Ignition timing - why do we need to advance the spark?

Almost all distributors that we meet have a set of bob weights and springs as well as a vacuum operated advance mechanism, but it may not clear why it is needed. Nic Houslip admits it was somewhat of a mystery to him until he sat down one day and thought about it.

In his two page article he explains that as the air-fuel mixture is compressed by the rising piston it is ready to be ignited. A spark across the points of the plug will do this very nicely, but the flame that it starts takes a finite, although very small, amount of time to propagate throughout all the compressed mixture. To ensure that the maximum burning rate and gas production occurs, it is necessary to fire the spark a short
while before TDC (Top Dead Centre) so that the resulting build up of gas pressure can push the piston downwards and turn the crankshaft.

Contact breaker distributors have another adjustment that electronic distributors do not have, that is also critical, it is called setting the dwell angle or dwell time. This adjusts the relative position of the contact breaker points and the cam that drives them to set the optimum closed time. The note describes how to check the dwell angle. With the original distributor fitted to the V8 engine in the Factory MGBGTV8, the dwell angle can be adjusted to 26 to 28 degrees whilst the engine is running using a dwell meter. On the side of the distributor there is a convenient hexagon headed screw on the distributor body - turn clockwise to reduce the dwell angle and anticlockwise to increase it. See the full note as V8NOTE543.
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