Great article. Not mine.

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.

Vacuum Advance Starts at inches Hg
Vacuum is measured in "inches of Mercury". Mercury has the chemical symbol "Hg." Thus, manifold vacuum is measured and referred to as "Hg". The "Start" spec for the vacuum control unit is a range of the minimum vacuum required to get the control unit to just barely start moving. When selecting this specification, consideration should be made to the amount of vacuum that a given engine produces, and what the load is on the engine at this specification. For example, an engine with a very radical cam may be under very light load at 7 inches Hg, and can tolerate a little vacuum advance at this load level. Your mom’s Caprice, on the other hand, has such a mild cam that you don’t want the vacuum to start coming in until 9 - 10 inches Hg. For most street driven vehicle performance applications, starting the vacuum advance at about 8" Hg produces good results.

Total Vacuum Advance at inches of Hg
This is the range of manifold vacuum at which the maximum vacuum advance is pegged out. In selecting this specification, you must consider the vacuum produced at cruise speed and light throttle application. If your engine never produces 20" Hg, you better not select a control unit requiring 21" Hg to work.

Max Vacuum Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is specified in DISTRIBUTOR degrees - NOT crankshaft degrees. When talking about these control units, it is important that you know whether the person you’re talking to is referring to the distributor degrees, or if he’s talking crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet, will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees (which is what you "see" with your timing light). Thus, a vacuum advance control unit with 8 degrees of maximum advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft advance. This would be an 8-degree vacuum advance control unit.
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.

From the Edelbrock Performer and Thunder Series AVS Carburetor Owners Manual:
Despite the enormous variety in engine designs, virtually all (spark-ignition 4-Cycle) engines have very similar A/F Ratio requirements. For fully warmed-up engines, the range of A/F is:

5 to 1
RICH BURN LIMIT: Combustion is weak/erratic.

6-9 to 1 EXTREMELY RICH: Black smoke and low power.
10-11 to 1 VERY RICH: Some supercharged engines run in this range at full power as a means of controlling detonation.
12-13 to 1 RICH: Best power A/F: Un-supercharged WOT.
14-15 to 1 CHEMICALLY IDEAL: At 14.6 the A/F is at the theoretical ideal ratio with no excess fuel or oxygen after combustion. Good A/F for part throttle cruise and light to moderate acceleration.
16-17 to 1 LEAN: Best economy A/F ratio. Borderline for part throttle drivability (worse than borderline if EGR is used).
18-19 to 1 VERY LEAN: Usual lean limit (Driveability).
20-25 to 1 LEAN BURN LEAN: Varies with engine and system.
Even though engines will run anywhere between 5 and 25 A/F, the usual target values for an un-supercharged engine are a fairly narrow range (Figure 1). A/F is about 12.5 for the WOT and 14.0-15.5 at part-throttle cruise. An intermediate value of about 13.5-14.0 is usually used for mid-range power (non-WOT acceleration).

Ignition System
In order to utilize precision A/F mixtures, the ignition system must produce a high voltage spark at just the right moment so maximum work is extracted from the explosive charge. Originally, the 440-3 engine was developed for use in the oil fields where they could chug steadily along at say 2100 RPM. In that situation, you could set the timing for that one speed and load and forget everything else. A vehicle however, is subjected to different loads and RPMs which result in the need for variable timing requirements. These timing requirements are established by two separate functions in a distributor. The first is centrifugal advance which is used to ensure maximum power can be produced throughout the normal operational RPM range of the engine. The second system is vacuum advance which is primarily used to maximize fuel economy. Problems in either advance system will create drivability problems (detonation, pinging, poor performance, overheating, etc.).

