This timing is added to the static timing. The vacuum advance extends the burn cycle even longer so that all the heat energy may be absorbed by the pistons resulting in more power yield per measure of fuel and more mpg. The sensing device is a spring offset, vacuum activated diaphragm. This diaphragm receives its vacuum signal from the carburetor or intake manifold. Early computer assisted vehicles retained the vacuum advance system but employed the computer to modify the signal to the diaphragm, thereby optimizing the system performance.
Total advance is the combination of initial, centrifugal, and vacuum.
See Figure 1. Figure 1 Vacuum advances are sensitive devices. By design, vacuum advances are spring offset to give zero advance. Only when a strong enough vacuum signal is present will it advance the ignition timing. The higher the vacuum signal, the greater the timing advance.