The distributor is separated into three sections: the upper, middle,
and lower. In the middle section, the corners of the spinning breaker cam
strike the breaker arm and separate the points some 160 miles an hour.
(standard ignition) High-voltage surges generated by the action of the
coil travel to the rotor that whirls inside a circle of high-tension terminals
in the distributor cap. At each terminal, current is transferred to wires
that lead to the spark plugs. Two other devices - the vacuum advance and
the centrifugal advance - precisely coordinate the functions of the points
and the rotor assembly as the requirements of the engine vary.
An ignition circuit consists of two sub-circuits: the primary, which
carries low voltage; and the secondary, which carries high voltage. The
primary circuit, controlled by the ignition key, releases 12 volts of electricity
from the battery or alternator through the coil to a set of breaker points
in the lower part of the distributor, or to the relay in electronic ignition
applications. When the points or relay are closed, current flows through
the chassis back to the battery, completing the circuit. When the points
or relay are open, the flow stops, causing a high-voltage surge to pass
from the coil through a rotor in the top of the distributor to the spark
plugs. Once the car has started, the voltage regulator protects the battery
from being overcharged by the alternator. The condenser absorbs part of
the low-voltage current when the points are open.
Changing to a "hotter" ignition system, or components, from a properly
adjusted and operating factory HEI system will not normally help performance in
any street/strip engine running less than 6000 RPM. A factory points system relies
on the points to furnish adequate current (power) to the coil to develop suitable
high voltage. Worn or mis-adjusted points, a defective condenser, a faulty ballast
circuit, or a defective coil will lower the available high voltage. Most aftermarket
ignition amplifiers are not dependent on the points to furnish the power for the
coil, and may allow a marginal points system to develop more spark, which in turn,
will allow the plug gap to be widened to .045-.050. A race engine with a wide
overlap cam and high RPM manifold may start and run better at low RPM with an
ignition system featuring increased spark duration and/or multiple sparks at
lower RPM. However, on all normal engines, only about 15,000-20,000 volts is
required to fire the spark plug gap, and while it is firing, the voltage will
never increase above that value. Correctly operating HEI and points system will
easily develop in excess of 25,000 volts to 5500 RPM, and the HEI will maintain
that level of voltage past 6000 RPM. When the plug initially fires, a small flame
kernel is developed at the spark. If a normal fuel mixture is available in the
chamber, the flame expands away from the spark, and the spark has no further
effect on that cylinder firing, regardless of the spark power or duration. The
most criticl aspect of any ignition system is to make it fire at the optimum
time throughout the RPM range in use. See next paragraph for timing information.
An aftermarket ignition advance kit will not automatically help performance.
Most of the lower cost kits actually degrade the advance curve. The desired
curve must be determined with a distributor machine, or on the engine using accurate
timing lights and properly located timing marks. The optimum total mechanical timing
(initial timing plus the mechanical advance provided by the springs and weights) is
the most critical aspect, and must be found by trial and error on each vehicle.
Vacuum advance has absolutely no effect on full throttle performance, because the
vacuum unit retards to zero when the vacuum drops below about 5 to 7 inches, and
most engines at full throttle have about 2 inches or less. However, vacuum advance
is desirable for improved drivability and gas mileage.
Changing to a cold heat range spark plug does not help performance on a normal
street/strip car, and very likely will degrade the idle. The heat range of a spark
plug is designed to keep the firing tips clean under all anticipated service in a
specific application. If the range is too hot, the tips may melt/erode under
extreme service, and if too cold, the tips will not be able to burn away the
deposits, thus causing misfire. Heat range has absolutely nothing to do with
the ability to conduct the spark across the tip. For race engines that develop
very high heat in the combustion chambers, a colder plug is needed to prevent
possible pre-ignition caused by the plug electrodes over heating. Again, the
colder plug by itself does not increase performance. Use the plug that is
recommended by your favorite plug manufacturer for your vehicle.
Computerized and Electronic Ignition
In an electronic ignition, a rotating reluctor and magnetic-pickup coil
replace the traditional cam, breaker points and condenser in the distributors
of cars equipped for electronic ignition. This system reduces the time
between tune-ups. The high spots of the reluctor interrupt the magnetic
field of the pickup coil and the permanent magnet. These interruptions,
or pulses, are transmitted from the pickup to a nearby electronic control
unit. There, the pulses signal a transistor to break the low-voltage sub-circuit
and release high voltage from the coil to the spark plugs.
