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LP Engine G430 (3.0L)
Igntion System
For the ignition coil to put out maximum secondary
voltage, the maximum primary current must be
reached before the primary circuit is broken. In the
breaker point system the length of time the primary
current is "ON," is controlled by the speed of the
breaker cam. This period is called "dwell angle" and is
given in the number of degrees of distributor shaft
rotation. Most V-8 engines have a dwell angle of 30
degrees before the points open and fire the cylinder.
This dwell angle remains the same regardless of
engine speed but as engine speed increases, the time
the points are closed decreases. This causes
available voltage and coil energy to decrease as
engine speed increases due to the reduced saturation
time of the ignition coil.
When the contact points close, the current does not
instantaneously reach a value of 4 amperes but it
takes several milliseconds for this value to be reached
(figure 6-6). At 1000 engine RPMs, the distributor shaft
rotates once every 0.12 second. Of this time, the
points are closed for 0.010 second or 10 ms for every
cylinder of an eight cylinder engine. This is sufficient
time for the primary current to build up to its maximum
current of just over 4 amperes.
When engine speed is increased to 2000 RPM, the
time the points are closed for each plug firing is
reduced to 5 ms. A duration of 5 ms allows the primary
current to build to 3.8 amperes. At an engine speed of
3000 RPM, the duration time drops to 3.3 ms and the
current to 3.2 amperes.
Two features were incorporated in the HEI system to
give it a higher energy level at higher engine speeds.
First, by decreasing the resistance in the primary
circuit, the time needed for the current to reach its
maximum is greatly reduced. It takes 10 ms for the
current to reach maximum in a coil that has a resis-
tance of 2.6 ohms. In the HEI system, the primary
winding has a resistance of 0.5 ohms. This allows full
current to be reached in about 3.4 ms.
Dwell Control Circuit
Figure 6-6 Current vs. Time
131-062
