Originally Posted by Silver Surfer
OK good point about the venturi effect, thank you. But back to the original question, does one method have pro's or con's?
The Holley vacuum diaphragm set up is tunable. It has different springs as well as the check valve system can be modified by using a lighter/smaller check ball or a check ball seat w/more or less air bleed bypass.
Downside to the Holley system is the diaphragm is hard to replace and costs quite a bit. The diaphragm can change over time by becoming stiffer w/age and this can affect the secondary operation. Also changing the spring can be a pain unless a quick change cover is bought or made. And modifying the check valve system is a trial and retrial deal that can be time consuming.
The Q-jet is also very tunable but it's a lot less user friendly, at least until one is familiar w/the Q-jet. The air valve spring is the first thing that most guys will work with to get the secondaries dialed in. But there are other things just as important if not more so.
The secondary air valve opening is damped by a vacuum chamber. This sometimes does double duty as a choke pulloff. The vacuum reaching the diaphragm is restricted by a normally nonadjustable orifice that can be drilled out to give a quicker response. Why no one has marketed an adjustable vacuum chamber for them is beyond me.
The amount of fuel entering the secondaries of a Q-jet through the secondary pull over tubes and orifices (acts as a passive accelerator pump) before the air valve is open far enough to draw fuel up from the float bowl through the secondary discharge tubes can be altered in several ways. This can have an effect on the opening rate as well.
As for the Edelbrock Performer set up (like the 1406), the air valve opens when the amount of airflow through the secondary is greater than the closing force of the counterweighted valve. The main problem w/this is the non adjustability. That's the main reason I'd use a Thunder series carb w/adjustable secondaries.
The upside is this set up has the least moving parts.