Ported or manifold Vacuum to Dist.

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Mattax said:
I'll check my '67 FSM for that carb.
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'67 AFBs, Vacuum advance connection is pretty high.
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The spark port is about middle of the transfer slots, way above the idle discharge port.
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Be interesting to see if other versions were different.
 
Dana67Dart said:
In a relative nut shell...
Manifold vacuum will be high at closed throttle.
Ported will be high at open throttle.

Some older Ford's I have had had both going to the advance canister. And it kind of averaged the two.

The CAP valve does some thing with both to slow the timming return to no vacuum, IIRC. There is a CAP factory manual on imperial club web site again IIRC
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Ported vacuum has the vacuum inlet above the throttle plate, so with the throttle closed it sees no vacuum.
Manifold vacuum has the vacuum inlet below the throttle plate, so with the throttle closed it sees full vacuum.
As the throttle plate opens ported and manifold vacuum are the same.
Full open throttle, ported and manifold vacuum will be near zero.
Manifold vacuum is used to add ignition advance at idle. Adding advance at idle is helpful to stabilize idle quality and to prevent misfires at idle on engines that have cams with overlap that results in reversion and exhaust gases getting back into the intake charge. That exhaust and fresh charge is more difficult to light off and burns slower when lit, so on some engines and some cams, it is useful to add advance to the ignition at idle.
Off idle, as the throttle is opened ported and manifold vacuum are the exact same.
 
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I run ported with my street/strip 10.3:1 iron headed 340, 236@.050 Plugs stay clean, motor runs cool, and no rattle using 93 pump gas. With a good tune either source works.
 
toolmanmike said:
Crazy you mention that but my original HP273 AFB carb only has one vacuum port for the distributor. Manifold vacuum. I should snap a picture tomorrow.
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A TON of Thermoquads are like that.
 
my68barracuda said:
Ported vacuum has the vacuum inlet above the throttle plate, so with the throttle closed it sees no vacuum
Manifold vacuum has the vacuum inlet below the throttle plate, so with the throttle closed it sees full vacuum.
As the throttle plate opens ported and manifold vacuum are the same.
Full open throttle, ported and manifold vacuum will be near zero.
Manifold vacuum is used to add ignition advance at idle. Adding advance at idle is helpful to stabilize idle quality and prevent misfires at idle and engines that have cams with overlap that results in reversion and exhaust gases getting back into the intake charge. That exhaust and fresh charge is more difficult to light off and burns slower when lit, so on some engines and some cams, it is useful to add advance to the ignition at idle.
Off idle, as the throttle is opened ported and manifold vacuum are the exact same.
Click to expand...

Probably the best explanation on here. Thank you drive through.
 
is ported vacuum really zero at WOT? I thought velocity through the venturi where the port is located would produce a vaccum, ala Holley vacuum secondary carb? Looks like this type of port is higher in the venturi (choke point) where as these ported pickups are lower....maybe the placement is key? Seems you would get tip in vacuum then it would get stronger as velocity increased past the port? I guess graph proves this wrong as throttle at 90 (WOT) produces same if not slightly lower vacuum using a standard location port right above the throttle blade.
Holley velocity sensing port placed high at choke point of smallest venturi diameter"...Vacuum for the diaphragm is supplied initially from one of the primary venturis. As rpm increase, so does velocity through the primary venturi, resulting in a progressively stronger vacuum signal..."

vacuumchart-L.jpg
 
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carb? Looks like this type of port is higher in the venturi (choke point) where as these ported pickups are lower....maybe the placement is key? Seems you would get tip in vacuum then it would get stronger as velocity increased past the port? I guess graph proves this wrong as throttle at 90 (WOT) produces same if not slightly lower vacuum using a standard location port right above the throttle blade.
Holley velocity sensing port placed high at choke point of smallest venturi diameter"...Vacuum for the diaphragm is supplied initially from one of the primary venturis. As rpm increase, so does velocity through the primary venturi, resulting in a progressively stronger vacuum signal..."

View attachment 1715407383[/QUOTE]
pishta said:
is ported vacuum really zero at WOT? I thought velocity through the venturi where the port is located would produce a vaccum, ala Holley vacuum secondary carb? Looks like this type of port is higher in the venturi (choke point) where as these ported pickups are lower....maybe the placement is key? Seems you would get tip in vacuum then it would get stronger as velocity increased past the port? I guess graph proves this wrong as throttle at 90 (WOT) produces same if not slightly lower vacuum using a standard location port right above the throttle blade.
Holley velocity sensing port placed high at choke point of smallest venturi diameter"...Vacuum for the diaphragm is supplied initially from one of the primary venturis. As rpm increase, so does velocity through the primary venturi, resulting in a progressively stronger vacuum signal..."

View attachment 1715407383
Click to expand...
i agree that the placement of the vacuum inlet has a part in ported vacuum response signal. You raised a good point.
If the ported vacuum inlet was in the Venturi it would have an increasing vacuum signal with increasing throttle opening.
I was discussing a ported vacuum inlet directly above the throttle plate used to not engage vacuum advance at idle, and to engage VA at part throttle. So when discussing ported vacuum, it is necessary to know the placement of the vacuum port or the vacuum response plot.
 
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There is an additional port in the passenger side secondary to increase the signal even more once the secondaries open. Both are located just below the main venturi.
 
pishta said:
is ported vacuum really zero at WOT?
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It's actually the same as manifold which is close to zero on a 4 bbl, but even then not zero.
pishta said:
I thought velocity through the venturi where the port is located would produce a vaccum, ala Holley vacuum secondary carb? Looks like this type of port is higher in the venturi (choke point) where as these ported pickups are lower....maybe the placement is key?
Click to expand...
I think the other two guys covered this, so I'll just throw in some illustrations,
On the carbs we've been talking about, the timing port (for vacuum advance) is just above normal closed throttle position. Sometimes they pick up a little bit of vacuum at idle, but for sake of discussion, essentially closed off. So this vacuum is created by the throttle restriction on the air the cylinders are drawing - except at idle.
upload_2019-10-12_20-30-49.png


Throttle plate bottom view of a Holley 650 Vac Secondary 4150.
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Yes, vacuum secondaries are opened by air velocity in the main body. So they are dependent on the air flow, in other words, amount of air consumed. But not dependent on load.
Vacuum secondary passage on same Holley pictured above.
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In the larger venturi versions there's often a slash cut tube extending from the passage into the lower part of the venturi.
This acts like a pitpot tube, amplifying the vacuuum created by the air flowing past.
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upload_2019-10-12_20-59-43.png
 
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