The key to adding ported vacuum is the accuracy of the hole drilled for it.
It needs to be just above the throttle plates at idle speed so woln't pull any vacuum, but provide FULL manifold vacuum with very slightly movement of the plates. This will have it do EVERYTHING that a manifold vacuum source would do except no timing added at idle and coasting.
I cover the topic in my book and have put out PLENTY of threads on various Forums about setting up distributor advance curves and adding vacuum advance.
This is a seriously miss-understood topic. To make things worse folks continue to regurgitate old/outdated and inaccurate information on the subject.
Was just reading a thread on it recently and someone posted that they switched from manifold to ported vacuum and picked up a butt-load of power at full throttle.....HUM...last time I checked neither one does anything at full throttle as opening the throttle plates all the way sends vacuum to at or near zero.
I've read gobs of similar threads where folks post that using ported vacuum made their engines ping at heavy part throttle because it kept adding timing as the throttle angle increased. Another myth as at every source located under the throttle plates vacuum is pretty much the same regardless of throttle angle.
So the real deal is that once you uncover a ported source, especially a correctly located one required to run vacuum advance, any vacuum reading you get there will be the same as any other source under the throttle plates at that same time.......Cliff
