Item Posts    Sort Order
Sticky Thread

Vacuum Advance.,. Ported or Manifold?

Posts: 17
Joined: 04/12
Posted: 01/28/14 08:24 PM

One of the interesting issues I see on different sites is how to hook up the vacuum advance to the distributor. Some will say it has to go to ported vacuum and others say straight to manifold vacuum is just as good. I know in the past, I have tended to use vacuum straight from the manifold as it was my belief that idle quality and off idle performance improved. It also seemed to me that since you use less throttle opening with the manifold vacuum supply, that you would use less fuel. Today I found a site that explains what is really happening to the vacuum signal from both sources.

Here is a chart of throttle position versus both manifold and ported vacuum:


This chart is taken from

As you can see both manifold and ported vacuum are virtually the same except at idle.....when the throttle is closed, manifold vacuum is at maximum, but when the throttle opens, ported vacuum is the same as it is from the manifold. So other than at idle, there is no difference between the vacuum signal from either source. So this begs the question, which is the best source?

Back in the old days, the vacuum advance was always from the best as I can remember! I really didn't remember when they started using ported vacuum, but I do remember my 74 Monte Carlo. It had a restrictive air cleaner and ported vacuum. I turned the air cleaner lid upside down and ran the vacuum advance to the manifold. In addition, I adjusted the idle mixture screws the way we did it before emissions and guess what....? Gas mileage went from 12 to 16.......!

So why did we change to ported vacuum? Believe it or not, the purpose of ported vacuum is to raise the temperature of the engine at idle to lower NOx emissions. If you are not really concerned with NOx, then what are the advantages of manifold vacuum?

Well for one, lower idle temperature and in addition, it will idle smoother and take slightly less throttle at low speed to maintain road speed. It should increase fuel economy.

Here is link to the entire article if you are curious.....

Thanks to for clearing up some common misconceptions.

When The Flag Drops.,.


The Bull ***t Stops.,.

P. Engineer, Engine Builder

Post Reply