what's the reasoning behind choosing ported or manifold vacuum for advance?

Maybe this will help...

It describes how the ported vacume works with the advance. Doesn't really say why it's better, other than there is no advance at idle by doing it this way.

NAte, I don't know if I can shed some light on this for you or not but, I am going to take a stab at it.

One of the big problems with performance cams is their lack of vacuum at idle. This results in a poor signal to the carb ports to control fuel flow. In order to get enough air to idle, the throttle valves have to be open too far and end up exposing the transfer ports. As you have noted, this also provides vacuum to the advance port as well as supplying the fuel/air mix. As the fuel mix flows the vacuum rises advancing the timing which will result in a lean condition producing a reduced vacuum which in turn reduces the fuel mix flow from the transfer intil the vacuum drops and the fuel mix richens again. It is a seesaw process that results in poor idle. The idle port is still the best place to get advance vacuum because that is related to engine load which determines how much advance it really needs. The problem is to get the throttle plates to cover the transfer slots at idle and cut off the vacuum to the advance. I have neve done this on anything other than a Holley as most of the Carter AVS type carbs are pretty well dialed in. On the Holley's, you can drill a 1/8" hole in each of the primary throttle plates about half way between the shaft and the edge of the plate. This allows it to draw air away from the transfer port while the port is closed. The vacuum is then not available to the advance at idle making the vacuum signal to he idle ports more consistant and smoother.

Again, I do not know if this procedure works as well on the AVS as it does the Holley. If you do this and it does not work, you will need to solder the holes or rivit them. I do not know Edelbrock's posittion on this but, Holley tech has no problem with it in performance apps.

Also, if it has an idle adjustment on the secondaries, you may want to be sure they are adjusted correctly. If you have a secondary vacuum adjustment, you can sometimes help by opening them a hair more such that some idle mix is available from there which would give you more control over the primaries and the vacuum signal.

Good Lugnut

The good things about manifold vacuum:

-More advance at idle = cooler exhaust, less fuel burn at idle

-Less advance during cranking- easier cranking

The bad things about manifold vacuum:

- Lots of engines will bog badly (just flat fall on their face) when coming off-idle, because the vacuum advance decreases as the throttle opens, especially when its opened quickly

- More nitrogen oxide emissions at idle

- sometimes you get a mild misfire at idle

Bottom line- if a modified engine seems happier all around with manifold vacuum, by all means use it.

Nate Nagel wrote:

First of all the gasoline available today is not the same as what that engine and compression is going to like much. The first thing you need to do is verify the vacuum advance diaphragm is good and not leaking. If there is a leak in the diaphragm or vacuum lines none of the following applies. When you say the RPM increases when you connect to manifold vacuum that could be because the timing gets advanced at idle or it could be because the engine is sucking air thru the vacuum line.

As for your direct question. The only way you are going to know for sure what the difference between port and manifold vacuum is is to put a vacuum gauge on each and compare. I suspect if you did that you would find that there is not much difference except at idle. Under load and open throttle the ported vacuum may be just a slight bit lower. If the engine knocks (pings) then hooking up to ported vacuum should help prevent that. If it doesn't knock then manifold vacuum shouldn't hurt anything and may even help. This assumes you have a muffler that works well enough to hear a ping if there is one. Ideally you want the timing set so that it is just bit retarded from the point where it pings under moderate and heavy load.

Hooking the vacuum advance up to manifold vacuum could cause hard starting and rough idle when the engine is hot. That might be a problem in summer but not winter. That might be a problem if there is a heater for the air intake, but probably won't be a problem if the engine is sucking cold air. Heat at the base of the carb and intake manifold may be a factor also. Mostly they started hooking the vacuum advance up to ported vacuum instead of manifold at the same time they started putting heat stoves on the intake and the octane of gasoline started to drop.

-jim

If it runs smoother at idle hooked up to manifold and the distributor vac advance is working then that suggests your base line timing may be a little late.

In general it should probably be hooked up to ported vacuum