light, and some understanding of what actually going on.


OX1 12-29-2016 06:54 AM


Originally Posted by Frango100 (Post 105670)
Thanks for that Dave. But my 2004 Jeep is using the manifold air pressure (MAP) as a prime input to the PCM for injection pulse calculations. Modern cars use MAP or MAF as a primary input.
But i will have a look at the vacuum advance to see if its doing anything while applying light vacuum and if not, order a new one from Rockauto.

Many of the new ecoboosts have no mass air sensor. My 2.0 fusion has two MAP's, pre-post throttle.

Frango100 12-30-2016 09:07 AM

So just checked the vacuum advance actuator and its completely shot. When applying a light vacuum, you can hear the air being sucked in via the distributer housing opening. So its time for a new one. Don´t know if there are different brands available, but is there any better then others?

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