[PeteF]

There are two schools of thought on this, but this is the way I understand it. Pre-emission engines (like these), used manifold vacuum. This gives you full vacuum advance at idle, but no vacuum advance under hard acceleration (load), where the centrifugal advance takes over. This provides more advance at idle and cruise for improved mileage (more complete burning of a lean mixture), while protecting the engine from detonation under acceleration. Typical setting would be 10 deg initial, 24 deg mechanical(in the distributor) and 15 deg vacuum advance. The maximum advance would not exceed 10+24 = 34 deg under load. When emissions became mandated, vacuum systems went to ported. This reduced emissions (NOx I think), but if you run this way, you have to adjust your settings to prevent too much advance under load, because the ported vacuum provides advance with venturi velocity. If you run 10 initial+ 24 mechanical+15 vacuum under load = 49 deg total, which may drive you into detonation. Therefore, you would reset to either less initial timing or recurve the distributor to have less mechanical advance. A third option (what I did) is to remove the vac advance and run 10 initial + 24 mechanical only. Mid 60s high performance Fords ran centrifugal only/no vacuum. You give up 2-3 MPG, but have a simpler, straightforward advance mechanism where you get advance proportional to rpm only. Either way will work with the proper settings. And this will surely bring on more comments and opinions. You can Google the subject and learn more, but the differences of opinion are there too. Good luck