Adjustable Vacuum Advance

Bewy said

From my 1972-78 MM, p. 2-457. 'When coolant temp at idle reaches 225*F, the valve opens and applies MANIFOLD VACUUM directly to the dist....This increases engine idle speed & provides additional engine cooling.'

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Now your purposely taking that out of context. Yes. Absolutely many manufacturers used manifold vacuum as a way to overide the retarded initial timing that was being used for emissions control. The purpose of the late timing was to put more heat into the cylinder walls. The CTO overide was there to add back timing at idle to prevent overheating in certain traffic and climate situations. My '85 AMC v-8 also has a nonlinear valve that mixes manifold vacuum and timed vacuum for certain warm up situations.

The point is that the use of ported vacuum is not something that was developed for emissions controls. It was used back as far the 1940s. There are many ways to shape the advance curve. Using mechanical advance for the rpm related, and the leaving the vacuum advance for the density related lets each of them do what they do best. Using manifold vacuum to increase timing at idle sometimes works out. It works out best when the manifold vacuum isn't much affected by changes in idle rpm and load.

If you are familiar with the books, then you'll know that they are based on a lot of emperical testing and development. If you are familiar with Shrinker's work you know he was testing with a 5 gas analyzer on dyno along with a scope and of course, at the track.