Quote:
Originally Posted by poorboy
You want it hooked to a port that increases the vac as the rpm's go up. If you hook it to a manifold vac port it will be at constant full advance in the timing. Just start the motor and use a vac gauge or hold your finger over the port. Raise the rpm it should increase the vac as you raise the rpm.
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Sorry, this is just not correct. Vacuum typically
decreases as RPM increases. At WOT if the carb is sized correctly there should be no vacuum
The simple answer to the question is that if you have a stock or near-stock cam, you should connect to ported vacuum as no additional vacuum advance is needed at idle. Ported vacuum is taken above the throttle butterflies, and so is less than manifold vacuum at idle.
If you have an aftermarket cam that's a little warmer, connecting the distributor advance to manifold vacuum will provide additional vacuum advance at idle which can improve idle quality.
Once the butterflies are open to any extent, manifold vacuum and ported vacuum will be virtually identical and so there is no performance/fuel economy/etc. difference - the only reason for additional advance at idle is to smooth out a lumpy cam.
BTW, this has been hashed out a million times - search is your friend