Is there any automotive feature more misunderstood than vacuum advance? It’s unobtrusively been doing its job of increasing fuel mileage, sharpening throttle response under light loads, and keeping down engine temperatures of most non-computer-controlled cars for almost a century, and its operation is simpler than one might think.
What Is Advance?
For best power and economy, an internal combustion engine needs ignition advance; in other words, the spark plug needs to fire before the piston reaches Top Dead Center (TDC), which is why ignition timing is, in most applications, measured in degrees Before Top Dead Center (BTDC). The faster the engine goes, the more advance it needs, because the mixture needs time to combust completely. If the spark plug is fired too late, the piston already will be on its way down the cylinder by the time the flame front has expanded, wasting energy in the form of heat transferred to the cooling system. The later the timing is, the greater the surface area of the cylinder being exposed by the descending piston, and the greater the area that is subjected to the expanding gases. If the timing is late enough, that heat can even affect the exhaust port in the cylinder head. How early the ignition system needs to be fired depends on engine speed and load. That’s why most distributors have a centrifugal advance mechanism: As the engine speeds up, the advance weights are “thrown out” against spring pressure, advancing the points cam and therefore the timing. This is called the timing “curve.”
Before the early 1930s, engines had to operate on low quality gas, and thus had to use extremely conservative timing curves to avoid detonation. This obviously reduced an engine’s peak power output, but it also reduced fuel mileage because an engine at light load has different needs than an engine at full throttle.
Vacuum Advance
By the early 1930s, engineers had realized that by using an engine’s vacuum to augment spark advance (more on that in a minute), they could have the best of both worlds. Since an engine can handle more spark advance under light-throttle conditions, they were able to advance the timing when the engine could handle it without adding extra centrifugal advance and damaging the engine under heavy loads and high speeds.
Engine Vacuum
The vacuum advance system, by definition, depends on the engine’s vacuum. As the pistons travel down their bores and the intake valves open, a pressure differential is created in the intake manifold that is used to run many different systems, including (in the case of most carbureted and early fuel-injected engines) power brake boosters, automatic transmission modulators, and the distributor’s vacuum advance.
This pressure differential, or vacuum, is measured in inches of mercury (inHg). One pound per square inch (psi) of pressure differential will move a column of mercury 2.036 inches, so 20 inHg means that the pressure differential in the intake manifold is roughly 10 psi (less than atmospheric pressure). Most engines, unless they have high-performance camshafts with longer duration and overlap than stock, have somewhere between 15 and 22 inHg of vacuum in the intake manifold at idle and light cruise. These high levels of vacuum are partially the result of throttle blades that are either closed or nearly closed, as atmospheric pressure is being more-or-less partitioned from the intake manifold plenum under these light throttle situations.
Fuel Mixtures and Burn Time
Before explaining how vacuum advance uses this pressure differential to save fuel and eliminate light acceleration sag, it’s important to expand upon why OEMs used it in the first place. First, the stoichiometric air/fuel mixture of gasoline is 14.7 parts air to one part gasoline, or 14.7:1. Varying percentages of ethanol can change that figure slightly, but it’s possible for an engine to run perfectly well at leaner mixtures (in the 15:1-16:1 range) under light loads, such as cruising down the highway.
By nature, leaner air/fuel mixtures burn more slowly than rich mixtures (in the 13:1 range, for example), so the mixture can still be burning as the exhaust valve opens, causing a lot of that heat to enter the cooling system, as mentioned previously. Adding extra advance by using vacuum advance, sometimes up to 20 degrees, starts the burn earlier so more of it is being used as energy to drive the car rather than being lost through the cooling system as extra heat. This extra energy can also reduce the necessary throttle input, making the car even more efficient. The result is better economy and (potentially) lower cooling system temperatures.
