DWELL ANGLE:

Dwell angle is the number of degrees the distributor cam turns during which time the breaker point are closed. During the dwell period, a magnetic field is built up in the primary winding of the coil. However, time is required to build up a full strength magnetic field. When a full strength magnetic field is produced, the coil is said to be “saturated”. To be assured of coil saturation at low engine speeds presents no problem because of the relatively slow rotation of the distributor cam. At high speeds, however, unless the distributor points are adjusted to provide a sufficient dwell period, coil saturation will not be attained.

In a six-cylinder engine running at idle speed of 400 rpm, the ignition system must produce 20 sparks per second to fire all of the cylinders. With an engine operating at this speed, dwell angle is not very critical because more than sufficient coil “saturation” time is available. However, with the engine running at 4000 rpm, or the equivalent of  90 miles per hour or more, 200 sparks per second would be required to fire all the cylinders. This is the speed at which dwell becomes extremely critical. If the dwell was reduced only slightly from the required amount, the engine would begin to misfire at high speed because the coil does not have time to becomes sufficiently “saturated”.

IGNITION DISTRIBUTOR

Although dwell is not critical at low speed, point gap becomes very important. With the engine cranking, there must be sufficient point gap or the points will arc excessively and the engine will not start readily. Also, if an engine is operated with too little point gap at low speed, the points will deteriorate rapidly. If the points open rapidly, and do not open wide enough, an arc will continue across the contact points using energy that would normally create a spark at one of the spark plug. When an arc occurs the engine usually misfires because the energy of the primary circuit is dissipated preventing sufficient secondary circuit voltage build-up.

Dwell angle directly affects ignition timing. Under certain conditions the rubbing block on the movable breaker arm may wear. As a result, the dwell angle increases which in turn causes the ignition timing to be late.

One of the largest single causes of breaker point failure is the lack of cam lubricant. Point rubbing block wear can be appreciably reduced by applying a thin film of high-temperature cam lubricant to the distributor cam when servicing the distributor. It is important that the proper lubricant be used since it must be able to adhere to the cam surface at high cam speed; resist melting at high temperature; resist chemical reaction with the polished steel cam; effectively control moisture to prevent rust formation on the cam; resist drying out with age. Ignition point spring tension plays an important part in the performance of the ignition system, and must be within specified limits. Excessive pressure causes rapid rubbing block and cam wear, while insufficient pressure will permit high-speed point bounce which in turn, will cause arcing and burning of the points and misfiring of the engine.

CONTACT POINT DWELL TEST