A 4 barrel carburetor is a carburetor with four venturis and four throttle butterflies. They evolved from the 2 barrel carburetor, which evolved from the 1 barrel carburetor. The history of carburetion goes back about 130 years.
When four-cycle engines were first developed, fuel was a different substance from that used today. Early engines didn't use carburetors, but a wick submerged in fuel. This was like the kerosene lamp familiar from camping trips. Air entering the engine passed the wick and absorbed fuel as vapor. This mixture was then ignited, in the cylinder, by a spark or hot wire. The expanding hot gases pushed the piston down, turning the crank and generating power. These early engines turned very slowly, and it didn't take much fuel to operate them.
As engines became larger and more powerful, they required more fuel than simple evaporation could provide. Someone, there is little evidence to support any one man as the inventor, devised the carburetor. The carburetor used the vacuum in the intake tract of the engine to draw fuel out of a bowl. Soon, carburetors became so efficient that they needed to be throttled back to keep them from running away. The invention of the throttle resulted in higher vacuum, but lower air velocity. It was soon understood that it was the velocity of the air passing through the carburetor that pulled the fuel out of the bowl. This took advantage of the venturi effect, drawing fuel into the air stream with the low pressure created by the high speed of the air.
As engines turned faster, and generated more power, the diameter of the carburetor throat increased. At a certain point it became so large that the venturi effect was reduced in strength. To keep this essential process intact, more than one carburetor was fitted. It quickly became obvious that a carburetor with two venturis was cheaper to produce than two separate carburetors, and the two barrel carburetor was born.
The 4 barrel carburetor is actually a pair of 2 barrel carburetors joined together in one piece. The primary barrels act as a 2 barrel carburetor during cruise and slow running. When more power is needed than the primaries are capable of producing, the secondaries open. These additional barrels are opened either mechanically or by engine vacuum. The vacuum method works better with automatic transmissions, as it is more easily adjusted.
4 barrel carburetors, while capable of making more power than a 2 barrel carburetor, will generally return higher gas mileage. This is because the smaller primary venturis operate more efficiently in the cruise mode. If one goes about with the pedal against the floorboard, mileage will suffer. This makes the 4 barrel carburetor the best choice for a replacement or new application on any engine that will accommodate it. 4 barrel carburetors are also generally less expensive. Tuning and adjustment of them has become widely known, and getting a new one going rarely takes more than an hour or two.
