Okay, I think I am beginning to "get it." Those articles are really good - thanks for sharing and insight. I will put off reading those other threads for now as they would probably mess to much with my head. I think I now know that using the manifold vacuum is not necessarily a "bad" thing. Do you think Edelbrock indicates to use ported vacuum so that they do not run afoul of the EPA? Classic cars here in Illinois are exempt from emission testing.
So here are my takeaways, comments and questions.
1) A lot of advance timing is required at idle. Much more than just 6*BTDC. And this is why folks really bump up their initial timing anywhere (from what I have read in threads here) all the way up to 20* BTDC. I assume that these people are using ported vacuum. And, at idle there is no ported vacuum so it does not matter if they disconnect and plug the hose.
2) When you are using ported vacuum and have a high initial timing of say 15* BTDC, when you accelerate, you get the initial + Vacuum + increasing centrifugal advance which will give you a whole lot of total timing advance. This would seem to give you more than needed or desired.
3) If you use manifold vacuum for advance and you set your initial timing to 6* BTDC (with the vacuum hose disconnected and plugged) you will wind up with your idle timing at about 20* BTDC.
4) Given the scenario 3 above, this would allow for (and the engine would like) a leaner idle mixture adjustment. So, maybe ~two turns out versus my current of 4 plus turns out).
5) In 1968, Ford used the dual diaphragm advance. I am using just a single advance. Does this really change anything in the conversation?
6) With my vacuum currently using ported and my initial at about 11 degrees, the car accelerates and cruises very nicely with no pinging. But, a smooth idle is a little elusive and the idle mixture being so rich troubles me. I use 10% ethanol 89 octane fuel. Nearest non- ethanol gas is about 50 miles away.
7) So, should I try the manifold vacuum and if so, should I set initial timing to 6* BTDC with the line disconnected and plugged? And then maybe start with a mixture of two turns out?
Thanks so much for the help and education, P51.
I'm not sure I can exactly answer all your questions accurately but here is how I understand it:
When you have the vac advance canister hooked up, the only difference between ported and manifold is at idle. Once you crack the throttle blades even a little (enough to expose the ported vac port to the manifold vacuum), the vac advance is the same whether you are using manifold or ported vacuum. So, the only question is whether you want the advance engaged at idle or not. Some engines do fine with manifold vac, some do not. For example, if you have loppy cam with very low or highly-varying vacuum, using manifold vac will not give you a good idle. With a highly-varying manifold vac at idle the advance would be going "in and out". Or if you have low manifold vac, the vac advance might be partly engaged causing it to act squirrelly. But for a stock engine with >15" *steady* vacuum, hooking the vac advance to manifold vac should work.
Bottom line: Manifold vacuum uses less fuel, engine runs cooler, and it produces more emissions. But ported vac is a lot more forgiving.
As for " initial + Vacuum + increasing centrifugal advance" (per above) being very high, you see that at idle when rev'ing the engine because the engine is not under any load. When you're actually driving and accelerating the load on the engine is much higher, your throttle blades are open a lot more, and, therefore, your vac advance has likely *dropped out mostly or completely*. So, you will only be seeing approximately "initial + centrifugal" when the engine is under load. When cruising you are not loading the engine much (just keeping the car moving at a set speed doesn't take all that much hp), the throttle blades a just cracked a little, and the vac advance is again engaged (it is the same whether using either manifold or ported vac). Yes the advance seems large but the amount of fuel being consumed by the engine is so low in this case that the time it takes to burn in the combustion chamber is longer allowing for much more advance (refer to the the Timing 101 article Timing)
As for setting the initial to 6* BTDC, I personally would leave it at 11* as you now have it and then try both manifold and ported vac to the distributor to see what you engine "likes" better. One thing to check is manifold vac (with a vac gauge) at idle 11* initial with vac canister disconnected. If the vac is lower than ~15" or if it is bouncing around a lot it may mean that you have a vac leak (which might explain the idle screw setting??). If you don't have a steady vacuum you may be better off with using ported vac to the disti.
Edit - As to your question "Do you think Edelbrock indicates to use ported vacuum so that they do not run afoul of the EPA?"
I think that given that hooking the vac advance to ported vacuum "just works" for a large variety of engines/mods, Edelbrock recommends it so that it doesn't get as many calls to their tech support and/or people complaining that their product doesn't work.
Four *full* 360* turns out (not "half turns") on the idle screws still seems very excessive to me. I have never seem much more than 2.5 full turns out.
Aside: I also only have a single-port vac advance canister on my disti