Hi Ryan,
I see that Frank has answered most of your Q's, so I'll just clarify a
bit.
High vacuum (pressure lower than atmospheric) in the intake manifold
advances the spark in addition to the mechanical advance, which is
RPM-related. When you suddenly open the throttle to accelerate, the vac
rises to near atmospheric and the spark is retarded until the engine
starts catching up to the throttle setting. One thing you get out of
this is that you can run more spark advance without it pinging at when
you lug it. Also can get away with lower octane fuel, within reason.
Highest vac is when coasting down a hill in gear. Kind of in between
is steady speed on the road. The vac unit tries to suit the amount of
spark advance to the engine's load and mechanical advance unit tries to
suit it to the RPM. So to suit the advance to both engine load and RPM
you need both units working.
Merry Christmas,
CR
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