Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Ported vs. Manifold vacuum
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Old 08-16-2012, 11:05 AM
BogiesAnnex1 BogiesAnnex1 is offline
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Originally Posted by View Post
So do I need to retard my timing before I hook up manifold advance so it doesnt ping. Also where is that port located
Maybe or just turn the idle down.

The difference between ported and manifold is timing when there is or not vacuum advance. The ported sources from just above the throttle plates when they are nearly closed at idle, so the port doesn't see manifold vacuum until the throttle is opened enough to expose the port to the manifold vacuum.

The manifold vacuum port goes directly to a locating under the throttle blades so it sees manifold vacuum all of the time.

Choosing which port to use is a function of the cam timing, idle speed control, and idle emissions requirements. For a mild cam that doesn't need to pass emissions using full time vacuum advance will let you have sufficient idle speed with less throttle opening and idle fuel flow, but this tends to run idle NOx emissions up.

If this engine uses the timed port, the idle speed will naturally want to be lower and will have to be adjusted up to proper speed with more fuel and air delivered by the carb. This often results in run-on when the engine is shut off, tends to increase unburnt HC emissions but reduces NOx, often develops a stumble or jerk at the transition point.

Once the timed port is exposed to the manifold vacuum by the throttle blade opening, either it or the full time source will work the same at the vacuum diaphragm.

For a wild cam which port is moot as the throttle blade is typically open at idle beyond the position of the timed port so it and the full time port will behave the same unless the throttle blade is drilled to supply idle air in which case the sources go back to operating as they would with a mild cam.

In large measure which to use will be that which makes you and the engine happiest.

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