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Old 01-31-2013, 08:26 PM
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Whether to use ported or manifold vacuum is a subject that has filled many threads. Suffice to say it depends on the combo and how much initial timing is being used. In cases where </= 12 degrees of initial timing is used (and the vacuum advance is limited to 10-12 degrees of added advance like is recommended for a performance ignition advance setup), manifold vacuum almost always works better than ported vacuum.

A cam that's barely able to supply enough vacuum to operate power brakes in a 350 sized engine is big enough to need timing in the area of 16-18 degrees as a starting point. When using a stock distributor this will usually mean the mechanical advance has to be limited to keep the total timing (initial plus mechanical, no vacuum added to this figure) from going higher than 36 degrees BTDC for non fast burn heads. Fast burn heads (Vortec, etc.) use 30-32 degrees BTDC as a starting point.
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