ported or manifold vacuum for advance?
Vacuum advance is designed to increase effeciency at part throttle cruise. It is not designed to work at idle, nor is it designed to work at full throttle.
I know many people who swear that manifold vacuum makes their car run better. That's cool.
I would advise working on your distributor advance (it is very simple) so that without vacuum advance connected, you get the advance you want at idle and at 2500-3000 RPM.
You want 16 degrees initial, and 36 degrees total. Your distributor needs to have a mechanical advance of 20 degrees. (36-16=20).
Now your distributor is setup to provide you with great power performance.
But, if you drive on the street, 36 degrees is not going to be nearly enough at light throttle cruise. Many engines need another 10 to 15 degrees of timing at part throttle to achieve lower EGT's and better mileage.
Here is where ported vacuum comes into play. Ported vacuum provides another 10-15 degrees of timing, and is available only during the time when the throttle is barely opened, say just above idle to 1/4" or so at the max.
So, you are cruising around at 3000 RPM, your total advance is 51 degrees (36 degree mechanical + 15 degrees vacuum as an example) your car is purring along doing great. You stomp on the gas, and the timing drops back to 36 because the throttle is now past the point where ported vacuum is available, and no pinging occurs.
The timing numbers used as an example here are what I use on my 466 in my signature. It picked up several MPG's with the vacuum advance correctly adjusted, and I could tell that I needed to press the gas pedal less with it hooked up, than without it at cruise.
Good luck, hope I didn't confuse the issue.
\'71 Torino GT 466/C6
11.3:1, Edelbrock CJ RPM heads, 250/262 @ .050 solid flat tappet, Weiand CJ stealth, Holley annular 850 DP, Hooker 6115 headers, TCI streetfighter converter, C6, 3.89 31 spline trak-lok.