Reply to "Vacuum advance, ported or manifold, age old question answered"

dave2811Member

Nice chart. I think it is important to understand that the concept of ported vacuum came into play through emission control requirements. Retarded timing at idle has emission benefits because it heats up the exhaust. Ported vacuum might also be a benefit in a performance engine to stabilize the idle with a lumpy cam. Variations in idle vacuum may make manifold ported advance difficult to stabilize if the timing is jumping around.

3/18/126:35 PM

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