Todd Mullins wrote:
>
> > What Bob said:
> > Manifold vacuum is taken from the intake manifold, farily close to the
> > intake valves, thus insuring maximum available vacuum all the time.
>
> No, not all the time. Vacuum taken just behind the butterflies and
> vacuum taken in the middle of the manifold should net the same effect:
> In general, high vacuum with closed throttle and vice versa.
>
> > This
> > is to the benefit of vacuum operated accesories -- power brake booster
> > on LBCs.
>
> Wrong again. Brake boosters only need big vacuum during braking, which
> is exactly the moment at which vacuum is HIGH on a
> manifold/Bob-Allen-"ported" system. It's actually pretty neat. You get
> the most assistance during panic stops, when you're usually coming WAY
> down from speed.
Well, shoot, for awhile there we were dancing so well together!
I still contend that manifold vacuum tries to make maximum available
vacuum available to accesories. The example was power brake boosters. I
stand by my statement.
The vacuum feed for accessories has a one way valve just after the
manifold inlet. The purpose is to have the system 'store' vacuum until
needed. In the case of power brakes, you should have vacuum assist for
at least several minutes after the car has shut off. Without an
operational one-way valve, the power brakes will seem tempermental based
upon what the motor was doing just prior to using the brakes. Two-footed
drivers would be punished big time.
I don't believe that power brake boosters are engineered to take
advantage of instaneous vacuum levels in the manifold. (i.e. designed to
be sensitive to throttle position.)
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. 'Ported vacuum' which I believe is very near (but not
before) the throttle butterflys will vary according to throttle position
-- that ain't something the power brake booster wants to know about but,
to an earlier point, can be useful to the distributor.
Bob Allen, Kansas City, '69CGT, '75TR6
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