01-04-2007, 07:23 PM
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CC Member
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Join Date: Mar 2001
Location: Cinnaminson,
NJ
Cobra Make, Engine: Cobra Fibercraft Bodies 427 S/C, 351W disguised as a 427.
Posts: 391
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Not Ranked
Frank, you have explained exactly what I have thought. The vacuum advance works best when connected to direct manifold vacuum. Vacuum advance is only necessary for street driven vehicles and is useful under idle/mild throttle conditions. The idea is to add vacuum advance under light load/part throttle conditions where the engine can tolerate it to increase fuel economy. Adding this advance results in more torque at part throttle operation. Adding the advance at idle helps reduce NOx emissions and debatedly promotes a cooler running motor at idle. When you get on the throttle, manifold vacuum drops rapidly and the vacuum advance is out of the picture. Under full throttle conditions the air/fuel mixture is denser and requires less advance until engine RPMs increase and by then the mechanical advance has kicked in.
Since I am running a stock distributor with Pertronix guts and a Crane adjustable vacuum advance canister I set my initial timing at 12* BTDC and then dialed in an additional 14* of vacuum advance at idle with the hose connected to direct manifold vacuum.. Mechanical advance is set at 26*.
It looks like the final concensus of opinion is to use direct manifold vacuum rather than ported.
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