60 ENGINE AND ENGINE REBUILDING
economy. As engine load increases, engine
vacuum drops, which allows the ignition tim-
ing to retard, thereby preventing detonation
due to over-advanced timing. Some special
high performance models such as the 427/L88
do not have vacuum advance systems. Fuel
economy was not designed into such "off-
road" engines.
NOTE:
Do not remove the vacuum ad-
vance for street usage—fuel economy will
suffer, along with engine performance if not
set-up properly.
Another timing feature of the distributor is
the centrifugal advance mechanism. This
mechanism alters the ignition timing by ro-
tating the distributor cam (a small amount)
independent of the distributor shaft. Again,
the relationship between the distributor shaft
and the point opening is changed due to this
rotation. The centrifugal advance mechanism
consists basically of two small springs and two
specially shaped weights attached to both the
distributor cam and the shaft. These compo-
nents are mounted underneath the ignition
rotor. Instead of vacuum, this mechanism uses
engine rpm as a guide for advancing the ig-
nition timing. As engine rpm (and distributor
shaft rpm) increases, the weights are moved
gradually outward (against the spring ten-
sion) by centrifugal force. As the weights
move outward, the distributor cam is moved
to a more advanced timing position. When
the engine rpm drops, the springs gradually
pull the weights back to their low-speed po-
sition, thereby reducing the amount of tun-
ing advance.
ELECTRICAL OPERATION
The point set, or breaker points as they are
sometimes called, is the switching device of
a conventional ignition system. The "pri-
mary" side of the ignition coil is connected to
the battery, through the ignition switch, a re-
sistor, and various wiring. When the ignition
switch is in the "run" position, voltage from
the ignition switch passes through a resistor,
which lowers the battery voltage to the coil
from 12V down to about 6-8V.
NOTE:
If this resistor is defective, the full
12V from the battery will quickly burn the
breaker point contacts and shorten the life
of the ignition coil.
When the ignition switch is in the "start"
position, the resistor is bypassed. This pro-
vides a full 12V to the ignition system to aid
in starting the engine. Voltage flow after
the resistor (or bypass
wire) is through the primary (low voltage)
windings of the coil, to the breaker point set.
When the points are closed (grounded), a
magnetic field is produced within the pri-
mary windings of the ignition coil. When the
points open, the coil voltage no longer has an
easy path to ground. The magnetic field col-
lapses and transfers the voltage from the pri-
mary windings (outer) of the coil to the "sec-
ondary" windings (inner) of the coil, through
induction. Since the number of secondary
windings in the ignition coil is much greater
than the primary, the voltage is multiplied to
roughly 20-25,000 volts. This high voltage
then travels out of the center tower of the
ignition coil, through the high tension lead
(secondary coil wire) to the center tower of
the distributor cap. The voltage is then dis-
tributed by the ignition rotor to the outer ter-
minals of the distributor cap (in the "firing
order"), through the plug wires and finally,
the voltage jumps the gap of the spark plug,
causing ignition of the air/fuel mixture in the
cylinder.
The condenser, which is also attached to
the breaker plate (grounded), acts as a tem-
porary voltage storage unit when the points
are closed. This helps to prevent arcing be-
tween the point contacts when the points are
opened. Rapid metal transfer between the
point contacts is indicative of a bad conden-
sor.
The Breakerless Ignition
Systems (Including H.E.I.)
Both the transistorized and H.E.I, systems
operate in basically the same manner as the
conventional ignition system, with the excep-
tion of the type of "switching device" used.
As stated previously, the switching device of
a conventional ignition system is the breaker
point set. In the breakerless ignition sys-
tems, a toothed iron timer core is mounted
on the distributor shaft. The timer core is
mounted on the distributor shaft. The timer
core rotates inside of an electronic pole piece.
The pole piece has internal teeth (corre-
sponding to those on the timer core) and con-
tains a permanent magnet and pick-up coil
(not to be confused with the ignition coil). The
pole piece senses the magnetic field of the
timer core teeth and sends a signal to the ig-
nition module which electronically "calls the
shots" concerning control of the primary coil
voltage. The ignition coil operates in basi-
cally the same manner as a conventional ig-
