None of my vacuum advance units have ever retarded the timing. They drop out their added advance when vacuum drops, but the timing only returns to the initial or mechanical advance. I haven't had one retard the timing beyond that.
Determining what the engine likes is not that difficult with a tach, timing light, and a hand held vacuum pump.
I am talking total timing. Dropping out is retarding. Your cruising on the highway at part throttle. What is vacuum? How much of your total timing is mech and how much is vacuum? What happens when you punch it to pass? Vacuum goes away and total timing retards. The motor does not want all that timing under load and starting to accelerate. As RPM climbs mechanical runs up and vacuum timing is 0 and your at your total around 32-34. Prior you where probably around 44-46 total timing maybe more. The vacuum can adjust time up and down based on load. Mechanical adjust up and down based on RPM of motor. Both are factors in determining what timing is needed for peak efficiency.
Go read the Chrysler master training material on this. That will explain what the Chrysler engineers were doing, and why things changed for emissions. It really explains what the motor needs at various operating conditions.
So I have read were some race guys use a low adv vacuum can to help their car idle after start. The high initial keeps car from starting hot. So they dial back initial to start hot, but vacuum can on manifold gets total back to were it is needed for idle. Once they get on it, the vacuum drops out and they are on the mechanical. So you use it as you thing your combo needs it. Manifold versus ported is basically an on off switch for the vac can. Ported is off at idle an on at a specific point of throttle blade opening. Manifold is on all the time.
