Vacuum advance Ported or not?

[Goose]

Just a curiosity question.. Are most of you running your dizzy vac advance from an unported source? or ported..(You know what I mean.. vacuum all the time or only above a certain throttle position?? And why any advantages to either way..?? I'm curious. Mine was ported on a 1406 edle carb now I'm trying it direct..

[mrtazwrench]

most are ported stock, don't want the vac advance kicking in at idle.

[tufcj]

You want the advance to increase as the RPM increases. Ported vacuum is drawing from the venturis, as airflow increases, so does vacuum. When you're at idle, the carb is on the idle circuit, so no vacuum.

Manifold vacuum will be somewhere around 12-20" at idle, depending on cam and timing, maybe less. You will still get a strong vacuum at low load cruising (like going down the highway). But when you open the throttle like a jackrabbit start, pull a trailer, or even pull up a long grade, manifold vacuum will drop to near 0, as will your vacuum advance. That's opposite of what you really want.

The biggest problem with manifold vacuum is you may have to back down initial advance, because you'll have vacuum advance coming in at idle.

Just put a vacuum gauge into a manifold port and drive for a while, you see what I'm saying.

Bob
tufcj

[Goose]

See I knew you guys would know..It's one of those questions that just never occured to me before..Thanks for the enlightenment. Hey just to let you guys in CO know theres a 84 J-10 w/258 6cyl t-176 trans and NP 208 in the "Pull and Save wrecking yard in aurora..Body/box fair all the interior and running gear was there 2 days ago..(except the seats which are now in nebraska with me!