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Old 06-25-2009, 09:20 AM
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blue54 blue54 is offline
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Good Morning. I choose ported vacuum as it is drawn off of the venturi so there is no advance until the throttle blades are opened. When shutting down the engine it cuts some advance to make it easier to stop. When under part throttle acceleration the vacuum advance is in play to help with low end power. When using manifold vacuum, vacuum drops during initial acceleration which can cause a power drop off. Sometimes it is appropriate to use manifold vacuum as it will allow for timing retard during acceleration and will possibly stop detonation in a motor that is right on the edge of too much timing for that engine.
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