Ignition Systems page 5Ignition Systems page 5

Dwell AngleDwell Angle

Dwell angle is the amount of Dwell angle is the amount of degrees of rotation of the degrees of rotation of the

distributor cam that the contact distributor cam that the contact breaker points are ……………. breaker points are ……………. Closed

Out of the 360° of rotation of the Out of the 360° of rotation of the cam, a typical setting for a dwell cam, a typical setting for a dwell angle for a four cylinder engine angle for a four cylinder engine

is approximately ……….is approximately ……….50º

If a four cylinder vehicle had a If a four cylinder vehicle had a dwell angle of 50°, calculate the dwell angle of 50°, calculate the

amount of degrees in one amount of degrees in one rotation the points would be:rotation the points would be:

• Open 160º

• Closed 200º

Complete the drawing to show the Complete the drawing to show the dwell angledwell angle

Dwell angle (points closed)

How many cylinders does the How many cylinders does the engine below have?engine below have?

Four cams on the rotor, so the engine has four cylinders

Having too large an air gap at the Having too large an air gap at the contact breaker point will cause contact breaker point will cause

what?what?

• The dwell angle will be less than it should be, this gives weak sparks, poor performance and misfiring at high speed.

Having too small an air gap at Having too small an air gap at the contact breaker points will the contact breaker points will

cause what?cause what?

• Excess dwell angle, output of coil is poor, timing incorrect, causing misfire and bad starting.

Contact breaker points have many Contact breaker points have many disadvantages when compared to a disadvantages when compared to a modern electronic ignition system, modern electronic ignition system,

name three disadvantages.name three disadvantages.

• They wear out fairly quickly. • The primary voltage must be kept low, this

means the secondary output is low.• Timing doesn’t stay accurately set for long

periods.

Questions Questions

1. What is dwell angle?2. What is point ‘bounce’?3. Why don’t contact breaker points stay

accurately set for prolonged periods?4. If the gap at the points is too large, what

does this mean to the dwell angle?5. If a four cylinder engine had a dwell

angle of 56º, what is the total dwell angle for one rotation of the distributor?

Answers Answers

1. Dwell angle is the amount of time in degrees of distributor rotation that the points are CLOSED. (the coil is charging)

2. Point bounce is what happens to contact breaker points at high speeds.

3. Because they are a mechanical linkage, and wear out.

4. The dwell angle is reduced.5. 56 X 4 = 224º