The dwell angle
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On all engines’ there is a figure for the cam dwell angle, the service manual shall have that figure.
The definition of dwell angle is the amount of time, measured as degrees of rotation that the contact breakers has been close in a distributor. A small dwell angle value indicates a shorter time where the contact breakers are closed compare to a large value.
The correct dwell angle is more important than the gap of the points.
The reason is that it is the dwell angle that controls the charging of the ignition coil.
If the charge will be too little the ignition coil will not be charged enough to give a sufficient spark at higher revolutions of the engine.
With the ignition coil that I’m using for the test the charge will look like this.
Note that the ignition coil has a resistor in series with the coil, the resistance is 1.6 ohm.
The oscilloscope is connected over the coil and the coil is fully charged when the voltage is 6 volt.
According to the figure it will take 4.4ms to charge the coil to 90%.
A 4-cylindric engine has typical 60 degrees of cam dwell angle when the points are closed, and the points are open 30 degrees. Totally it is 90 degrees for a 4-cylindric engine.
That means that the ignition coil will be charged 2/3 of the time.
At 5000 rpm the engine will fire 10 000 times. 2500 times for each cylinder.
Per second it will be 167 times. That gives 6 ms between each spark.
But the charge time of the ignition coil is 2/3 of the time and that gives a time of 4ms.
The ignition coil will be charged to 87% at 5000 rpm.
A 6-cylindric engine has typical a cam dwell angle of 35 degree when the points are closed, and they are open 25 degree. The total angle on a 6-cylinder engine is 60 degree.
That ends up that the points are closed 58% of the time.
At 5000 rpm the engine fires 15 000 times per minute or 250 per second, the time between the sparks will be 4 ms. The points are closed 58% of the time so that’s ends up to 2.3ms for charging the ignition coil. From the picture it will be 3.8V or 63%.
At 5000 rpm an 8-cylindric engine fires 20 000 times per minute or 333 times per second.
That’s end up to 3ms between each spark. To be able to charge the ignition coil 2ms the points need to closed 67% of the time.
This is a picture of a dwell measuring instrument with scales for 4, 6 and 8 cylinder engines.
Note that the way the counting of the angle are different depending of the number of cylinder the engine has. 45 degree on a 4-cylindric is the same as 30 on a 6-cylindric and 22.5 on an 8-cylindric engine. The higher value of the degree the longer time the points will be closed to charge the ignition coil.
If the dwell angle is adjusted the ignition timing need to be adjusted, changing the gap of the points affect the ignition timing but not the reverse.