Centrifugal (Mechanical) Advance
For the A/F mixture to burn efficiently as the engine RPM increases, the ignition spark has to happen sooner (measured in degrees before Top Dead Center). This requirement has to meet irregardless of engine load. The centrifugal advance system performs this function. The need to advance the timing stays constant up until about 3000 RPM. Additionally, the "Total Advance" must be limited to 34 degrees. Advancing the timing beyond these values can be detrimental to the engine. The term "Total Advance" refers to the sum total of initial plus centrifugal advance. If the spec says set the initial timing to 8 degrees then you can only have a maximum of 26 degrees centrifugal advance (8 + 26 = 34). The key thing to remember about centrifugal advance, is that it is directly related to engine RPM. Engine load (vacuum) has no effect on centrifugal advance.

The centrifugal advance system uses rotating weights to vary the timing. As the weights are thrown outwards, the timing of the spark is advanced from the initial set point to around 34 degrees before TDC. The rate of change vs RPM curve is controlled by 2 springs. Lighter springs allow the weights to be thrown outwards faster thereby achieving maximum centrifugal advance faster. Heavier springs not only reduce the speed at which the weights are thrown outwards, they also can increase the RPM at which maximum centrifugal advance is achieved. Most street applications use 1 light and 1 heavy spring. This allows fast initial advance to some point and then a slower advance to the final RPM point. Advance curve specifications are typically listed in degrees of distributor RPM which for a 4 cycle engine is 1/2 crankshaft RPM. Make sure you translate your reading correctly. I am presenting the data here in crankshaft degrees unless otherwise noted. The Centrifugal Advance Spec curve for a MOPAR 440-3 MH chassis is shown below. The solid lines represent the listed Centrifugal Advance spec curve as seen at the crankshaft. The dashed lines represent the base Centrifugal Advance curve plus 8 degrees initial advance added in (Total Advance). Referring to the + 8 degree spec window (dashed lines), you can see that the small spring initially allows the advance to start around 650 RPM (9 to 16 degrees advance) and come up fast until you get 20 to 24 degrees advance @ 950 RPM. The heavy spring slows down the rate of change until a "Total Advance" of 29 to 33 degrees is achieved @ 2500 RPM (Total Advance). The actual advance curve of your stock MOPAR 440-3 MH distributor should fall somewhere between the dashed lines. As shown in the figure, the initial advance setting (i.e.: 8 degrees) affects the entire mechanical curve. For the 440-3 engine, the centrifugal advance curve basically stays the same over the years however, it can be tuned for maximum efficiency and power. I will be getting into that after I finish discussing the basics.

Computerized engines replace the centrifugal advance with a base timing lookup table (i.e. the timing curve). The base value selected is still determined by engine RPM. That base value is then modified based on the MAP sensor value which performs the same function as the vacuum advance can. This process is performed several times a second.

MOPAR Electronic Distributor
Vacuum advance hose must be disconnected and plugged at carburetor in order to measure centrifugal advance.


GM HEI Distributor

Vacuum Advance
From the mystery aspect, the vacuum advance circuit and the design changes surrounding it win hands down. The vacuum advance circuit is based on engine vacuum therefore this circuit is not even used when you put the pedal to the floor. At Wide-Open Throttle (WOT) engine vacuum drops to near 0Hg. For racing applications, you will typically discover that either the vacuum advance circuit is not there or has been disconnected. Racers are typically not concerned with economy (MPG) and drivability. What the vacuum advance circuit does do is provide up to 20 degrees more advance during idle or cruise operations. This improves both efficiency (MPG) and drivability which are two very important concerns for a motorhome application. Disconnecting the vacuum advance will also shorten plug life (carbon buildup).

It is important to understand that vacuum advance is placed on top of centrifugal advance.
If you are running at 2500 RPM with 32 degrees total advance (8 degrees initial + 26 degrees centrifugal) and engine vacuum is inducing a total of 20 degrees vacuum advance, the sum total of advance is 52 degrees. In the days of leaded gasoline, 52 degrees was the absolute maximum value of advance. On todays gasoline, a maximum value of 50 degrees should be utilized Anything greater is detrimental to the engine. The amount of vacuum advance change is dependent on engine loading. As you step on the pedal, engine vacuum drops therefore vacuum advance decreases. As you approach a steady speed on level ground, engine vacuum increases therefore vacuum advance increases. The changes in engine vacuum are also accompanied by a change in A/F mixtures by the carburetor. Richer mixtures (13/1) used for power reduce detonation (ping) whereas leaner mixtures (15/1) promote detonation (ping).

