Date: Sat, 8 Feb 1997 12:39:19 EST
From: Russ Maak <russcm@JUNO.COM>
Subject: SPARK ADVANCE 2

              Heres the rest of the article

                                                           THE SPARK
ADVANCE

                                       By Petersons  Basic Ignition and
Electrical Systems

       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.



       There you have it in a nut shell, or three nut shells if you may.
        centrifugal advance only.
        vacuum advance only
        centrifugal and vacuum advance ( duel advance )

        Hope this helps answer a lot of questions about advances.


                     RUSS

                     56 oval
                     58 ragtop BAJA daily driver
                     63 convert. ( Snubs )
                     65 notch
                     and more
SPARK ADVANCE 2