Vacuum Advance Misconceptions
These parameters might help to clear up some misconceptions about vacuum advance. First, vacuum advance only works when there is a pressure differential between the intake manifold and the atmosphere. As soon as the throttle is opened wide enough, the atmosphere rushes in to fill the void, and the pressure differential (vacuum) drops to nearly zero; balance is achieved between the atmosphere and the intake manifold. Therefore, vacuum advance will also drop to zero as a result, leaving initial timing plus mechanical advance, which is usually in the 30- to 40-degree before top dead center (BTDC) range (see the chart above). Therefore, vacuum advance will not cause your engine to ping under full throttle, although it CAN make your engine ping in part-throttle situations if it supplies more advance than the engine needs or wants. These situations are much less likely to cause major engine damage, however, because the engine will be operating under a light load.
Remember, although an engine can be cruising down the road at 70 miles per hour at up to 50-plus degrees of advance, the vacuum advance does not work under heavy throttle situations; therefore, the total advance as advertised is always initial timing plus the total mechanical advance.
The Great Ported Versus Manifold Vacuum Advance Debate
If you’ve spent any time on the internet, you’ve realized that choosing between the two methods of controlling vacuum advance, ported vacuum advance versus manifold vacuum advance, is about as contentious as the Ford v. Chevy debate. There is, however, no such thing as the “right” vacuum advance; whatever the car prefers is the one that should be used. They both offer the same amount of vacuum advance; the only difference is when the distributor gets that advance. In the 1960s and 1970s, OEMs used both on various vehicles, regardless of automaker. For example, my 1963 Buick Riviera uses ported vacuum advance, while my 1965 Buick Skylark uses manifold vacuum advance. Both cars came from the factory with Carter AFB carburetors; both carburetors are set up to use different vacuum advance ports.
Manifold Vacuum Advance
The only difference between the two types of vacuum advance is the location in the carburetor bore where the distributor’s vacuum advance canister reads the engine vacuum. A manifold vacuum advance port is located beneath the throttle plate, so it is feeding vacuum to the distributor whenever the intake manifold has a pressure differential, including at idle. Having full manifold vacuum advance can help cool an engine that tends to run hot at idle (remember how lean mixtures tend to extend the burn?). However, some engines can feel “nervous” with too much idle advance; it all comes down to the engine combination. Also, manifold vacuum advance can increase emissions at idle, since the combustion chamber burn can actually be hotter. In some engine combinations, you’ll notice that hooking the distributor to manifold vacuum will increase idle speed to the point where it needs to be readjusted.
Ported Vacuum Advance
Ported vacuum advance takes its signal from a port just above the throttle plates, so it will only be fed vacuum advance when the throttle is cracked open a little, but not enough for manifold vacuum to drop. Therefore, it can be helpful under light acceleration and light cruise, even up to expressway speeds (as is manifold vacuum advance). The only real difference is that ported vacuum advance is not in operation at idle speed.
In fact, driveability is another reason to use vacuum advance; engines can run leaner mixtures and still feel “crisp.” Too much advance can cause detonation or other driveability issues, but the right amount can cover lean sags, so OEMs could tune carburetors leaner for better fuel mileage. Under heavy throttle, carburetors used a power enrichment system to supply the necessary extra fuel needed for heavy loads and high speed operation, times when the vacuum advance will not be in operation.
The Vacuum Advance Canister
The vacuum advance canister is mounted on the distributor itself, and it is made up of two chambers, which are separated by a diaphragm. The vacuum chamber is fed through a hose or a hard line to the vacuum advance port of the carburetor, and the other side of the diaphragm is open to atmosphere; therefore, the vacuum advance canister works in much the same way as a power brake booster. When vacuum is applied, the atmospheric pressure in the rear chamber overpowers the vacuum chamber and spring, moving the diaphragm away from the distributor at a rate determined by the engineers. Attached to the diaphragm is an arm that is connected to the distributor breaker plate, which moves either clockwise or counterclockwise (depending on the direction of distributor rotation), changing the points’ relationship to the distributor cam and advancing the timing. The same rules apply to cars with electronic ignition, but the relationship between their modules and triggers replaces the points and cam. The canister itself has a spring to control when the advance starts to “activate”: A heavier spring will require more vacuum to overpower it, and vice versa.
