What are the differences of square bore intakes verses spread bore intakes?

Answer 1

Square bore and spread bore are terms used for 4 Barrel caburetors and the matching intake manifolds the carburetors are mounted on. The term(s) mostly describes the carburetor in normal usage but applies to both carb and manifold.. Square bore carburetor's venturi's are of equal size, ie 4 holes of the same diameter. The primaries generally open first , the secondaries open next with vacum or air flow through the carb air horn or sometimes through linkage designed to provid sequencual opening. On some hot rod/very high performance applications all 4 venturi's open together, generally for strictly perfomance motors where fuel mileage is not considered. Square bore carbs generally offer a little more power and a smoother power range. Spread bore carburetor means unequal in size venturi. These carbs have 2 equal size small venuri's plus 2 equal size large- huge, venturi's. The small venturi's are called primaries and are used for normal low power application, idleing, crusing at highway speeds for just normal easy driving. The primaries generally run a little lean on the jetting. The secondaries are much larger and used when max power is needed. The secondaries begin to open when the throttle is normally somewhere near or past or well past half throttle and open up quite quickly. The secondarie are open by vacum or flow through of air through the air horn, by demand. Rarely are they open by throttle linkage alone except at full pedal. At full throttle both the primaries and secondaries are fully open. The secondaries jetting is normally set richer than the primaries. Spread bore carbs generally deliver better fuel economy. If you drive them hard you soon lose the advantage of the better fuel economy. The intake manifolds match the bore sizes specifically for each type carb. You can change back and forth between carb types w/an adapter, however there is a small loss in performance potential when the carb and intake maifold are not matched... adapters work but not quite as well as with a matching carb and intake manifold. Edelbrock and others make after market intake maniflods that have large bores for spread bores that will work quite well with both style caruretors.