Vacuum Advance - Manifold or Ported

[SuperHart]

Question to EXCALIBER. First, I appreciate your detailed discussion. Everything you mention makes perfect sense. I am still not certain about which port to use. I run a 351W with dual 390 cfm #8007 Holleys, a smooth RV cam and the motor makes 19" of vacuum at 900 RPM idle. I installed an Edelbrock adjustable vacuum advance module on the distributor and set initial timing to 10* with the vacuum hose off and plugged. The Edelbrock instructions said to connect the vacuum module to ported vacuum on the side of the carb. When I contacted Ford they suggested connecting to the manifold (direct) vacuum port so I would have maximum vacuum advance at idle. They said that doing it that way creates a cooler running engine at idle and reduces NOX emissions. Ford also said that is the reason why you should disconnect and plug the vacuum line when setting initial timing to remove the vacuum advance module from the equation.

Now, it seems to me that if I use direct manifold vacuum the signal is greatest at idle or when the throttle plates are closed and will give me vacuum advance at those times. If I use ported vacuum the vacuum advance will increase as engine RPMs increase because ported vacuum increases as engine air flow increases through the carb. This will give me vacuum advance along with mechanical advance as engine RPMs increase. I do know that as engine RPMs increase the engine likes more advance timing to make power but initial timing of 10* plus mechanical advance of 26* plus vacuum advance of another 22* (Edelbrock factory setting for the vacuum module) seems like an awful lot. Some "experts" have even suggested that I really don't need vacuum advance and I should just disconnect it and run only mechanical plus initial.

I hope I'm not being too dense here but I find all this confusing. Maybe I should just stick to what I know.