Vacuum advance mandatory for a street car?

Vacuum advance will improve engine performance at light throttle. It adjusts the timing per the engine load and associated mixture. At light throttle, light load, fuel mixture is leaner. Leaner mixtures burn slower, so more advance is required to get the burn started earlier. The advance related to the load component helps keep engine speed stable for varying load condition. An example is nearly stable vehicle speed while negotiating hilly terrain. An engine with insufficient advance will labor, and require more throttle to maintain speed. The retarded timing results in late fuel burn on the way out, so it heats heads, exhaust and engine compartment.

An engine with correct vacuum advance will have better throttle response, and better economy at low to medium throttle. Even cruise at highway speeds is light to medium throttle. Vacuum advance typically provides a maximum of 10 to 20 degrees adder, to the mechanical advance.

Here are some of the problems. When different carbs are swapped, the ported vacuum characteristics may change. If the idle speed is adjusted too high, it is possible the ported source will leak at idle. Vacuum advance diaphragms develop leaks, and fail to operate. Vacuum advance settings are a compromise, special fabrication may be required for improvement in advance rates. Much trial and error.

Electronic advance in modern cars use many controls to get the correct advance. There is much to gain with proper advance control, in performance and economy, when using a 3D RPM/MAP timing control.