MOPAR Vacuum Advance

GM HEI Vacuum Advance

Prior to 1972, most vehicles had the distributor vacuum advance connected to a manifold vacuum port. With this configuration, you applied full vacuum to the distributor vacuum advance at idle. This meant you had around 20 degrees additional advance above the initial setting applied at idle. When setting your timing and adjusting the carburetor idle mixture, you removed this extra advance by disconnecting and plugging the vacuum hose from the carburetor before doing adjustments. This works well for the racing crowd which needs the extra advance for the larger cams. In 1972, in order to meet tightening emissions requirements, manufactures moved the vacuum advance hose to what is commonly referred to as a ported vacuum port. This port is located just above the throttle plate and does not see engine vacuum at idle. With no vacuum applied at idle, there is no vacuum advance being introduced at the distributor. This allows the engine to run hotter at idle and burn NOx more efficiently. Once the throttle is cracked open, full manifold vacuum is seen at the ported vacuum port and vacuum advance operation from that point is the same as it was prior to 1972. Most performance tuning descriptions (aftermarket tech reps) will tell you to move the vacuum advance hose to the manifold vacuum port and plug the ported vacuum port. The only real adverse impact to doing this is that you may fail an emissions check. However what they neglect to tell you is it also means that you will need to modify your initial timing and carburetor idle screw adjustments methods (factory settings will no longer apply).

When moving to manifold vacuum advance, the critical issues are:

  • Total advance cannot exceed 50 degrees (Initial advance + Centrifugal Advance + Vacuum Advance)
  • All Vacuum advance must be implemented by 2Hg below maximum manifold vacuum (In drive for automatics). Failure to do this will cause the idle RPM to fluctuate due to manifold vacuum variations.
  • Initial vacuum advance starting point has to be adjusted such that it does not produce pinging.
While this seems daunting, it is all addressed in the procedures below. A side benefit of using the manifold vacuum port is the engine will run cooler.

Along comes 1975 and MOPAR implements Exhaust Gas Recirculation (EGR). From the distributor stand point, nothing much changed however the carburetors changed some and now a small percentage of burned exhaust mixture is being introduced into the intake mixture. The purpose is to reduce combustion temperatures during part throttle (cruise) conditions which in turn reduces NOx emissions. Unfortunately diluting the A/F mixture also robs the engine of power. Additionally, the EGR valve can get clogged up and stick resulting in a large vacuum leak. Make sure you check it if installed. Operation of the EGR circuit is a bit complicated but it basically applies a small venturi (not manifold) vacuum control signal from the carburetor to a vacuum amplifier on the rear of the engine. The vacuum amplifier amplifies the signal and controls the EGR valve via a heat operated sensor valve located by the thermostat. The sensor valve prevents EGR valve operation while the engine is warming up. When an EGR based system is utilized (not disconnected), maximum vacuum advance can be increase to allow up to 56 degrees (initial + mechanical + vacuum).

Along comes 1977 and as a cost cutting measure (remember the first Chrysler Bankruptcy in 1979?), MOPAR does something real dumb to the 440-3. In order to meet tightening emissions requirements, rather than redesign the heads for greater efficiency, MOPAR drops the pistons down and reduces the compression ratio. This robs the engine of more power. Many people end up driving themselves nuts trying to squeeze more power from the engine wondering why it is not as powerful as 76 and earlier engines. Naturally they blame emissions control systems but the drop in compression ratio is the real culprit. Sadly, MOPAR stops producing a motorhome chassis in 1979.