Downsides?
There aren’t many downsides to using vacuum advance in a stock or street-oriented performance car. Engines with large camshafts might not do well with vacuum advance because they don’t create much manifold vacuum (a large camshaft tends to have a long “overlap,” when the intake and exhaust valves are both open at the same time, and an engine can’t build vacuum with the exhaust valve open). This greatly reduces the signal to the vacuum advance canister, meaning that unless there is a very light spring in the canister, the advance will not operate. Some canisters have rates that are adjustable by a screw, but those are getting uncommon.
Race engines rarely use vacuum advance because most of the time on the racetrack is spent at full throttle, where vacuum advance will not work. On the street, engines that ping or surge when the vacuum advance is applied might need their maximum vacuum advance limited, a modification that is outside the scope of this article.
Summary
Vacuum advance is a good thing in most driving situations. Modern aftermarket carburetors almost always come with ports drilled above and below the throttle plates, so the owner can choose which application, ported or manifold, will be better with their combination. Without vacuum advance, you’ll probably have to use richer carburetor jetting than you otherwise would (or replace air bleeds, idle feed restrictions, etc.), because as anyone knows who has had a vacuum advance diaphragm fail, driveability issues generally result. Can an engine run well without vacuum advance? Of course, but fuel mileage at the very least will usually suffer. As a personal anecdote, I could tell last summer when the vacuum advance canister had failed in my 1965 Skylark (which, once again, uses manifold vacuum advance) because the engine temperature at idle crept up very quickly on a hot day. Replacing the canister put everything back to normal.
So, don’t unhook that vacuum advance canister just yet: It’s helping more than you think.
Related Reading
The Brain Is In The Carburetor? Understanding Ford’s Forgotten Loadomatic Distributor (by me)
Automotive History: Chrysler’s CAP, The First Effective Exhaust Emission Control (With ’63 Dodge and Dart Road Tests) (by Daniel Stern)
Another great Sunday morning read.
In my hot rodding youth I was fortunate enough to have access to an old Sun distributor machine which allowed me to learn and understand how this worked. I spent a fair bit of time at it and eventually got pretty good at setting up a curve. I quickly learned that what worked on my 327 with a warm cam and a 4 speed didn’t do so well on say, a 390 in a Ford pickup with an automatic hauling a camper. I don’t remember who made the adjustable vacuum advance canisters but I used them quite often. It was amazing the difference proper ignition curve tuning tuning could make once I understood the relationship between centrifugal and vacuum advance. Today? all done by a computer and maybe a chip….
Thanks for this well-written easy to understand explanation of the black magic that happens in distributors. I’ve had experience with both extremes: the vacuum-only advance Loadomatic in my ’66 F100 and the mechanical-only advance Bosch distributors in my old VWs. And a few with both. But I was never adept with a timing light, and I damaged one of my VWs by cranking in too much advance. It’s hard to hear detonation in an air cooled VW, but after a very long grade I did start hearing the raw exhaust on one of the cylinders from having pulled the studs out of the case. But it ran really strong until that happened!
Vast majority of air cooled VW’s had vacuum only. Venturi vacuum port provided a carb airflow signal proportional to rpm, with an additional internal port right at the throttle plate for early advance to prevent “bog”. Emission controls led to further mods to this system, including vacuum retard at idle.
I have a Bosch vacuum and mechanical advance one on my ’74, more like American cars.
Paul’s mechanical advance “009” is a common mod, but originally came with early Type II’s that ran full throttle much of the time, with 36hp in a relatively large vehicle.
You’re right about the VW dizzies. One of the ones I had was modified with an 009, but the other did have vacuum advance.
As mentioned older VW’s (air cooled) mostly ran better with the dual advance mechanisms added in the 1968 “Autostick” model .
