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Timing And Vacuum Advance 101SPARK TIMING AND CENTRIFUGAL AND VACUUM ADVANCE IN TERMS

THAT NON-ENGINEERS CAN RELATE TO

B Y J O H N H I N C K L E Y

In this day and age, when modern automotive powertrains are com-

puter-controlled and engines don’t even use distributors anymore,

the knowledge of what distributors did and how they operated to

control ignition timing has begun to fade; for those just entering the

classic automotive hobby, the function of the distributor and the notion

of “timing” is even more mysterious. To keep your classic Corvette run-

ning reliably and at maximum efficiency, some knowledge about the

principles of spark timing and how it’s controlled is essential. The objec-

tive of this article is to de-mystify the principles of spark timing and to

explain why and how your distributor-equipped Corvette’s spark timing

is controlled and varied to suit changing driving conditions.

PHOTO BILL ERDMAN

1 Here’s the distributor, that wondrous device that hidesback there in the dark under your ignition shielding and contains the two totally independent, but complementary, spark-advance systems that determinehow your Corvette performs.PHOTO JOHN HINCKLEY

2 This is the centrifugal advance mechanism that hidesunder the rotor – the “autocam pivoting weights,” andtension springs. The faster it spins, the more the weightsmove outward, advancing the spark timing.PHOTO JOHN HINCKLEY

3 The pin and slot in the “autocam” that limits maximumcentrifugal advance weight travel is visible directlyabove the end of the condenser.PHOTO JOHN HINCKLEY

I won’t get into the gory details ofcombustion theory, but let’s understanda little about what happens as the pistonis traveling upward on the compressionstroke toward the point where the sparkplug “lights the fire.” Before we light thefire, let’s talk a little about what we’relighting – the fuel/air mixture that’s been

metered by the carburetor and atomizedin the intake manifold as it heads foreach cylinder’s intake valve.

FUEL/AIR MIXTURE AND “BURN

RATE”: At idle and steady cruisingspeed, the load on the engine is low,and the air/fuel mixture is “lean” (more

air/less fuel); when accelerating, theload on the engine is higher, and it’s feda “rich” air/fuel mixture (more fuel/lessair). These are two very different condi-tions, as a lean mixture burns relativelyslowly, and a rich mixture burns faster.Remember this distinction – it’s a keyfactor in ignition timing.

Back in the cylinder, with the piston rising and compressing the air/fuelmixture, the idea is to fire the spark plugat just the right moment such that themixture is ignited (starting the “burn,” asthe flame front proceeds across the cylinder) and the rapidly expandinggases reach peak cylinder pressure justafter the piston reaches TDC (top deadcenter), exerting maximum force to pushthe piston down on the power stroke formaximum efficiency.

SPARK TIMING: Referring back to theburn rate comparison, slower-burninglean mixtures need to have the “fire lit”earlier in the compression stroke(because they take longer to reach peakcylinder pressure) than faster-burningrich mixtures (which take less time toreach peak cylinder pressure). Witheither mixture condition, the objectiveis to reach peak cylinder pressure atexactly the same point after TDC, whichmeans they have to be “lit” at differentpoints during the piston’s upward travel

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2 3

1

When someone says their initial timing is set at 10 degrees,that means the distributor is set to fire the spark plugs

when the crankshaft is 10 degrees of rotation before the piston reaches top dead center, which is 10 degrees of advance.

– this is what “spark timing” is all about– managing the point at which the sparkplug fires under different operating conditions. This point is expressed as“spark advance” in degrees of crank-shaft rotation before the piston reachestop dead center. When someone saystheir initial timing is set at 10 degrees,that means the distributor is set to firethe spark plugs when the crankshaft is10 degrees of rotation before the pistonreaches top dead center, which is 10degrees of advance. This is the “initial”or “base” spark timing which is checkedand set at idle during a traditional tune-up(with the vacuum advance disconnected);it’s fixed at the point where it’s set byclamping down the distributor hold-down bolt, and doesn’t change – it’salways there.

