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.
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
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.