Right
Passenger side port is timed (carb) drivers side port on carb is manifold
Sent from Bill's iPhone!
On Oct 19, 2008, at 8:18 AM, "Paul Hunt" <paul.hunt1@blueyonder.co.uk> wrote:
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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