EARLY SPARK TIMING: In the daysof the simple, low-compression, inefficientModel T, spark advance was set manually to a fixed level with a lever onthe steering column. About all the driver did was to “retard” (delay) thespark timing while turning the crank tostart the engine, then move the lever to“advance” the spark timing once theengine was running. If the driver forgotto retard the spark when cranking andleft the lever in the advanced position,the engine could “kick back” while theoperator turned the crank to start theengine, which could result in a brokenarm (an unforgettable lesson in sparktiming). Once the engine was running,the operator could vary the sparkadvance with the lever for best performance(such as it was), or just leave it alone(which most operators chose to do).With that big, low-compression, slow-running, low-powered engine, little damage could be done by improperlysetting the spark advance.

Fast-forward to the 1960s and high-compression 350-horsepowerCorvette engines howling at 6500 rpm.Suddenly the spark timing equation ismuch more complex, and spark timing

errors can result in scattering expensiveengine parts all over the street at oneextreme, and poor performance andfuel economy at the other extreme. Anautomatic device has to recognize theentire spectrum of operating conditionsand manage the complexities of sparktiming in a manner completely independent of, and transparent to, thedriver, who has other things to keep himoccupied – like traffic, flashing bluelights in his mirrors, etc.

THE DISTRIBUTOR AND ADVANCE:

This wondrous device that handles allthat work is the distributor, which livesquietly in the dark, at the back of theengine, hidden forever under theCorvette’s ignition shielding, demandingonly an occasional set of points, a condenser, and a rotor to continuedoing its job. Let’s talk about the twodifferent ways the distributor managesspark timing while you’re watching traffic and grabbing gears – centrifugaladvance and vacuum advance.

The centrifugal advance mecha-nism under the rotor in the distributoradvances spark timing based solely onengine rpm (it’s driven at half crankshaftspeed); a pair of weights pivots on pins,retained by little coil springs. The fasterthe shaft turns, the more the weightstend to pivot outward (centrifugalforce), and the rate at which they moveoutward is controlled by the tension ofthe little springs. Lighter springs letthem move fully outward at relativelylow shaft rpm, and stronger springsrequire higher shaft rpm for full outward movement. The pointed “tail”of the weights, at the pivot end, bearagainst a cam (called the “autocam”)attached to the top of the distributorshaft and, as the weights move outward, the eight-sided cam thatopens and closes the contact points

(which trigger the coil to fire the sparkplugs when the points open) is“advanced” so it opens the points earlier than when the weights are fullyretracted (as they are at idle). In mostdistributors, this mechanism providesup to 20-25 (crankshaft) degrees ofspark advance when the weights arefully extended. The maximum advancethis system can provide is limited by a bushing installed over a pin, whichmoves in a slot in the lower plate of the autocam. The system is designed sothat the weights don’t begin to moveuntil slightly above normal idle rpm, so the initial timing can be set accurately without any influence fromthe centrifugal advance mechanism.

CENTRIFUGAL ADVANCE CALIBRATIONS:

There are many different calibrations ofweight configurations and spring tensions specified for productionCorvette distributors, depending on theperformance level of the engine, manualor automatic transmission, etc. Thepoints between the rpm at which theweights begin to move and the rpm atwhich they’re fully extended, providingmaximum advance, is referred to as the“centrifugal advance curve,” which istailored to each engine combination.The key point to remember here is thatthe centrifugal advance mechanismadvances and retards spark timing in response only to engine rpm, andnothing else. Its function is to advancespark timing as engine rpm increases.As upward piston speed increases withrpm, effectively shortening the time forthe compression stroke, the spark has tofire sooner, as the air/fuel mixture stilltakes the same amount of time to burnas it does at lower rpm. In effect, thecentrifugal advance mechanism handlesonly the basic physics of lighting the firesooner at higher rpm so peak cylinder

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4 Two different sets of centrifugal advance weights andthree different strengths of tension springs are shown.How these are combined determines the “advancecurve” for each specific engine combination.PHOTO JOHN HINCKLEY

5 A typical vacuum-advance distributor, showing the vacuum diaphragm on the right and the centrifugaladvance mechanism at the top. These two systems areindependent, but work together.PHOTO JOHN HINCKLEY

4 5

pressure is still reached at the samepoint just after TDC.

Now we have the basic physicshandled, but we still need another systemto manage spark advance based on allthe variations of driving conditions andengine load variations experienced innormal operation. This is handled by thevacuum advance system.

