Thread: Vacuum Advance

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Here's my point. In this case, Dennis says he has to run very low initial timing because of the high manifold vacuum. When he drops the hammer, ported and manifold vacuum are essentially equal, and he's left with that low initial. If he uses ported, he sets a higher initial, and the immediate drop-off isn't as severe.
The following article was written by Kenneth Steinbach on another forum, and I find it quite logical.

Centifugal advance is strictly related to engine speed. Vacuum advance is related to engine load. Under light load, ie. low throttle angle conditions, cylinder pressure is relatively low and the less dense air/fuel mixture tends to burn slowly. Therefore additional ignition timing is necessary to burn all the mixture, hence vacuum advance. On the other hand, at wide throttle openings engine load is high, manifold vacuum is near zero because the engine is no longer sucking past partially closed butterflies, cylinder filling and pressure are greater, and the denser mixture burns faster. The vacuum seen by the vacuum advance diaphragm is not enough to overcome the spring which opposes the vacuum signal, so no vacuum advance occurs. The issue of ported vs. manifold vacuum is very simple, and 99% of what was said in the stangnet forum is wrong. The main reason for using ported vacuum rather than manifold vacuum is to reduce hydrocarbon emissions at idle. If you have an extra 10-15 degrees of advance at idle you'll gain nothing in performance but will double or triple your HC emissions. Another issue is idle quality. If you have a lumpy cam, vacuum at idle tends to be unstable, resulting in a constantly fluctuating vacuum signal which would cause erratic vacuum advance activity. A third reason to use ported vacuum is idle speed. If you increase timing at idle by 15 degrees, idle speed is going to increase by several hundred rpm. If you try to compensate by turning down the idle speed screw on the carb you may close the throttle blades too much and disturb the relationship between the blades and the idle transfer slot which will result in an off idle stumble. As far as ported vs. manifold at WOT, there's no difference. Neither produces enough vacuum to actuate vacuum advance.

But again, the dyno will tell.

OK, Dennis, it's your responsibility to tell us the results. Don't make us hunt you down.

. . . then, there's daylight saving time . . .