There's no "size" to an intake manifold. There are physical dimensions such as runner diameter and length. Some manufacturers change those to change the "personality" of engine response, but mainly you have to look at bolt pattern for the carburetor. Some manifolds have two sets of drilled and tapped holes to accept two different carburetor flange sizes, but you also have to look at the size of the holes for the throttle blades. A lot of people swear by GM's "Quadrajet" four barrel carburetor. Those have real big secondary throttle blades so the manifold has to have big openings as well. You can spot those a mile away by their two different size holes.
Holley and Edelbrock both make aftermarket carburetors that bolt right onto those GM manifolds. Some of them also have two sets of mounting holes to fit more applications.
A 650 is a good all around choice for normal street and highway driving. Your present two barrel has to flow enough cfm to sustain high speed highway driving and high-speed acceleration. That volume makes it somewhat inefficient at low speeds. The four barrel carburetors have smaller primaries which improves low speed efficiency, and the larger secondaries gives it the higher volume needed at higher speeds.
Friday, October 5th, 2012 AT 7:10 PM