Ported Vacuum vs Manifold Vacuum
Thread Starter
Racer
Joined: Apr 2005
Posts: 494
Likes: 0
From: Virginia
Ported Vacuum vs Manifold Vacuum
Getting ready to set the timing on my rebuilt 73 – 350. Have always used Ported Vacuum for the distributor advance, but based on what I read on this forum, currently have the distributor connected to Manifold Vacuum. This car is a weekend cruiser and occasional drag strip car, so fuel economy is not my number one priority, performance is. Should I stay with the Manifold Vacuum? 
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
Absolutely,..she'll idle better, run cooler, and you'll have better off-idle performance. Some think they're on man-vacuum but not. Use your vacuum guage to make sure.
Last edited by 73, Dark Blue 454; Feb 6, 2007 at 02:05 PM.
Le Mans Master
Joined: Mar 2004
Posts: 7,502
Likes: 1,510
From: Little Rock AR
It makes no difference in "performance" either way. Manifold, ported, or none at all will all have the same HP and track numbers. Vacuum advance has no downside, only upside.
Manifold is better when just driving/idling around town but you need to select a different vacuum can if you want it to work right. If you do not want to spend the effort and $10 for a new canister then ported is what you need. You can just hook it up to what ever you have and go.
Aren't you glad I did not type a long technical explanation.
It has been done here quite a few times so I will save you from having to read all those old techy detailed threads. 
-Mark.
Manifold is better when just driving/idling around town but you need to select a different vacuum can if you want it to work right. If you do not want to spend the effort and $10 for a new canister then ported is what you need. You can just hook it up to what ever you have and go.
Aren't you glad I did not type a long technical explanation.
It has been done here quite a few times so I will save you from having to read all those old techy detailed threads. -Mark.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,392
From: At my Bar drinking and wrenching in Lafayette Colorado
Use whichever sourcve produces the best quality idle - not neccessarily fastest idle. In most cases, this will be manifold vacuum, but it depends on how big your cam is and how much initial advance you're running. Just give the engine what it wants for best operation. If you have to pass emissions, use ported.
