The Short Answer
Spreadbore and Squarebore are terms used to describe the 2 major designs of 4-barrel carburetors. In a spreadbore design, the primary bores are smaller than the secondary bores. In a squarebore design, both the primary and secondary bores are the same size.
The Longer Answer
The bores (also known as barrels) are parallel chambers going from the top of the carburetor body to the bottom (base) of the carburetor body. They are the chambers where fuel and air are mixed together to create the combustion charge that makes the engine run. At the bottom of the carburetor body there are butterflies, which are movable valves that open and close allowing varying amounts of the fuel/air mixture to enter the engine. These can be seen in Fig 2 below.
Most 4 barrel carburetors have a pair of primary bores and a pair of secondary bores. The primaries are worked directly from the accelerator pedal; as the driver pushes down the pedal the primary butterflies open. As the accelerator pedal is pushed further, the primary butterflies are fully open and a linkage on the side of the carburetor causes the secondary butterflies to open. When the accelerator pedal is fully depressed both the primary and secondary butterfly valves are fully open.
The why of it
Spreadbore carburetors (such as the Rochester Quadrajet and Carter Thermoquad, to name just a couple) came about in order to improve the drivability and fuel economy of passenger cars, while preserving the capability to flow sufficient air to operate the engine at full power when the accelerator pedal is floored. Using smaller primaries spreadbore carburetors offer quicker throttle response and better fuel economy at low power settings (such as in-town driving) than squarebore carburetors. At full throttle settings both designs (if properly tuned) are effective in getting fuel and air to the engine. Higher performance carburetors (such as Holley competition carburetors) are optimized to provide maximum airflow under full power settings, at the expense of low power setting throttle responsiveness and low power fuel economy, and generally use a squarebore design.
Can I swap a squarebore / spreadbore carburetor?
Yes, however it's not as simple as unbolting one and bolting on another. The intake manifold may be set up for one design of carburetor or another, and an adapter is generally required to complete the job. We offer an adapter that allows for installing a spreadbore carburetor onto a squarebore manifold, or vice-versa.