Winginit has just posted an excellent description of how vacuum advance works in this thread in the 335 Series forum. I think it is a good read and is helping me understand the situation.

 

I know I've read this before...

Wingingit is John Z from some Camaro board over a decade ago???
Timing & Vacuum Advance 101 - Team Camaro Tech


Otherwise, his post is pure cut & paste with no attribution.

 

He poo-poo's ported vacuum advance, but as you said Gary, if you really turn the timing control upside down you could get rid of it. It would be a lot of work, calibrating distributors has become a lost art.

 

Yes, I assumed that it wasn't written specifically for us. But the overall ideas should still apply. And while I'm not, at least yet, bought into the manifold side of the idle discussion, I am intrigued and want to explore it further. And, I'm old enough to remember that manifold vacuum was the stardard before emissions concerns came along. So, maybe there's something to it?

As for lost arts, I fear that distributor curving and carb tuning are both in that category. In fact, even the ability to find someone to rebuild a carb is waning. Makes me wonder about the wisdom of building project vehicles with carbs vs EFI. However, there may be no one to work on the current aftermarket EFI systems when the need arises later.

 

And it seems to be posted info on many forums across the net. The vacuum canister part Number info seems to apply to GM products of course.

If we with Fords were to use the info it may be good to note that most vac canisters of our era are generally adjustable which is info that isn't widely expressed during the typical DS conversion.

 

"Spark" or ported vacuum has been around since at least 1950, long before any sort of exhaust emission consideration. It may be that most cars and trucks had manifold vacuum signal for distributor advance prior to the 1970s? No idea. But ported vacuum is not a smog era invention.

Modern vac canisters are adjustable but afaik only for the total amount of advance with a given vacuum signal, but not when the advance comes in. Performance engines as the GM engineer notes have relatively low idle manifold vacuum and so care must be taken in the selection of the vacuum advance canister to prevent idle "fluttering" if manifold vacuum is utilized.