Thread: super advance running better

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pirated from netwelding.com....

The Chevy instructions also say to use "Ported Vacuum" to connect to the distributor vacuum advance diaphragm. This means there will be no vacuum present at idle. This port comes from above the butterfly's in the carb. Therefore until the throttle is open, there is essentially no vacuum in this port to activate the distributor advance mechanism. However and old Peterson publication entitled "Basic Ignition and Electrical Systems" reviews the history of "Manifold" versus "Ported" vacuum. As they indicate, "Ported Vacuum" came about during the start of pollution controlled vehicles to reduce nitrous oxide emissions with lean air/fuel idle mixtures. They state that by using "Manifold Vacuum" the spark at idle will occur sooner and "less cylinder wall is exposed to the heat (of combustion), thereby lowering coolant temperature." Interpreting their statement, with more advance at idle, the combustion starts on the upper portion of the cylinder on the compression stroke. Flame front travels very slowly at the low cylinder pressure that occur when idling. Therefore the cylinder head and upper cylinder walls will absorb most of the combustion heat and the lower portion of the cylinder on the power stroke will be cooler. This transfers less total heat to the cooling system. In fact, on this 502/502 motor an additional ~ 10 degrees advance occurs at idle when the "Manifold Vacuum" port is connected to the distributor vacuum advance. This provides about 25 degrees advance in total when idling or when cranking! The engine idles considerably faster compared to when the "Ported Vacuum" port is employed. The throttle butterfly's have to be closed about 1/2 turn on the idle adjustment screw to maintain the ~900 RPM idle speed. All else being equal, less butterfly opening means less fuel consumed which equates to less total heat to transfer to the coolant.

pirated from carcraft.com...

Where’s the right vacuum port to connect to the vacuum advance, you ask? Most street carbs are equipped with both ported vacuum ports and manifold vacuum ports. Vacuum is generated by the engine’s pistons traveling to the bottom of the cylinders—air is sucked in to fill the voids, which in turn creates a signal to the carburetor to meter fuel. Manifold vacuum is present anytime the throttle blades aren’t fully open, including idle when they’re nearly closed. Ported vacuum isn’t “seen” at all when the engine is idling, because it’s taken from a port above the throttle blades.

Some gearheads have near-religious views on ported versus manifold vacuum. Patterson connects the vacuum advance to either source, and chooses the “right” port after test-driving both hookups. He does say that full manifold vacuum should help tip-in response, and this setting made Smith’s Mustang feel crisper off-idle. But if your car idles shaky and “nervously” with manifold vacuum advance, it’s a sign that there’s too much initial advance. So give the ported vacuum a try.