vacuum advance

vacuum advance
A mechanism on the side of the distributor that automatically varies the instant at which the spark occurs as a function of intake manifold vacuum. Vacuum advance provides the additional advance that is needed when the engine is operating at part throttle. At part throttle less air-fuel mixture gets into the cylinders and the mixture takes longer to burn after it is ignited. Because the mixture burns more slowly, the piston will be past top dead center and moving down before the mixture has a chance to burn and produce high power. As a result much of the power in the fuel will be lost. The vacuum advance mechanism consists of a flexible spring-loaded diaphragm connected by a linkage to the breaker plate on which the points are mounted. The sealed side of the diaphragm is connected by a tube to the carburetor. The throttle valve is below the vacuum passage in the carburetor air horn so there is no vacuum advance when the engine is idling because the throttle is closed. However, when the throttle is partly open, intake manifold vacuum pulls the diaphragm in and this causes the breaker plate to rotate a few degrees and advance the timing. With wide-open throttle there is very little vacuum in the intake manifold so there will be no vacuum advance. In most instances the vacuum advance is disconnected before checking the timing and point gap.

Dictionary of automotive terms. 2015.

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Look at other dictionaries:

  • vacuum advance mechanism — See vacuum advance …   Dictionary of automotive terms

  • vacuum advance unit — See vacuum advance …   Dictionary of automotive terms

  • ported vacuum advance — (PVA) A series of restriction devices between the distributor advance unit and the carburetor advance port to ensure that there is no vacuum advance during idle, but increasing ignition advance as the throttle opens …   Dictionary of automotive terms

  • control vacuum advance — See speed control vacuum advance …   Dictionary of automotive terms

  • speed control vacuum advance — A system which cuts off vacuum ignition advance when a car is cruising at low speeds, typically under 35 mph; for this purpose, a solenoid operated valve is hooked into the distributor advance vacuum line, which is governed by a speed dependent… …   Dictionary of automotive terms

  • advance — [1] The act of changing the ignition timing so that the spark occurs earlier in the cycle. The opposite is retard. [2] It may refer to the device which makes this adjustment. Also see automatic advance centrifugal advance electronic spark advance …   Dictionary of automotive terms

  • Vacuum switch — Prior to effective engine computers, engine vacuum was used for many functions in an automobile. Vacuum switches were employed to regulate this flow, and were commonly controlled by temperature, solenoids, mechanically, or directly. They operated …   Wikipedia

  • vacuum delay valve — (VDV) a valve used by GM to bleed ported vacuum to the vacuum advance unit through a small orifice and control vacuum advance rate. Used to retard or delay the application of a vacuum signal. Also called delay valve …   Dictionary of automotive terms

  • vacuum — An enclosed area in which the air pressure is below that of the surrounding atmospheric pressure. See air valve carburetor constant vacuum manifold vacuum ported vacuum advance pulse vacuum hublock venturi vacuum …   Dictionary of automotive terms

  • vacuum retard delay valve — (VRDV) delays a decrease in vacuum at the distributor vacuum advance unit when the source vacuum decreases. Used to delay release of vacuum from a diaphragm a momentary vacuum trap …   Dictionary of automotive terms

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