Ported or manifold Vacuum to Dist.
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.
Throttle plate bottom view of a Holley 650 Vac Secondary 4150.
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.
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.
It's actually the same as manifold which is close to zero on a 4 bbl, but even then not zero.
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.
Throttle plate bottom view of a Holley 650 Vac Secondary 4150.
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.
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.