VACUUM ADVANCE: The vacuumadvance system consists of a vacuumdiaphragm mounted on the distributorbody. The diaphragm is spring-loaded inthe zero-advance position, and has a rodthat connects to a hole in the breakerplate, which is the movable plate thepoints are mounted on. When vacuum isapplied to the diaphragm, it pulls on therod, which in turn pulls on the breakerplate, rotating it with respect to the eight-sided cam on the distributor shaft whichopens and closes the points. Whenviewed from the top, the distributor shaft(and the eight-sided cam for the points)turns clockwise. When the vacuumadvance rod pulls on the breaker plate, itrotates the breaker plate (and the points)counter-clockwise, which “advances”the opening of the points (triggering thecoil to fire the spark plugs). A typicalvacuum advance unit, when fullydeployed, will add about 15 (crankshaft)degrees of spark advance over andabove what the distributor’s centrifugaladvance system is providing at themoment, which depends on engine rpm.They are two independent systems, butthey work together to provide the correctamount of spark advance.

CONTROLLING VACUUM ADVANCE:

Let’s look at how the vacuum advancesystem is controlled. Referring backagain to burn rates, remember that leanmixtures burn slower and rich mixturesburn faster. Engine load conditions (idle,

steady cruise, acceleration) result inhow lean or rich the air/fuel mixture is(the carburetor handles this), and thebest indicator of engine load is intakemanifold vacuum. At idle and steadycruise, engine load is low, and intakemanifold vacuum is high due to thenearly closed carburetor throttle plates.Under acceleration, the throttle platesopen wider, and intake manifold vacuum drops. It is essentially zero atwide-open throttle. As a result, intakemanifold vacuum is a “free” indicator ofengine load, which correlates nicelywith fuel mixture being supplied – leanmixture at high vacuum, and rich mixture at low vacuum.

At idle, the engine needs additionalspark advance in order to fire the lean(and exhaust-diluted) idle fuel/air mixture earlier in the cycle in order todevelop maximum cylinder pressure atthe proper point after TDC for efficiency,so the vacuum advance unit is activatedby the high manifold vacuum, and addsanother 15 degrees of spark advance ontop of the fixed initial timing setting. Forexample, if your initial timing is set at10 degrees, at idle it’s actually 25degrees with the vacuum advance connected (a properly calibrated centrifugal advance mechanism will nothave started to move yet at idle rpm).

The same thing occurs understeady highway cruise conditions. Themixture is lean, takes longer to burn, theload on the engine is low (it takes onlyabout 40 horsepower to cruise at50mph) and the manifold vacuum ishigh, so the vacuum advance unit isagain deployed and adds 15 degrees of spark advance over and above whatever the distributor centrifugaladvance mechanism is providing at thatengine rpm. If you had a timing lightconnected so you could see it as youcruise down the highway, you’d see

about 45-50 degrees of spark advance –your fixed initial advance of 10 degrees,20-25 degrees provided by the centrifugal advance mechanism, andthe 15 degrees added by the vacuumadvance unit.

When you accelerate, the fuel/airmixture is immediately enriched (by theaccelerator pump, power valve, meteringrod piston, etc.), and that rich mixturenow burns faster and doesn’t need theadditional spark advance anymore.When the throttle plates open, the manifold vacuum drops, and the vacuum advance unit diaphragmretracts to its zero position, “retarding”the spark timing back to what is beingprovided at that moment by the fixedinitial timing and the centrifugaladvance mechanism. The vacuumadvance doesn’t come back into playuntil you back off the gas and manifoldvacuum increases again as you return tosteady-state cruise, when the mixtureagain becomes lean and needs morespark advance for fuel efficiency.

VACUUM ADVANCE CALIBRATION:

There are also many different calibrationsof vacuum advance units. Some beginto deploy at different vacuum levelsthan others, and some provide moredegrees of advance when fully deployedthan others. The original calibration wasselected based on the intake manifoldvacuum characteristics of that particularengine/transmission combination andhow it was expected to perform in dailyuse. Vacuum advance units were connected to full manifold vacuum fordecades. In the late ’60s and early ’70s,when emissions began to become anissue, many were instead connected to“ported” or “timed” vacuum sources.We’ll discuss this aberration a little later.

