FAQ: Ignition System
11. What is "dwell angle", and how do I measure it?

Dwell angle 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 coil.

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

 

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