OK Newb, You asked.
THE SPARK ADVANCE
By Petersons Basic Ignition and Electrical,
It's not enough merely to direct a spark to each cylinder; it
has to get there at precisely the right time. At low engine speeds the
spark plug fires at approximately 3 to 10 degrees of crankshaft rotation
before the piston reaches the top of it's compresion stroke. This gives
the fuil/air mixture an instant to " light up " before it starts to
expand and drive the piston down on it's power stroke. The fuil/air
mixture takes just about the same to get " lit " at high engine speeds ,
but since the engine is turning faster, the piston would have already
started down before the expanding gases could develop thier " push." An
engine operating under these circumstances would definitely have a steady loss of power as engine speed increased. For this reason the spark has to be fired earlier at high rpm than at idle . Doing this is the job of the
distributors centrifugal spark advance mechanism.
Before 1930 all cars had manual spark advances. This usually
took the form of a lever mounted " turnsignal style " on the steering
colume. When the driver moved out on to the highway, he advanced the
spark timing by pulling the lever downward on it's notched quadrant. For
starting, driving at low speeds, or ascending steep hills, the spark
lever was raised a few notches to retard the timing slightly. When
cranking the engine, it's spark control had to be placed all the way up
in the full-reyarded position.
Although some all-out compatition sports cars and grand prix
racers retained thier manual spark controls untill the 1940s, such units
have subsequently been replaced by " automatic " advance mechanisms in
all modern automotive applications. A number of worthwhile reasons forced
this general changeover. Probably most important for the average driver
was that " broken arm " type accidents- which frequently occurred when
hand-cranking an engine without fully retarding the manual spark
control-were all but eliminated by automatic advance devices. Crank
starting, you may recall, was a feature retained on many cars for over 20
years after introduction of the electric starter.
Another reason for abandoning manual spark controls was the
greatly increased compression ratios and higher rpm of later-model
engines. 35 years ago a real idiot could knock the tops of all four
pistons out of a Model A Ford by lugging the engine on a steep grade
without retarding the spark. Think of what his grandson could do to a 454
Corvette without the safeguards built into the automatic advance unit of
it's distributor.
There are currently three spark advances in commen use. The
first is the pure centriugal type, found mostly in high-performance hot
rod distributors, Ford high-perfomance cars, 68 and 69 L88 427 Corvetts,
VW transporters, early VW Beetles, Porsches and the Formula Vee race
cars. This unit operates quite simply by just advancing the spark as
engine revs increase, and retarding it as rpm appoaches an idle. It's
chief disadvantages are relatively poor low-speed economy and the
possibility of excessive advance under certain full-thottle conditions.
The pure vaccum-advance distributor has had wide use both in the
U.S. and abroad, but in recent years it's only signifigant application
has been on Fords economy 6s with Loadomatic distributors, and late VW
passenger cars. Vacuum advances on VWs function well enough under most
everyday driving conditions but tend to limit top speed and full-throttle
acceleration. This is the reason they are usually replaced with
centrifugal advance distrbutors when VW engines are installed in race
cars or dune buggies.
For many years the only distributor used on Ford passenger cars
was the Loadomatic. It was made by Holly Carburetor Co. the same people
who made the famous 4-barrel carburetors.
The third type of distributor uses both centrifugal and vacuum
advances. This " dual advance " type is found on the vast majority of both domestic and foreign cars. The vacuum advance unit receives it's vacuum supply from one of two sources - from either below the throttle valves ( intake manifold vacuum ) or from a location above the carburetor throttle valves ( spark-ported vacuum ). Manifold vacuum is the highest at idle and decreases as the throttle valves open but spark-ported vacuum functions differently for it is lowest at idle but increases with throttle opening. During idle and part throttle running, manifold vacuum advances the spark for more economical operation but, when the throttle is opened, vacuum advance is not provided, thereby reducing the chance of engine " ping " or damaging detonation. Since 1966 most manufacturers have switched over to spark-ported vacuum for emission
reasons. Spark-ported vacuum is more tailored to the performance and
economy demands of the engine. It is not available at idle or any other
time the throttle closes, so the spark is not advanced. As the throttle
opens, vacuum increases and timing advances to increase performance and
economy when you need it most - that is, as the throttle opens. The
centrifugal advance ensures the best settings for maximum acceleration
rates and top speed. The two advance methods work independently of each
other, according to the demands of the engine at the time.
All vacuum advance units on duel-advance distributors are controlled
by either intake manifold vacuum or spark-ported vacuum. Regardless of
vacuum source, all single-diaphragm and single-action diaphragm units
operate in the same manner. A tube running from a special vacuum port
above or below the throttle valves is connected to a vacuum chamber on
the distributor. Inside the vacuum chamber is a thin diaphragm that
separates the chamber into two halves. One side is exposed to vacuum and
the other to the ambient pressure of the outside air. When there is
vacuum in the chamber, air pressure deflects the diaphragm against its
spring loading so that a rod attached to it can advance the timing of the
spark by rotating the breaker plate. This changes the relative position
of the breaker point rubbing block and the distributor cam. When vacuum
drops, the diaphragm in the vacuum chamber is not so greatly deflected,
retarding the spark timing slightly. The only significant departure from
this system is the distributor on older Chevrolets 6-cylinder engines. On
these cars the vacuum unit moved the entire distributor body, not just
the breaker plate.
The centrifugal advance works by changing the position of the cam in
relation to the distributor shaft. This is accomplished by means of two
governor weights held close to the distributor shaft by small springs. As
the shaft's speed of rotation builds up the weights tend to fly apart,
stretching the springs. Pins on the weights act against a plate fitted to
the base of the distributor cam. The further the springs allow the
weights to fly out, the further the cams position-and the spark timing -
is advanced. The precisely calibrated tension of the springs is therefore
the prime controlling factor in the operation of a centrifugal advance.
Auto makers provide data on advance curves, etc.in the shop manual
for your make and model car. Applying this information, however, requires special testing equipment that is not generally available to the home mechanic. The important thing to remember is that any defects that
thorough machine testing might uncover are usually the direct result of
dirt, lack of lubrication, wear or faulty parts in the distributor. Most
repairs are just a matter of cleaning, lubing or replacing.
It's been known 'round these parts. 