Baselining Current Ignition
Verifing Timing Chain
The distributor is driven by the cam. Given that the distributor is adjustable you can compensate retarded ignition timing due to chain wear however, the cam is not adjustable. Most manufactures used nylon toothed cam gears which wear faster than steel ones. If excessive chain assembly wear is discovered then even though both gears must be replaced, it is advisable to replace the chain assembly with a double-row timing set. A double-row timing set is longer lasting than a normal timing set. If chain slack is severe, then the chain can jump a tooth which can also result in 1 or more bent valves. Ford 460 and the Dodge 360 owners should be aware that these 2 engines can exhibit excessive slack within 40,000 miles. The following procedure can easily be performed when you change plugs however it should initially be done at 20,000 miles from new or replacement and every 10,000 miles thereafter.

  1. Remove the spark plugs and distributor cap.
  2. Using a socket and long handled ratchet on the harmonic balancer bolt, turn the crankshaft clockwise until the timing mark is aligned with the TDC mark. Do not reverse direction to align the marks. If need be, go all the way around again.
  3. Use a piece of tape or other material to form a precise reference point between the distributor body and the rotor.
  4. While closely watching the reference point on the distributor and rotor (enlist an assistant if necessary), turn the crankshaft in the opposite direction (counter clockwise) very slowly until the rotor just begins to move.
  5. The difference from TDC to the current position of the harmonic balancer mark represents chain slack in degrees. If the value is greater than 9 degrees, then the chain assembly should be replaced. Values of 15 degrees or more can result in the chain slipping a tooth.
Measuring Existing Centrifugal Advance Curve
From Timing is everything [MoPar Muscle Magazine April ’98 Article]

  1. Setting out to modify the advance curve without knowing what you've got is like picking up a date in a smoky Vegas lounge at closing time. The modifications may work out, but you might be disappointed with what you end up with. You need to check the RPM at which the centrifugal advance begins to kick in, how much it advances for an incremental RPM increase and at what RPM it has finished advancing or is "all in." To do this, you need only a timing light, tach and degreed damper. Mark the damper yourself as described in the MoPar Engine Book or stick on the MoPar Performance timing tape. Use graph paper to plot the point to generate a nifty looking curve. It will give you something to hang on your fridge besides your three-year olds’ artwork.
  2. With a marked damper: the timing can be read up to 60К using the -0- mark on the timing tab as a reference.
  3. With a dial back timing light, the timing offset can be read off the dial or digital readout referenced to the -0- mark on the timing tab.
  4. Disconnect and block the vacuum advance and read the timing at idle. Bump up the RPM while reading the timing until the advance just begins to kick in. Record the RPM. If it's right off idle speed, try backing down the idle and start again.
  5. Note: Centrifugal advance on a most stock engines does not start until at least 1000 RPM.
  6. Continue to increase the RPM in even steps of 500 rpm, recording the timing until it no longer advances. Zero in on the exact RPM at which the timing is "all in." Subtract the initial timing and you've got the centrifugal advance curve. The total timing reading at maximum advance minus the initial advance is the total amount of centrifugal advance, or how much is "in the plate." If you're mathematically challenged like us, you can set the initial timing at TDC and read the centrifugal curve directly without having to subtract the initial setting.
Measuring Existing Vacuum Advance Curve
As with centrifugal advance, to tune vacuum advance requires its curve be mapped. Again, this can be done with the engine used as the distributor machine, only this time using a hand vacuum pump to check the rate of advance for various vacuum levels. The procedure is simple enough: Pull up vacuum with the hand pump while checking the timing. Note the vacuum pump's gauge point where the advance just begins to kick in and record this. Move the vacuum level up in 1 inch increments, recording the vacuum level and how much the timing has correspondingly been increased. There's a trick here: Lower the idle speed to well below the RPM at which the centrifugal advance kicks in each time the vacuum pump level is pulled up. Otherwise, as the vacuum advance brings up the timing, the RPM will increase causing the centrifugal advance to come into play and invalidating your readings.