[patrickk1]

4bbl carbs have small primaries and large secondaries. If you keep your foot out of it, you can cruise on just the primaries. The secondaries kick in when you put your foot into it (assuming it's not a double pumper). The quadrajet 4bbl has even smaller primaries that most 4bbl carbs. To me, if tuned perfectly, an appropriate sized 4bbl would have a mpg advantage because the smaller primaries would allow a more precise air/fuel ratio at cruise speed but when you hit the gas, more fuel is going to get dumped down the carb from the large secondaries. Some people say 2bbl get better mileage, some people say 4bbls do. But it could be that some 2bbl just die off around 60mph and its a struggle to get to 70, so you rarely travel at that speed with a 2bbl, which would lower mpg due to wind resistance of the vehicle. I am speaking of carbs in general, not specific to the charateristics of how these v6's run.

More air/fuel would get to the engine from the larger secondaries, if they are open. Less air/fuel if you are going through just the primaries.