Technically speaking the carb makes no vacuum of it's own.....The engine makes a vacuum on the intake stroke of a 4 stroke engine. What Paul is trying to describe is at what point the carb gets it's vacuum signal....you have Manifold ported and direct, and then venturi etc.... Venturi vacuum is caused by the venturi effect and depends on the total mass flow through the carburetor.
Guys, as air flows through the venturi there is a low pressure area developed that puls fuel from the fuel bowl. That same vacuum signal is what is used to operate the secondary throttle plates. The primary signal comes from the primary venturi but as soon as the secondaries start to open the signal is tied into both venturis. If it relied on manifold vacuum then the secondaries would never at wide open throttle - there is no manifold vacuum! They would open at an idle! That is when vacuum is highest - except when you are decellerating. Think about it...
I get the whole Venturi effect thingy Paul. well at least as good as somebody can that is not into fluid dynamics and the such.... IMO opinion it should be called just Venturi so as not to confuse most people. But again with out the Engine's Vacuum there would not be Venturi Vacuum as such a carb can not operate with out an engines vacuum/suction.... In other words for the Venturi Effect to take place in the Venturi(which is little more than a restriction, much like an orifice tube or expansion valve in an A/C system) there has to be either a vacuum source ( Engines are air pumps in /out intake/exhaust etc) or air forced through with a pump on the pressure side.... Blow through carbs with a supercharger would be an example of the force fed (of course they do both)....
The only time engines have vacuum is when the throttle plates are closed or cracked open, At wide open throttle there is no vacuum.( unless your carb is to small) This is when atmospheric pressure comes into play. In theory you have 14.7 psi of boost at sea level. But there is no vacuum at wot. As far as using a vacuum secondary with blow thru, I dont believe it will work,(unless you use a carb enclosure) Because it will be pressurizing the secondary pod, and or the check ball is going to seat and not let it do anything. It might be made to work by sealing the bottom of the chamber and rerouting the boost to act on the bottom of the diaphragm, but then why not just go with a mechanical secondary and not forgo all the trouble.
See now here is a guy who knows what he is talking about..... Yes you are correct I was trying to be ubber simplistic:bananaman and if you read my answer closely(I had to re-read it myself) I was not saying you could use a vac sec carb....I was merely stating that the Venturi effect is a result of the engines pumping action......Technically it is sucking in "That atmospheric pressure" and or pressure is being pushed through( I used a blower as an easy example) so people could see what venturi vacuum was!!!anyway I can see you have studied well... I really enjoy this kind of stuff if it pertains to the automtive world in the least bit
