On the subject of ported vs manifold, everyone has an opinion about which one to run. The OP was correct in the context of running a stock engine with all the emissions and before the 1960s there was no ported vacuum so they all ran on manifold vacuum. Nowadays we have both ported and manifold, so just because all vacuum advances used to run on manifold vacuum is besides the point. On vehicles where emissions are not an issue and you can set your timing however you want, the choice of ported vs manifold is not so set in stone. I run ported and here’s why.
Vacuum advance has no bearing on performance beyond idle, off idle and partial throttle light engine load situations. Performance distributors have no vacuum advance and all-out race distributors have no advance whatsoever. You set your distributor for max advance and you’re always all in.
Now on these stock or close to stock distributors: if you run your distributor on manifold vacuum you are getting the maximum amount of vacuum advance at idle. When you put your foot into the gas pedal and begin to accelerate, your manifold vacuum drops and so does your vacuum advance. Accelerating off the line is when you need more advance to make more power, but because your manifold vacuum is dropping your vacuum advance is actually retarding the timing. The vacuum advance will retard the timing as you’re trying to accelerate—the complete opposite of what you want. It also follows that when you take your foot off the gas pedal your manifold vacuum increases and that’s when the vacuum advance is at its highest again—when you DON’T need it on deceleration.
If you run ported vacuum and everything is set up correctly then you have no vacuum advance at idle, but when you begin to accelerate you get a sudden burst of vacuum, which gives you a burst of advance, which translates into a burst of torque as you’re getting into it.
If you’re running on manifold and switch to ported, you need to set your initial timing a little bit higher to compensate for the fact that you have no vacuum advance at idle. I set mine to 16°. I also used a dial back timing light to verify that with the initial timing set that high I wasn’t exceeding 36° of total timing. You will also have to readjust the idle mixture screws and possibly set your idle stop screw a little higher, but it’s not a big deal. The result is more torque when you get into it off the line and once you get going it’s really doesn’t behave significantly different than running on manifold vacuum.
Again, there is no right or wrong about ported or manifold and everyone has their reasons for each. Just adding my two cents to the discussion.
