Those vacuum modulator valves kind of provided a "best of both worlds" sort of function with the vacuum advance. It's the only emissions device I still use on my '79 W72 besides the DS-TVS that it works in conjunction with. The valves are calibrated to meter exactly 9" of MANIFOLD vacuum to the vacuum advance, which for those late 70's cans that added a ton of timing (as high as 25 degrees all in by as little as 11-12" of vacuum for some models), added around 12-17 degrees of timing at idle on top of the base timing setting. Then, once the PORT vacuum signal exceeds that calibrated 9", the vacuum to the vacuum advance is equal to whatever the port vacuum is. So basically, the vacuum advance gets 9" of manifold vacuum at all times, and then gets port vacuum past 9" - it ultimately still ends up the same, either way, since port vacuum is no different from manifold vacuum once the throttle plates open up wide enough. You can see for yourself by hooking up two vacuum gauges to a manifold port and a metered port - rev up the engine and eventually they will give identical readings.
Anyway, whether you use port or manifold vacuum for your engine depends more on your needs. If you have a big cam with a tight LSA that causes mondo overlap, you'll need manifold vacuum to the vacuum advance so you can have that additional timing at idle so that it can idle smoothly, and it will also idle cooler engine temp-wise. There are ways around this such as richening your carburetor (which is challenging to do for the idle circuit on a Quadrajet, assuming that's what you're still using) or increasing your base timing a lot (which can cause hot start issues), but most people get around it by just hooking the vacuum advance to manifold vacuum. The only difference between the two is that port vacuum doesn't give you any advance at idle. Port vacuum was originally implemented to ensure a consistently smooth idle and reduce NOX emissions since they were coupled with very low base timing to ensure increased exhaust temperatures, which when combined with the A.I.R. pumps, reduced NOX emissions.
So again, if you're trying to 100% emulate stock operation, then hook to a ported source. But if you have a camshaft, deleted emissions equipment, or even bolt-ons, you'll need to make adjustments to your engine. Here are some common ones...
• Headers/free-flowing exhaust: Car will run leaner due to the increased scavenging effect. Up the carb two or three jet sizes.
• Deleted EGR: Richen carb and reduce your cruise timing 4-6 degrees. EGR actually reduces combustion temps with the introduction of inert gases into the combustion chamber, which is one reason stock vacuum advance cans could get away with throwing in so much timing on some of the later 70's cars. Take away those reduced combustion temps, then you increase likelihood of pinging unless you drop your cruise timing. It's as simple as adding a vacuum advance limiter plate. I deleted my EGR and installed a limiter plate on my original W72 vacuum advance. Instead of adding in 25 degrees of advance by 11", it now only adds 19. Still a lot when considering I use stock base timing (18 degrees), but then I have very heavy advance springs just like stock and so my centrifugal curve is very gradual and only adds around 18 degrees by near redline, by which point vacuum advance is usually out of the question, anyway since when the throttle is nearly wide or wide open, there is no engine vacuum. Remember, engine vacuum is a measurement of the negative pressure *RESISTANCE* caused by the closed throttle plates. When the throttle plates are vertical (i.e. open all the way), there is no more resistance and the pressure differential equalizes between inside the engine and the atmosphere... in other words, no more vacuum.
• High LSA camshaft: That lumpy idle is great, but you need to make changes to accommodate it. Increasing timing is the age-old way to do it since it's easy. Hook to a manifold vacuum source for the vacuum advance. The only issue is, you need to make sure your vacuum advance can is designed to be fully deployed at a vacuum signal LOWER than the vacuum your camshaft is able to provide, or else you'll have a fluctuating idle (this is one of the reasons ported vacuum was introduced). The other way to accommodate a fat camshaft is to richen your carburetor, otherwise you'll have to open up the throttle plates so wide by turning in the idle speed screw that you'll be idling off the primary metering circuit rather than the idle circuit. This is a big issue with Quadrajets since they're emissions carburetors and by design are meant to meter very lean mixtures. The modifications to a quadrajet to handle a big cam are fairly invasive.
If you've already deleted emissions equipment, it's safe to assume you have other mods done. You basically want to turn your '78 400 into a '70 400. Up your carb jet sizes, hook to manifold vacuum, ease down on the amount of total vacuum advance for when you're under load to prevent pinging, and have fun.
