yellovette

I was recently told that my Holly carb did not have a "ported" vacuum connection for the distributor can.

What exactly is ported vacuum? I've got a vacuum connection that seems to work just fine.

redwingvette

Ported vacuum is vacuum that is low at idle and increases as you give it gas. Attached to your distributor it will cause the vacuum advance to move quickly to increase performance and drop off when you let off on the gas for a smoother idle.

SIXFOOTER

Ported vacuum is taken from above the throttle plate. For performance you need to run manufold vac.

694speed350

run manifold vacuum.

yellovette

Thanks guys. I understand the above and bellow the throttle plate explanation now, but your individual answers, (the way I read em') seem to contradict.

redwingvette; says for performance, ported vacuum increases timing quicker.

Sixfooter; says, for performance you need manifold vacuum.

I thought that when you accellerate you're opening the throttle plates and your vacuum drops to nothing. That's when your mechanical advance takes over.

Am I confused?

yellow 72

Try it both ways, some motors like a lot of timing at idle(manifold vac).
Either way at WOT there is no vacuum advance.

noonie

You are right. Under full throttle your vacuum advance should not be working.

With ported vacuum at idle (when your throttle plates are completely closed) you will have no vacuum to the dist, hence no timing advance, thus allowing the engine to usually idle smoother. First widely adapted for smog engines.

With manifold vacuum at idle, you have distributer vacuum and timing advance. Where depends on which cannister you use. In most cases this results in a quicker revving response.

At varying light cruising throttle positions (where your dist uses the vacuum the best) there is always a certain mix of vacuum and both ported and manifold vacuum act essentially the same.

Generally shoot for manifold vacuum, but in some cases an engine will idle better with ported. You may have to try several different cannisters to get a good result when switching between the two.

yellovette

Thanks for the good info guys. I'm heading to the garage with my vacuum gauge in hand.

stingr69

Manifold vacuum is the pre-emissions way of varying timing in response to engine load. It backs off the timing when under load and it increases timing when at low load conditions such as cruise.

The only reason we have ported vacuum is because it reduces emissions. That is all. It does the same function as manifold vacuum advance except it does not provide any advance at idle. Ported vacuum advance will increase idling temperature which is good for emissions. Otherwise it operates just the same as manifold vacuum during operating conditions.

The car will usualy idle better and at a lower temperature with manifold vacuum but polute more. You have to select a vacuum canister to match you combo in order to enjoy the benefits of manifold vacuum advance.

If you do not know how to select a proper manifold vacuum advance canister, you should stick to ported vacuum advance. It is what you probably have on your car now as factory equipment. The ported vacuum advance is more goof-proof as it is less "application sensitive" as compared to manifold vacuum advance You can just hook it up and go.

-Mark.