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On 10 Dec 2002 at 22:59, Greg Merritt wrote: > With a vacuum advance system, when you open the throttles, > the ignition timing retards, no? It "kicks in" (advances) under a > strong vacuum (i.e., throttle shut), right? This is exactly where the confusion starts. Manifold vacuum acts the way you describe here, but very few distributors actually use manifold vacuum. Most vacuum advances use vacuum from carefully drilled holes somewhere in the carb, usually near the throttle or in the venturi. The holes are positioned to give a particular vacuum response as a function of various conditions. The fuel economy drillings are near the throttle valve so that there is high velocity (remember Bernoulli's Principle) there when the throttle opening is small, but none when the throttle is either closed or fully open. Systems that use vacuum advance for the major source of advance want to respond to the load on the engine, so they put the drilling in the venturi where the vacuum is simply a function of the amount of air drawn into the engine. Ports used for vacuum retard in some late cars are drilled near the throttle valve in such a place that they see full manifold vacuum when the valve is closed, but that plate crosses over them quickly as soon as the throttle is opened. I think in our cars, the only place that uses true manifold vacuum is the vacuum modulator for the AT. -- Jim Adney jadney@vwtype3.org Madison, WI 53711-3054 USA ------------------------------------------------ To unsubscribe, E-mail to: <type3-off@vwtype3.org> For more help, see http://vwtype3.org/list/