Even tho it is for racing, I have consistantly used a Vacuum Secondary Holley 600 CFM carb for the last 10 years. If the carb is set up correctly it is a lot smarter than I am. I did work with the dash pot springs to get the optimum control for my use. It was originally built by the Carb Shop in Ontaria Ca. and since been refreshened by Sean Murphy in Huntington Beach.
From my experience, stick shift cars can get away with mechanical secondaries but automatics perform better with vacuum especially off the line. Good Luck
