[jhow66]

Vac. advance does one thing--advances your timing while you are under cruise conditions which improves gas milage 2-4 mpg. When you step down on it the vac. goes away causing the time to drop back to whatever you have set your initial timing plus what ever the mechanical advance is adding at that rpm. An example--12* initial + 22* mechanical (most HEI have this much)=34* total advance at wide open throttle. Same setup at cruise--12* + 22* +vac. advance *'s (some GM's have as much as 20*)=54*. This assumes that you are turning enough RPM to have all the mechanical in. There is no way you could run this much advance at wide open throttle as it would ping itself to death. You can run one without vac. advance but you will lose gas milage and smoothness at cruise. Race engines are not cocerned with gas milage so no vac. advance needed. Hope this helps a little.