Whooeee!
Once again the list fails the questioner! The guy asked for a
treatise on what vacuum advance is and how it works. He received no
coherent answer that I saw on the list. I hope someone answered him
backchannel.
The answer is that vacuum advance attempts to modify the mechanical
spark advance as a function of engine load, detected by measuring the
intake system vacuum level. When intake vacuum drops it retards the
spark. When vacuum rises it advances it.
The idea is that the mechanical advance system detects engine speed,
the vacuum diaphragm detects engine load. Between them they tailor the
spark advance for the best (hopefully) approximation to the engine's
needs.
In later model engines both methods, along with VA disabling schemes,
were used to lessen the engine's pollution output, often at the cost of
lowering power output.
Charley Robinson
'69 B
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