Join Date: Jan 2019
Location: Sedgwick, Kansas
Think of it this way: The springs and limiters (and direction you have the dizzy clocked) come together to provide Mechanical advance. It only cares about how fast the engine's spinning.
Vac advance always seemed a bit backwards to me, when I was first trying to understand it. "Wait, when the engine's flat-out, it does nothing?"
But basically, that's it. Ignore vac advance, mostly, for setting your timing. Make sure it's disconnected, and that the hose (or vac nipple) is plugged so you have no leak. Vac advance is only an accessory. It does have a profound effect on how smoothly your engine idles, and on your gas mileage, but it will never affect full-throttle operation, and it won't keep your motor from running.
Ported vacuum (the weirdo port on your carb that takes off just above your closed throttle plates) is confusing to a lot of people. It has zero vacuum with the blades closed, but kicks up to full manifold vacuum as soon as they begin to open. A lot of 70s smog engines use this to keep the exhaust hot and make the catalytic converters function better. (Genius! Let's dump more unburnt fuel out the tailpipe in order to reduce emissions!) It's also useful for some cars with a lot of overlap on the cam, because the fluctuations in manifold vacuum can be strong enough to cause surging and stalling from the vac advance while it's just trying to idle.
There are a lot of very knowledgeable people that swear you should always use ported vacuum. It's an ongoing argument that may never be resolved. But to my thinking, it makes no sense to have "zero vacuum" and adjust idle to be perfect running on straight mechanical advance, then transition to a whole bunch of vacuum advance for cruise timing. Such a situation would work fine at full throttle, but not for everyday driving. I believe the best compromise you'll ever get with ported vacuum is an idle that's a bit retarded, and cruise that's probably just a bit over-advanced.
For most cars straight up manifold vacuum works best. If you have it all set right, what works best at idle will also work best at low-throttle cruise, giving you smooth idle, crisp throttle response, and best possible mileage. Under full throttle, the additional advance 'goes away', leaving you with a base mechanical curve that won't ping or detonate, allowing best possible throttle response, safely.