My engine has a standard aftermarket HEI with a Vaccuum advance. After my build was complete I set the idle timing at around 8-10 degrees and I never hooked up the vac adv to the carb. This was because as I recall the timing advanced by just the springs to an acceptable amount(Im sorry I dont remember the rpm or the degrees. My uncle helped me and it was last summer) My question is do I need to hook up the vac adv? What are the advantages or disadvantages of using it? Also what should final timing read and at what rpm for a 355? Sorry for the newbie questions

 

Vacuum advance will improve part-throttle response and cruise mileage, and reduce plug fouling.