It depends totally on the combination. For the most part, I've had the best success with ported vacuum. I "figured" since this slant 6 only pulls 6hg at idle it might benefit from manifold vacuum. I tried it. It didn't run as well so back to ported it went. Most street applications I imagine will respond better to a ported signal, but there are some exceptions. Chevy even used manifold vacuum straight from the factory on a few of their cars. The big block SS Chevelles are but one example.
