Can anyone give me some advantages and/or disadvantages between vacuum secondary 4bbl's and mechanical secondary 4bbl's? I'm in the market for a holley 650cfm, and I've always used mechanical, however, I'm being told I should use vacuum for this particular application. So I'm curious what differences there are, and what effects they have on performance. Also, are vacuum secondaries adjustable? Thanks in advance!!!

SteveT

 

Vacuum operated is definently more streetable! The secondaries will only open when intake pressure dictates that it is needed,and,yes it is very adjustable with different springs available.Mechanicals are better for radical engines above 5,000 rpms.
Phillip

 

Phillip is "right on". The only thing I might add is the vacuum operated carbs will give you more low end torque and work better especially with automatics where stock stall speeds are approx. 1600 RPM. Save the "mechanical" for the race track.