The vacuum that opens the secondaries on a 4 barrel or Six pack comes from a hole just below the start of the venturi, not down by the throttle plates. If you have a Holley its kind of inline with the boosters. You have to look hard to see it. Once the plates open a certain amount the ported vaccum goes away. I run my VA off manifold vacuum, the main benefeit is the advanced timing @ idle. My 440 makes about 16" @ 900 rpm with about 25 degrees advance. Sent my dist to Don @ FBO who beleives maifold vacuum is correct. You can't just plug it in one or the other and hope one works better, it has to be setup properly.