Todd Mullins wrote:
>
> Robert Allen writes:
>
> > The vacuum feed for accessories has a one way valve just after the
> > manifold inlet.
>
> Damn. I forgot about that.
>
> > When I said 'maximum available vacuum' the key was 'available'. I
> > believe that you'll find the most vacuum the closer you get to the
> > intake valves.
>
> I still don't see why this is necessarily true. Perhaps instantaneous
> vacuum levels are higher nearer the valves, but integrating over the
> time periods during which vacuum-actuated systems - uh - actuate, I
> would think the pressures would reach equilibrium. Does anybody here
> actually know?
>
> > 'Ported vacuum' which I believe is very near (but not
> > before) the throttle butterflys will vary according to throttle position
>
> So will manifold vacuum. Perhaps the average level of vacuum might be
> higher nearer the valves, but it still varies according to throttle.
I think we are pretty much in agreement and may have talked this through
to the point of splitting hairs.
If we believe that manifold vacuum is merely taken off a convenient
point from the manifold, fairly close to the valves, and that there is a
one-way valve whose purpose is to harvest vacuum whenever there is more
available on the engine side of the valve than the accessroies side:
then we may have the 'manifold vacuum' issue put to bed and we are in
agreement.
As for the ported vacuum, it is my understanding that its source is as
close to the throttle butterflys as possible such that, at idle, the
port is completely on the 'engine side' of the carb and, yes, ported
vacuum and manifold vacuum should read exactly the same.
However, as the throttle is opened, the butterflys begin to pass over
this ported source. At some small rotation of the butterfly, there is
still a large amount of manifold vacuum as the throttles are mostly
closed while the motor is trying to accelerate.
At this point, the ported vacuum source is now getting 'outside' the
engine as the throttle plates have passed over the opening. So now the
ported source sees more atmospheric pressure than manifold pressure and
the vacuum level subsides. This, then, can be exploited by the vacuum
advance distributor. I believe during these transitions, the 'ported
vacuum' and 'manifold vacuum' sources will see different pressure
values.
To the point: Manifold vacuum works to harvest all available vacuum
regardless of current engine parameters. Ported vacuum tries to 'read'
the engine speed and throttle opening to adjust ignition timing.
Bob Allen, Kansas City, '69CGT, '75TR6
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