The short-lived electronic ignition system was a transition from the
points and condenser system to the computerized ignition system. It came
into widespread use in the mid-1970s, but there are still a few engines
that use electronic ignition.
The starter circuit is activated when the ignition switch is turned
on. This opens a second switch in the solenoid, permitting a second flow
of electricity from the battery to the starter motor.
The engine cranking circuit is made up of a battery, starting motor,
ignition switch, and electrical wiring. When the ignition switch is placed
in the "start" position, the solenoid windings are energized and the resulting
shift lever movement causes the drive pinion gear to engage the flywheel
ring gear, and cranking takes place. When the engine starts, an overrunning
clutch (part of the drive assembly) protects the armature from too much
speed until the switch is opened. At this time, a return spring causes
the pinion gear to disengage from the flywheel.
A spark plug is a device, inserted into the combustion chamber of an engine,
containing a side electrode and insulated center electrode spaced to provide
a gap for firing an electrical spark to ignite air-fuel mixtures.
The high-voltage burst from the coil via the distributor
is received at the spark plug's terminal and conducted down a center electrode
protected by a porcelain insulator. At the bottom of the plug, which projects
into the cylinder, the voltage must be powerful enough to jump a gap between
the center and side electrodes through a thick atmosphere of fuel mixture.
When the spark bridges the gap, it ignites the fuel in the cylinder.
Spark Plug Wear
The spark plugs ignite the fuel mixture in the cylinders
by means of a burst of high-voltage electricity carried from the distributor.
The ability of the spark to ignite the fuel is badly affected if the plugs
are damaged or the spark gaps are abnormal. It is therefore important to
examine used spark plugs closely and to clean them periodically. The gaps
of old and new plugs should also be checked before installing them. There
are three basic types of spark plug fouling: "carbon" fouling, "high speed"
or "lead" fouling, and "oil/carbon" fouling.
Carbon fouling is caused from low-speed operation or a
fuel mixture that is too rich. It causes missing or roughness and creates
soft black soot that is easily removed. Lead fouling is caused by tetraethyl
lead used in some fuels and by extended high speed operation. Lead compounds
which are added to the gasoline have a bad effect on some spark plug insulators.
At high temperatures, it is a good conductor and may give good results
under light loads, but often fails under full loads and high combustion
temperatures. In some cases, it is possible to run the engine at a speed
just below the point where missing will occur; then, increase the speed
(always keeping below the missing speed) to burn off the lead fouling.
Lead fouling appears as a heavy, crusty formation, or as tiny globules.
The third type of fouling is found on engines that are
so badly worn that excess oil reaches the combustion chamber past the piston
ring, or the valve guides.
In all cases of fouling or wear, it is best to replace
the plugs. To avoid having to replace plugs one at a time as they wear
out, always replace the entire set, even though only one plug may be bad.
Plugs should normally be replaced about every 12,000 miles.
Spark Plug Wires
The spark plug wire carries 20,000 or more volts from the distributor
cap to the spark plug. Spark plug wires are made of various layers of materials.
The fiber core, inside the spark plug wire carries the high voltage. The
older design of spark plug wires used a metallic wire to carry the high
voltage. This caused electrical interference with the radio and TV reception.
Some spark plug wires have a locking connection at the distributor cap.
The distributor cap must first be removed and the terminals be squeezed
together, and then the spark plug wire can be removed from the distributor
To reduce interference with radio and TV reception, ignition systems
are provided with resistance in the secondary circuit. Resistor spark plugs
or special resistor type ignition cable may be used.
To work effectively in modern ignition systems, it is important that
the resistor ignition cable is capable of producing a specifically designed
resistance. The cable must also have enough insulation so that it can withstand
heat, cold, moisture, oil, grease, and chafing. High tension electricity
passing through a cable builds up a surrounding electrical field. The electrical
field frees oxygen in the surrounding air to form ozone, which will attach
to the rubber insulation if it is not properly protected. Ozone causes
the rubber to deteriorate and lose its insulating qualities. Electrical
losses will seriously weaken the spark at the plug gap.
As the rotor rotates inside the cap, it receives the high voltage from
the ignition coil, then passes it to the nearest connection, which is a
metal projection in the cap, which is connected to a spark plug.
The distributor cap should be checked to see that the sparks have not
been arcing from point to point within the cap. The inside of the cap must
be clean. The firing points should not be eroded, and the inside of the
towers must be clean and free from corrosion.
