holley carbs

ok thanks, that cleared up alot, now im looking at buying a 650 double pumper and right now just have a used 600 holley with vacum secondaries on my 360 to see how i like the holleys. I like the 600 with v/secondaries alot better than my AFB. What is the advantage to running mechanical secondaries against vacum secondaries? would this be a good rout to go?(buy the 650 DP?)