[olddog]

I'm no expert, but I have read a good deal on the subject.

Ported vacuum came out of emissions regulations. The goal was to use the throttle plates to block off the vacuum at idle. Retarding the timing at idle causes the combustion chamber to run hotter and reduce emissions it idle. It also uses more fuel. Long idle times can cause hot spots and pre-ignition.

Modern EFI computers also reduce timing at idle. Partly because timing is used to help control idle speed. Emissions and warm up are also in the mix. They do it intelligently taking into account many factors.

Based on the EFI strategies, it would seem that ported vacuum would make controlling idle speed a little easier, as with the timing pulled in to make maximum torque, the throttle may need to be practically closed at idle. Although I thought engines ran just fine on manifold vacuum back in the old days. I think I prefer manifold vacuum, but there could be a logical argument for ported.

The amount of ported vacuum at a given light load condition driving down the road may be quite different than manifold vacuum, so the vacuum canister may need set up differently to change from one to the other.