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In Reply to: vacuum advance vs. vacuum retard posted by Corey Turner on September 25, 2000 at 11:10:49:
At least from the way I understand it, if you are grabbing manifold vacuum (between the throttle butterflies and the head), then you are reading the degree to which the throttle is closed. If you are grabbing carb vacuum (outside the throttle butterflies) then you are reading the degree to which the throttle is open.
So when you open the throttles, manifold vacuum drops, and your vacuum retard diminishes. I don't think you want to drill into the carb bodies themselves to plumb for carb vac.
Me, I don't even have a distributor anymore, so this is all based on past conjecture.
Corey
The design behind a vacuum advance (from a stock point of view) is to provide ignition advance as load (vacuum) is at a constant or is decreased (more vacuum as throttle closes). As load increases, timing needs to be retarded to avoid knock. This is done by vacuum dropping and decreasing the advance that the vacuum advance is giving the dist. To do this the vacuum advance must be hooked up to a vacuum port on the air filter side of the throttle plate. As the throttle plate is opened, it regulates how much advance the vacuum advance unit will get (this is called ported vacuum). If it is hooked up on the manifold side of the throttle plate then full manifold vacuum is obtained at all times and the vacuum advance on the dist. will be getting full advance all the time, thus negating the design of the dist. Also when setting ignition timing, point gap or dwell must be set first, vacuum advance must be disconnected, and idle speed set properly.
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