Steam: Its Generation and Use

A three-chamber vacuum reducer valve is used in the distributor spark advance control system to provide a source of vacuum for advancing the spark firing during engine overheat conditions. The source of vacuum is greater than the ported spark vacuum found upstream of the throttle plate in a carburetor intake and less than the manifold vacuum. One input to the reducer is the manifold vacuum and the other input is atmospheric pressure which are combined by means of a regulator in the valve for producing an output vacuum which is reduced from the manifold vacuum.