This isn't true it all. Any book that states that is wrong.
All you have to do is look at various pre-emissions shop manuals and cars, especially Chrysler products.
For example, from 1959 (Session 136) of the Chrysler MTSC.
View attachment 1715749843
While some car companies, on some engines, did use manifold vacuum, they did so to get the best timing at idle they could with the distributors they were using.
But you can see in the manuals, on the cars, and in the information to the techs, Chrysler did not need to add vacuum advance at idle. Another example here.
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from Ignition System Analysis (Session 259) from the Master Technician's Service Conference
However its interesting to note that on some early Clean Air Packages (CAP) Chrysler did use a manifold vacuum connection to the vacuum advance. This manifold connection was made to the vacuum advance through a special valve which operated during engine braking. This was to insure the leaner closed throttle mixtures were given enough advance to burn thoroughly for reduced emissions. See 1967 Chrysler CAP System: Master Technicians Service Conference (Session 241) for more details.
This later was dropped as unneccessary for automatic equiped cars.
It is true that retarded timing can put too much heat into the cylinder walls. And it is true that retarding the timing at idle was a technique used to better consume HC, especially early in the emmissions era. Further into the emissions era some vehicles used manifold vacuum that connected when coolant temperatures become excessive. But again this was to offset the retarded initial timing. Sometimes, when stuck with using an emissions era distributor advance curve, using manifold vacuum at idle may be a fine solution. It all depends on the specific situation.
Ten years ago I too thought ported vacuum was an emissions related development. See "Carb Issues", scroll down to #11, 17 (me), and responses #13, and #19-22 where Shrinker et al set me straight.
edit: Also very much worth reading is Shrinker's post 10 where he explains what is going on at idle with timing and fueling.
