An engine's air/fuel ratio must be accurately controlled under all operating conditions to achieve the desired engine performance, emissions, driveability, and fuel economy. Modern Hilborn electronic fuel injection systems meter fuel very accurately and precisely, and closed loop fuel control based on feedback from an oxygen sensor (or O2 sensor) lets fuel-injected engines run considerably cleaner than comparable carbureted engines. Properly designed fuel injection systems can react faster and more precisely to rapidly changing inputs such as rapid throttle movements, and can tailor fuel distribution to closely match the engine's needs across a wider range of operating conditions such as load, ambient temperature, operating temperature, fuel quality, and altitude (i.e., barometric pressure). You got that?

Carburetors Versus Eight-Stack Injection
Hilborn EFI specialist Star has written extensively on electronic fuel injection and he's given us permission to quote a bit of technical info that will hopefully be of interest-especially to those with a more technologically adept psyche than I.

"The carburetor can best be described as an air/fuel mixer that uses a differential in pressure to provide fuel at an established metered amount. Although easy to define, the actually workings of a carburetor are complicated, enough so that very few people are able to maximize its potential.

"With air movement toward the intake valve in the intake manifold, a result of piston movement and valve timing, the main venturi of the carburetor has air flowing past the booster creating a pressure drop. This pressure drop causes fuel to be pushed into the booster supplied by the fuel bowl via the main well of the carburetor. The shearing of the fuel as it enters the air stream out of the booster causes the fuel to separate into smaller particles, where it is picked up by the air and carried into the intake manifold."

The legendary Smokey Yunick says, "The carburetor is a big restriction in the intake system." This is because in a round column of airflow, flow speed is fastest toward the center of the column." The design of the carburetor places the booster toward the center of that column in order to receive the strongest signal, or pressure drop at the booster, for maximum booster performance. Since airflow is impeded, the direction of part of that flow is diverted into eddies of spinning air that will disrupt the rest of the airflow around it.

Float bowl volume is essential for correct air/fuel ratio. With a needle and seat size a nominal .110 inch for gas, coupled with fuel pressure of 6 to 8 pounds per square inch at the needle and seat, it is difficult to keep the bowl filled. Carburetors use atmospheric pressure to provide lift of the fuel, therefore as the bowl empties, an increase in pressure drop is required to lift the fuel up the main well into the booster, compromising consistent air/fuel ratio. Fuel bowl problems also manifest themselves in applications that incur aggressive changes in direction, as in road race and autocross applications.

The cfm ratings for carburetors were introduced as a way to identify correct carburetor sizing for specific applications. In actuality, only the throttle blade size is needed to identify size, since a venturi will only flow so much air at a certain depression. Any published cfm ratings, regardless of product, should also include their depression in inches of water. Unlike the cylinder head aftermarket, where the standard for cfm ratings is 28 inches of mercury for pressure drop, there is nothing published for the carburetor aftermarket. As an example, all of Holley's 750 cfm carburetors use a 1.375 primary and secondary throttle blade, yet there are many companies that claim up to 950 cfm with the same throttle blade size. These types of exaggerated specifications only lead to confusion of the end user on what size carburetor will fit his needs. Qualified carburetor shops will sell a carburetor by throttle blade size and not by exaggerated cfm ratings.