Robert Allen writes:
> Barney Gaylord wrote:
> > Ported vacuum is low at idle, comes on strong on acceleration at low
> > speeds, then increases slightly with engine speed. Manifold vacuum is
> > high at idle, drops dramatically under hard acceleration, climbs again at
> > higher RPMs, and hits the highest when you close the throttle. Ported
> > vacuum with the vac-advance dizzy and manifold vacuum with the vac-retard
> > dizzy have roughly the same effect. Spark advances when you hit the gas.
> > As I recall, the primary reason for using the manifold vacuum retard
> > setup is to retard the timing dramatically on overrun to prevent
> > backfire.
>
> Hmmm. I was thinking just the opposite. Perhaps I have my definitions
> screwed up. Ported vacuum is usually taken just behind the throttle
> plates of normal carbs. Vacuum only present at part throttle openings.
> Thus *high* at idle, moderate at cruise, and negligible at full
> throttle.
I'm with you, Bob. It sounds to me like Barney's thinking of "ported"
vacuum as vacuum taken just in FRONT of the butterflies, which would be
exactly opposite of manifold vacuum. It also seems to me that your idea
of "ported" vacuum (just behind the butterflies) and my idea of manifold
vacuum would result in largely the same effect at the dizzy, as you said
"*high* at idle, moderate at cruise, and negligible at full throttle".
In fact, if you reverse the words "advance" and "retard" in Barney's
paragraph above, you arrive at my current understanding of the system.
> Somebody figured out that, as long as the engine isn't being stressed
> (low throttle openings) the engine can benefit from additional advance
> thus vacuum advance from a ported source.
Sure thing. When you're hard on the gas, you want the timing backed off
a bit to prevent pinging.
> 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.
Also note that my '84 Volvo gives the brake booster its vacuum from
directly aft of the throttle body (Bob-Allen-"ported" style). It works
extremely well. The airflow redirection flaps (well, whatever they're
called - you know, the buttons that send air to the floor, to the
windscreen, and recirculate) also get vacuum from behind the throttle
plate, but they use a vacuum reservoir for low-vacuum conditions.
--
Todd Mullins
Todd.Mullins@nrlssc.navy.mil On the lovely Mississippi (USA) Coast
'74 MGB Tourer in pieces, like me
'84 Volvo 245GL B23F M46
(witty quote here)
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