What is "dwell angle", and how do I measure it?
is a measure of the duration of time that the primary circuit of
the ignition system is closed to energize the primary windings of
the coil. It expressed (and measured) in degrees of rotation of
the distributor rotor, hence the use of the term "angle".
In actual operation, as the distributor mechanism rotates, the points
(or electronic module in electronic ignition systems) are closed
for a certain number of degrees of rotation, and open between these
points. Simply as a matter of interest, this means that the total
number of degrees during which the points are closed, plus the total
number of degrees that they are open, will equal 360 degrees.
In four cylinder engines, there is usually ample time for the primary
circuit to be open and closed (to energize the coil) four times
during each revolution of the distributor, which makes the dwell
value less critical than in 6 and 8 cylinder engines. In four cylinder
engines, there is more of a concern over having too much dwell time
(during which time the coil is energized) which can result in high
coil temperature and premature failure.
In electronic ignition systems, the dwell value is fixed at approximately
50 degrees. In conventional systems, (considering the difficulty
involved in checking and adjusting for dwell) most folks simply
rely on the setting of the point gap to provide the proper dwell
time. In the event that you want to check your dwell, here is a
reprint from the ignition chapter of our service and overhaul manual:
CHECKING FOR PROPER DWELL ANGLE
The dwell angle specified for the Atomic 4 is 31 to 34 degrees for
late models (Delco), and 38 degrees for early models (Prestolite).
To check for dwell angle, proceed as follows:
1) Connect the dwell meter as per its manufacturer's instructions:
Generally one of two leads is connected to the positive terminal
of the battery and the other lead to the negative terminal of the
2) Start the engine and read the dwell angle (making sure meter
is set on 4 cylinder).
3) Adjusting dwell on the Atomic 4 is somewhat of a trial and error
process. If dwell angle is too high (points closed for too many
degree of rotation), you will have to carefully reset your points
to a slightly larger gap. If the dwell angle is too low, the points
will need to be set slightly closer. Adjust the gap .002" or .003"
at a time for best control.
4) Restart the engine and re-measure dwell angle. Repeat as above
until dwell angle is correct. - Updated: November 4, 2003