THE ADVANCE COMBINATION:

Now we have two different advancesystems working independently, butcomplementing each other, to managespark timing – centrifugal, based on

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6 A typical vacuum advance unit. The rod at the bottomis connected to the vacuum diaphragm, and connects tothe movable points breaker plate at the other end. Whenvacuum is applied, it advances the breaker plate.

7 The vacuum advance unit, attached with two screws tothe distributor body. The tip of its actuating rod is visibleat the left, protruding through the movable breakerplate.

6 7

engine rpm, and vacuum, based on engine load and operating conditions. The centrifugal advance system is purely mechanical and is only rpm-sensitive; nothingchanges it except engine rpm. Vacuum advance, on the other hand, respondsinstantly to engine load and rapidly changing operating conditions, providing thecorrect amount of spark advance at any point in time, to deal with both lean andrich mixture conditions.

By today’s engine management terms, this was a relatively crude mechanicalsystem, but it did a good job of optimizing engine efficiency, throttle response, fueleconomy, and idle cooling, with absolutely zero negative effect on wide-open throttle performance, as the vacuum advance is inoperative under that condition. In modern cars with computerized engine controllers, all those sensor inputs to the computer change both spark timing and fuel/air mixture 50 to 100 times persecond, and we don’t even have a distributor anymore – it’s all electronic.

“PORTED” VACUUM: Now to the widely misunderstood manifold vs. ”ported”vacuum aberration. After 30-plus years of controlling vacuum advance systems withfull manifold vacuum, that “free” indicator of engine load and fuel mixture, alongcame early emission control requirements (seven years before catalytic convertertechnology was introduced), and all manner of crude band-aid systems were introduced to try to reduce hydrocarbons and oxides of nitrogen in the exhauststream. One of these crude, but effective systems was GM’s Air Injection Reactor(A.I.R.) system, which pumped fresh air into the exhaust ports to “afterburn” pollutants in the exhaust manifolds. The key to making this system work at maximum efficiency was retarded spark at idle; with retarded idle spark timing, the“burn” begins late, and is not complete when the exhaust valve opens, which doestwo things that were important for emissions. The incomplete burn reduced combustion chamber temperatures, which reduced the formation of oxides of nitrogen (NOX), and the significant increase in exhaust gas temperature ensuredrapid “light-off” and combustion of the hydrocarbons in the exhaust gas streamwhen the fresh, oxygen-carrying air was introduced from the air pump.

As a result, these engines ran poorly, and an enormous amount of wasted heatenergy was transferred through the exhaust port walls into the coolant, causingthem 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.“Ported Vacuum” was easy to implement – they just moved the distributor vacuumport orifice in the carburetor from below the throttle plate (where it was exposed tofull manifold vacuum) to above the throttle plate, where it was exposed to manifoldvacuum only after the throttle plate opened. This meant that the vacuum advancewas inoperative at idle (retarding idle spark timing from its optimum value), andthese applications also had very low initial timing settings; they were usually set atfour degrees before TDC or less, and some even had initial timing settings as muchas two degrees after TDC. The vacuum advance still worked at highway cruise, butnot at idle, which caused all manner of problems. “Ported Vacuum” was strictly anearly pre-converter crude emissions strategy and nothing more. Don’t believe anyone who tells you that ported vacuum is a good thing for performance and drivability – it’s not. Anyone with a street-driven car without manifold-connectedvacuum advance is sacrificing idle cooling, throttle response, engine efficiency, andfuel 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 mechanicswho don’t understand the principles and operation of vacuum advance either, sothey’re not alone.

THE BOTTOM LINE: Now that we’ve covered the why’s and how’s of spark timing and its control systems, you can appreciate what’s going on underneath yourignition shielding and how it affects performance and drivability. Checking theoperation of the centrifugal and vacuum advance systems during periodic mainte-nance and tune-ups can pay real dividends that you can feel in the seat of yourpants. Well, you say, “How do I do that?” Tune in next month, when we’ll show youhow to check out those systems, how to “map” your advance curves against theirdesign specifications and verify proper operation, and pass along some simple tipsand techniques for improving your Corvette’s performance by “tweaking” itsadvance systems for peak efficiency. ■

58 C O R V E T T E E N T H U S I A S T c o r v e t t e e n t h u s i a s t . c o m

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