Timed or manifold vacuum advance?
Vacuum advance is for economy only. The mechanical advance is for power.
Disagree. Deleting the vacuum advance does more than worsen fuel economy, it also negatively affects driveability. Remember, the combustion chamber and the fuel and oxygen molecules in it don't know or care what mechanism advances the spark. The effect of spark advance (however it is achieved), up to a point determined by the characteristics of the engine and fuel and some other factors, is to increase power output. If all you ever do is floor the accelerator, then vacuum advance won't do anything for you. But if you spend any time at part throttle, vacuum advance will do a great deal for you.
Let's spend an afternoon doing back-to-back test drives in street-driver cars with the vacuum advance present and base timing set at nominal spec, vacuum advance present, base timing at nominal spec, and mechanical advance curve increased, vacuum advance absent and no other adjustments, vacuum advance absent and base timing advanced, and vacuum advance absent, base timing nominal, and mechanical advance curve increased.
The second-case setup (vacuum advance present, base timing nominal, mechanical curve increased) will in most cases not only get the best fuel economy but also be the most driveable, with the most responsive part-throttle acceleration and most stable cruise. The first-case setup (vacuum advance present, nominal timing, stock mechanical curve) will in most cases be second-best.
blackhand said
It seems that a little more advance in some cases causes the mixture to burn at a lower temperature.
There are several means by which an advanced spark at idle causes lower engine temperature and a retarded spark at idle causes higher engine temperature, but this what you say is not one of them. An advanced spark tends to raise the idle speed, so the idle speed screw on the carburetor is backed off to bring the idle speed back down. Because the throttle plate isn't open as far, there's less air joining the fuel pulled in via the carburetor's idle circuit; the mixture for any given idle speed is richer and richer mixtures burn cooler. In some cases, factory-installed equipment (TIC valve) sent manifold vacuum to the distributor vacuum advance when the engine reached a certain temperature; this increased the idle speed which in turn increased the speed of the water pump and radiator fan, cooling things down.