Chevrolet: ignition timing, vacuum advance, timeing

Question
what is vacuum advance? and as rpm increases, should timing advance or retard? we put a timing light on #1 cylinder on my brothers truck and timing mark retards to 4 deg atdc.(from 0)

Answer
Joe;
 The vacume will cause the dist to go fwd. a bit. and with the timeing, you should remove the vacume line from the dist and plug it the vacume end.. than turn the dist to the factory timeing for the truck. and the idle must also be at the factory  setting , this way you will get the right timeing when doing so.  ok
there is a differnt way to time when the engine is at a higher rpm, but not for the truck.