When you connect the vacuum canister to a full source it provides addition advance at IDLE. A ported source won't be a vacuum source until you open the throttle just past idle, and then it is the same as a full source. Couple of my observations:
1. With an automatic car with a "big" cam the vacuum will be low at idle. If you connect the vacuum canister to a ported source you wont have the additional advance at idle which in turn has you opening the throttle plates more to obtain a good idle which causes even less vacuum at idle. Because of this the transition slots may be uncovered and you end up idling very rich. That problem can be corrected. The positive side is that with an automatic trans the idle will be stable when you drop it in gear because the throttle plates are opened supporting the idle speed versus on a full source the vacuum advance is supporting the idle speed due to increased timing which = increased speed which = closing the throttle plates to obtain the same rpm. The problem with that is when you drop it in gear while using the full source, the rpm drops due to the load and therefore vacuum drops which = less advance which = less rpm which could mean the vehicle stalls because of to low of an rpm. You could raise the rpm but then the trans slams into gear. My opinion is with a "big" cam and auto use ported source.
2. The pro's to using full source is that the vacuum advance provides additional advance at idle. This in turn provides for cooler temps at idle. Using a full source also increases idle speed which in turn causes you to close the throttle plates more which closes the transition slots so no problems there, and also provides for more vacuum at idle due to higher rpm with throttle plates closed more (versus the ported source) which = better throttle response. Probably not the way to go with an automatic but That is what I use on my 5 speed.
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