Bob MGT wrote:
> When one puts the pedal to the metal, does the manifold vacuum act to retard
> or advance the timing? Is the manifold vacuum higher or lower with the
> throttle open wide? Anybody know of a good book on this basic stuff?
This can open up a can of worms.
Generally speaking, vacuum advance will advance the timing thus "putting the
pedal to the metal" will cause the vacuum advance to retard the timing such that
only mechanical advance (the swinging plates underneath the distributor points
plate) will affect timing.
However, a couple of things mess with the traditional operation of vacuum
advance. A biggie is the SU "Constant Depression" carbs that try to always
maintain a constant vacuum in the manifold. Because of this, the source for the
vacuum was often from a port in proximity to the throttle butterfly (plate).
SU designs differed over the years. The vacuum port on some carbs comes in below
the carb casting such that the throttle plate tries to cover the hole at idle.
Some of them are positioned such that the port is ahead of where the throttle
plate closes and effects operation on coast down. Many American carb designs
(non CD) have simple ported vacuum just past the throttle plate. Finally, some
simply use manifold vacuum as the port comes off the top of the log manifold
(1275 Midgets).
So, anyway, vacuum advance will advance the timing when a vacuum is present but
how often it is present can differ based upon the intake design.
To confuse matters, some Zenith designs use vacuum retard, which sounds pretty
strange and it becomes very dependent on the location of the vacuum port. And,
finally, some dizzies had both vacuum advance and retard on the same distributor
to deal with emissions and other magic.
So you have to look over your car and the manual very carefully to figure out
what is right plus to determine if some DPO bodge changed it from the factory
specs.
--
Bob Allen, Kansas City, '69C/GT and other, assembled, cars
Fill in the blank: "Sometimes the best thing about a <blank> is the five minutes
of silence."
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