As Arron states many (? most ?) current VW hot rods / hobby cars don’t really run well because of the 009 distributors fitted .
In air cooled engines it’s also critical to set the full advance timing to prevent sub audible ping / detonation .
This article should be mandatory reading for anyone who works on older engines .
-Nate
Turbocharged Corvair’s came with a modified vacuum advance to act as a pressure retard for anti-detonation when manifold pressure exceeded atmospheric under boost.
“Vacuum” gauges commonly found on aircraft read as absolute manifold pressure, approx. 29.92 inches of mercury reading at rest.
My 68 Cougar, with the 302-4V, always got 12-13 mpg at it’s best which was mostly highway. It wasn’t till years later, when I got into opening distributors, that I saw the problem. Under the top plate of the distributor are weights and springs. In a Ford there are two choices available to you. Typically a 10L/15L and 13L/18L advance weights. My car was set on the 10L which means 20 degrees of mechanical advance. Combine with Ford’s initial at 6 degrees my engine had only 26 degrees total mechanical advance.
Inside the vacuum canister of the time was a spring, a fiber stop, and several washers which controlled the amount of vacuum advance. Even if I had 10 degrees the max on the highway would be 36 degrees at cruise. Once I saw that I left things at 10L but moved initial up to 15 degrees for a total of 35. Leaving vacuum advance untouched the car now got 17 mpg. Below is a picture of what the advance weight plate looks like and what goes into the earlier Ford canister prior to 1973. Oh, and an adjustable replacement for the canister internals are available so you can adjust your vacuum advance via a small hex wrench. Those advance parts I supplied to a 67 Mustang owner, with a 390, who had the wrong advance canister for his car. He never knew until two people, in one week, mentioned it to him and the other person supplied the correct canister.
Having played with 6 cylinder BMWs since model year ’69 and into the Motronic (computer controlled age), it took me a long time to wrap my head around vacuum “retard” having any positive impact on performance. Retarded ignition is bad, at least for performance, right? Yes, but… If it’s set up just on the manifold side of the butterfly, it serves to crank in a bunch, well, ~12 degrees for them at the time, a bunch of advance as soon as the throttle is opened, as its just on the other side of the butterfly, so once the throttle is opened, it stops retarding, in effect advancing. They used vacuum advance the same way, in reverse. At small throttle openings, it would see high vacuum just as the retard diaphragm stopped retarding. So at light throttle you got 20+ degrees advance all of the sudden. Which made for a very lively engine.
BMW specifically, and I’m sure others, made high output, high performance engines for the day with that as a critical part of it. IN retrospect I’m surprised they managed to pass smog, especially with a modest amount of extra equipment for the era.
Some of the earliest vacuum-controlled distributors were set up in a similar way. The idea was to essentially use the vacuum control as a safeguard against detonation, allowing more mechanical advance and then using vacuum to retard it if the throttle was opened too wide.
This is essentially the logic of modern electronic ignition systems with knock sensors: The computer will advance the timing as much as it can, but if it detects spark knock, it will back off to just below that threshold. The big difference is that with knock sensors, the computer can hear if there’s knock (rather than having to be set up to retard the timing in load conditions deemed likely to produce knock), and the plugs can be controlled directly rather than having to do this Rube Goldberg stuff to alter the timing curve of a distributor.
In his famous “How to keep your VW alive” book, John Muir argued in favor of mechanical advance, saying, “Vacuum advance never worked and never will, as the principle is wrong.” Your thoughts on that?
The song “15 Days Under the Hood” by Jack Tempchin includes a reference to vacuum advance:
https://youtu.be/lLKswxQ0NDI
I have to disagree with the esteemed Mr. Muir, and I wonder how many VW people were poisoned against it as a result of that comment! Vacuum advance is like anything else on a car; it sometimes needs tweaking to work correctly, and modern parts that don’t quite match up with factory curves don’t help.
He’s wrong. He’s wrong on the level of a flat-Earther. Every street engine using a conventional distributor should have vacuum advance.