Join Date: Sep 2000
Location: Washington, Michigan USA
There are many opposing theories on ported-vs.-manifold vacuum for advance. Ported vacuum was invented as an early approach to emissions control, with debatable results; the idea was that with retarded spark (no vacuum advance) at idle, more unburned hydrocarbons would be present in the exhaust gases, which would support fuller combustion of all exhaust gases in the exhaust manifolds with the Air Injection Reactor (A.I.R.) pump systems. With full manifold vacuum on the advance unit, your engine will idle smoother, will run cooler at idle (reduced exhaust gas temperature in the exhaust ports, which are surrounded by coolant passages), and will be more responsive off-idle, as the advance doesn't have to "catch up" with the increase in rpm - it's already there. That's why, in the late 60's and early 70's, when A.I.R. pumps were on almost everything, the first thing that was done to "de-smog" an engine was to plug the "ported" vacuum source and connect the advance diaphragm to full manifold vacuum (followed by busting the plastic caps off the idle mixture screws, re-setting idle mixture a bit richer, and re-setting the base timing).
'69 Z28 Fathom Green