Mechanical Advance Vs. Vacuum Advance
by Andrea Stein
Advancing the vehicle ignition timing ensures the spark plug ignites the combustion mixture at the best time for optimum engine efficiency and performance. Mechanical advance and vacuum advance both advance ignition timing, but vary in function and results.
Definitions
Mechanical advance uses the weights and springs in the distributor to rotate and alter the timing sensor shaft’s position relative to the engine’s position. Vacuum advance timing uses a manifold vacuum that reacts to the rotation of a position sensor mounting plate located in the distributor. As the throttle opens during higher engine loads, the vacuum diminishes, enabling the timing advance to move back to the base, or original advance.
Speed
Mechanical advance activates only during high engine rpm, which causes the distributor to rotate quickly enough to activate the advance. Vacuum timing senses changes in the engine’s load, or the demand placed on the engine for power, and offers greater timing advance during low loads.
Advantages
Vacuum advance offers greater better fuel economy and engine performance due to raised timing during periods of low speed such as gear shifting or stopping; it extends the combustion mixture burn cycle. Mechanical advance timing offers better engine performance in high speed applications such as race car driving.
