The height from the baseplate to the air cleaner base ring on both the Q-Jet , Hollies, and Demon carbs is 5.25 inches.
The reason the Holly or Demon type carbs give you more bottom end,
lies in the fact of the mechanical linkage controlling the secondaries, vs
the air flap on the Q-Jet. But then again, this depends on how you have the tension adjusted on the secondary air flap spring.
The Holly or Demon type carb will not give you the hit when the secondaries kick in like the Q-Jet, but will provide a more smooth constant
pull.
Generally speaking, the Q-Jet will give you better mileage than the Holly
type carb due to the fact the metering system is very percision for a
carb. We have to remember, GM spend a ton of money developing this
carb just prior to going all fuel injection for the EPA.
At the stock ZZ4 and 502 level of performance, both types are great carbs. On a race type engine, the Holly or Demon will usually provide
more performance.
Geez