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.