A distributor rotor is designed to rotate and distribute the high tension
current to the towers of the distributor cap. The firing end of the rotor,
from which the high tension spark jumps to each of the cap terminals, should
not be worn. Any wear will result in resistance to the high tension spark.
The rotor with a worn firing end will have to be replaced.
Rotors are mounted on the upper end of the distributor shaft. In this
connection, the rotor must have a snug fit on the end of the shaft. On
another design, two screws are used to attach the rotor to a plate on the
top of the distributor shaft. Built-in locators on the rotor, and holes
in the plate, insure correct reassembly. One locator is round; the other
The rotor is driven directly by the camshaft, but is "advanced" (turned)
by the centrifugal advance mechanism. Advancing the spark timing allows
the engine to run efficiently. A vacuum advance is also fitted on some
cars for the same reason.
The coil is a compact, electrical transformer that boosts the battery's
12 volts to as high as 20,000 volts. The incoming 12 volts of electricity
pass through a primary winding of about 200 turns of copper wire that raises
the power to about 250 volts. Inside the distributor, this low-voltage
circuit is continuously broken by the opening and closing of the points,
each interruption causing a breakdown in the coil's electromagnetic field.
Each time the field collapses, a surge of electricity passes to a secondary
winding made up of more than a mile of hair-like wire twisted into 25,000
turns. At this point, the current is boosted to the high voltage needed
for ignition and is then relayed to the rotor.
Primary current produces a magnetic field around the coil windings.
This does not occur instantly, because it takes time for the current and
the magnetic field to reach maximum value. The time element is determined
by the resistance of the coil winding or the length of time the distributor
contacts are closed. The current does not reach the maximum because the
contacts remain closed for such a short time, and more so at higher engine
speeds. When the breaker points begin to open, the primary current will
continue to flow. This condition in a winding is increased by means of
the iron core. Without an ignition condenser, the induced voltage causing
this flow of current would create an arc across the contact points and
the magnetic energy would be consumed in this arc. As a result, the contact
points would be burned and ignition would not occur. The "condenser" prevents
the arc by making a place for the current to flow. As a result of condenser
action, the magnetic field produced and continued by the current flow will
quickly collapse. It is the rapid cutting out of magnetic field that induces
high voltage in the secondary windings. So, if the condenser should go
bad, the high voltage needed to jump the gap at the spark plugs will not
be possible. This could cause a no-start condition or a driving problem.
Breaker Point (Standard) Ignition
The ignition distributor makes and breaks the primary ignition circuit.
It also distributes high tension current to the proper spark plug at the
correct time. The distributor is driven at one half crankshaft speed on
four cycle engines. It is driven by the camshaft. Distributor construction
varies with the manufacturers, but the standard model is made of a housing
into which the distributor shaft and centrifugal weight assembly are fitted
with bearings. In most cases, these bearings are bronze bushings.
In standard ignition, the contact set is attached to the movable breaker
plate. A vacuum advance unit attached to the distributor housing is mounted
under the breaker plate. The rotor covers the centrifugal advance mechanism,
which consists of a cam actuated by two centrifugal weights. As the breaker
cam rotates, each lobe passes under the rubbing block, causing the breaker
points to open. Since the points are in series with the primary winding
of the ignition coil, current will pass through that circuit when the points
close. When the points open, the magnetic field in the coil collapses and
a high tension voltage is induced in the secondary windings of the coil
by the movement of the magnetic field through the secondary windings.
The design is to provide one lobe on the breaker cam for each cylinder
of the engine; i.e., a six cylinder engine will have a six lobe cam in
the distributor; and an eight cylinder engine will have an eight lobe cam,
so every revolution of the breaker came will produce one spark for each
cylinder of the engine. However, on a four cycle engine, each cylinder
fires every other revolution so the distributor shaft must revolve at one
half crankshaft speed. After the high tension surge is produced in the
ignition coil by the opening of the breaker points, the current passes
from the coil to the center terminal of the distributor cap. From there,
it passes down to the rotor mounted on the distributor shaft and revolves
with it. The current passes along the rotor, and jumps the tiny gap to
the cap electrode under which the rotor is positioned at that instant.
This cap electrode is connected by high tension wiring to the spark plug.
As the rotor continues to rotate, it distributes current to each of the
cap terminals in turn.