Electronic fuel-injection systems fall into two categories: throttle-body injection or multi-point injection. A throttle-body housing has as many as four electronically controlled fuel injectors, depending upon manufacture and engine power requirements. These injectors aspirate outside air into the fuel spray from the computer operated injectors in order to promote thorough mixing of the atomized fuel and combustion air in the intake manifold. A throttle-body is similar in appearance and function to a carburetor only in that it is mounted atop the intake manifold, and serves to introduce a mixture of fuel and air into the manifold for distribution via the intake runners to the intake valves. Simple in design, throttle-body-based fuel-injection systems are much more economical to operate than carburetors and have enjoyed many years of success. However, because they employ the principal we refer to as air/fuel pre-mix-defined as the process of achieving fuel and air mix away from the combustion zone in order to achieve satisfactory performance and efficiency-successful throttle-body injection systems must rely completely upon proper intake manifold design. When fueling through the intake manifold, the internal design of the manifold dictates how well and how evenly atomized fuel and combustion air are mixed, and then introduced into each cylinder for combustion. a poorly designed intake manifold will allow atomized fuel to adhere to and collect along rough internal surfaces, as well as permit the formation of air eddy-currents around sharp turns inside the manifold, both of which can interfere with the combustion process.

Unlike throttle-body systems, multi-point-more commonly called multi-port-electronic fuel-injection systems comprise a single air door and an electronic fuel injector for each cylinder. In a multi-port system, a fuel injector is mounted at the end of each intake runner or port to introduce pressure-atomized fuel directly into the combustion chamber in each cylinder. Air for combustion is introduced through an air-door (similar in appearance and function to the air-handling components of the throttle-body) and is aspirated through the intake manifold into the combustion chamber. In this system, the primary mixing of pressure-atomized fuel and combustion air takes place in the cylinder, rather than in the intake manifold, in a process referred to as air/fuel post-mix. While multi-port injection systems still depend upon a proper intake-manifold design for optimum performance, the elements of intake design peculiar to fuel handling can largely be ignored. The particular advantage of multi-port systems compared to throttle-body systems, is that pressure-atomized fuel is introduced nearer the point of combustion, thereby permitting cleaner, more efficient firing than with a throttle-body system with a minimum of unburned fuel carried into the exhaust. More sophisticated, sequentially fired, multi-port injection systems allow for the precise coincidence of fuel injector opening and intake valve opening, yielding maximum power and minimum waste from each cylinder. While multi-port injection systems are superior in operation and performance to throttle-body systems, they can be substantially more expensive to purchase and install. Thus, they are not necessarily a cost-effective alternative in every electronic fuel-injection application. Performance Injection Equipment employs throttle-body injection systems in small-block applications and uses the more robust multi-port injection systems for big-blocks.