Originally Posted by stroker444
When i run ported advance the idle sounds much different than when i use manifold vacuum advance. the idle sounds much better when i use ported, it sounds more lopey i guess, but the off idle performance isnt very good. when i use manifold it helps off idle, but even if the idle rpms are the same, the sound is very bland. has anyone else noticed this or am i crazy?
Ported (or timed) vacuum source simply uses the throttle blades to obstruct the vacuum source so that the vacuum advance is not functional till the throttle is opened, usually coming off idle and going to the transition feeds.
So if you connect the vacuum advance to the timed port when the engine is idling there no more advance than the base setting. If the connection is made to a manifold port, then as soon as the engine is started full manifold vacuum is applied to the vacuum advance mechanism so that what ever amount of vacuum advance is built into the unit, usually around 10 to 14 degrees, is added to the base timing setting. This will affect the sound of the idle and the idle speed. More advance will make the exhaust sound crisper and the idle speed will come up. On a gas meter the NOx will be low and the HCs high when idling on ported vacuum. The reverse will be shown when idling on manifold vacuum where the NOx is higher and the HCs lower. If you have to pass emissions using the timed port works better as excess HCs are easier to clean up than excess NOx. From an idle quality stand point a large cammed engine will want more initial advance than a smaller cammed engine simply because the effects of large amounts of overlap and late closing intakes tends to dilute the fresh mixture with exhaust and pumps some back into the intake. This lowers the number of fresh mixture molecules in the cylinder which slows the burn rate (i.e. makes the engine act as if the timing was retarded) so the additional idle vacuum advance provides more time for the combustion reactions to complete before the exhaust valve opens, while, also, allowing the cranking advance to be kept moderately enough that the starter isn't being back loaded by early firing cylinders coming on the compression stroke.