A mechanical adv. is in your stock dist. and most every other dist. out there. Some racers lock it out so the timing never changes do not do this on your car. A vacuum adv. is good on a street application because it ads even more advance when cruising at light throttle or when coasting downhill at highway speeds at even lighter throttle IE it helps with fuel mileage. You may or may not want vacuum adv. I personally do not its more of a preferance on a mild build, and more of a nuesence (forgive my spelling) on a perfomance build.