Are you sure you have this right? Normally the two sources are carb and inlet
manifold. Inlet manifold vacuum is near maximum at idle, rises slightly as
the throttle starts to open simply because the engine gets more efficient,
then gradually tails away as the throttle is opened further. Carb gives zero
vacuum at idle, rapidly rising to a maximum as the throttle starts to open,
then is the same as inlet manifold i.e. gradually reducing to zero as the
throttle is opened further. How much vacuum you get *does* depend on engine
speed to some extent, but as engine speed is controlled by throttle opening
(except on the overrun) it's the throttle that controls vacuum, and the
reducing vacuum with increasing throttle is designed to offset the greater
tendency of an engine to pink under wider throttle openings by reducing the
amount of advance on wider throttle openings.
It's *centrifugal* advance that increases as engine speed increases, and that
takes advantage of the fact that as engine revs increase there is a *reduced*
tendency to pink, all other things being equal, so it can take more advance to
improve efficiency of combustion and hence performance.
Which vacuum source is used simply depends on the emissions requirements -
inlet manifold vacuum gives lower idle emissions as the engine runs more
efficiently with more advance. However it has a downside that it makes the
engine slightly more difficult to start as some advance is applied even during
cranking. Carb vacuum is usually preferred, if manifold vacuum were better
for other than emissions reasons it would have always been used.
PaulH.
----- Original Message -----
The passenger side vacuum port is ""timed" meaning vacuum increases with
engine speed.
The drivers side port is the "full" vacuum port meaning the vacuum decreases
as engine speed increases.
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