Most people understand that if the car has a mechanical contact beaker then the contact breaker point's Gap has to be set. This gap in itself is not the important issue, it is the Dwell Angle that is effectively being adjusted.
Overview
The dwell angle is the number of degrees of rotation of the cam/distributor during which the points are closed. During each rotation of the cam/distributor, the points must open and close once for each cylinder. The points must stay closed long enough to allow the coil primary current to reach an acceptable value, and open long enough to discharge and produce a spark.
As the crankshaft rotates and pistons go through their cycles, the distributor's function is to precisely control the timing of the spark. Each cylinder's spark must occur at a precise time during the rotation of the engine. The distributor is driven off a gear on a camshaft or an idler pulley in the timing chain, such that it rotates at half the speed of the crankshaft. The drive method is dependant on engine manufacturers. The engine makes two complete revolutions in the same time that the distributor makes one 360 degree revolution.
Functionality
With the primary circuit being switched on and off repeatedly each coil has to be designed for a particular application so that it operates efficiently.
For a four stroke 4 cylinder engine running at 2000 RPM, 4000 sparks must be supplied every minute. The time available to make and break the primary circuit each time is very short. As engine speed rises, the time available is even shorter. It is very important then to ensure that the length of time current flows through the primary winding is sufficient to create the necessary magnetic field.
In contact breaker systems this brief period of time is the period during which the contacts are closed. This is referred to as the “dwell angle”. It is usually specified in degrees of distributor rotation.
This closed period is influenced by the setting of the contact breaker gap. Once this has been set, the angle remains fixed regardless of engine speed.
A large gap gives a small dwell angle.
A small gap gives a large dwell angle.
The manufacturer's recommended gap provides the specified dwell angle for each application and those recommendations should be followed.
Summary
The term dwell angle is not use in today's ignition system since in an electronic circuit there is no angle. Instead the time the primary circuit is charging is basically referred to as "charging time" or in some instances, "closed time".
Source: CDX Global & Wikipedia - en.wikipedia.org
