MicroSquirt.com read only forum (Jan.15,2018)

How important is this?

With and without air conditioning...

Vac advance emulation is quite important for maximum fuel economy. I'm not sure where you're going with the AC reference though...

-Mike

Thanks, Glen - my initial message was quite unclear!

Vacuum advance is designed to provide maximum advance under light load (lowest MAP/highest vacuum) and retard timing as the engine is loaded (MAP goes up) to prevent knocking.

If you are using the MegaSquirt to control timing, you should disconnect the vacuum advance (plug the hose at the other end) and adjust your timing map to accomplish the same function. That would be discussed in the ignition tuning forum.

if the ignition timing used is the vacuum advance of the distributor, would that also have a big effect on the VE or amount of fuel on different loads?

Ignition timing is the result, or output. In a mechanical distributor, there are two inputs that adjust timing; engine speed (centrifugal advance) and engine load (vacuum advance).

MegaSquirt accomplishes the same thing using a table, with engine speed (derived from the crank or cam sensor) and load (from the MAP sensor) as the independent variables, and the amount of advance from base timing as the result. Using a table gives you precise control over advance at all points on the timing map.