Fuel injection systems come in two main forms. One is the TBI (throttle body injection). In this type of fuel injector system, there is a single or dual barrel throttle body and either one or two shower head injectors mounted above.
In the TBI, the fuel is atomized and distributed over the throttle plates and through the intake manifold. Throttle body injection is more efficient than the old-fashioned carburetor, but like the carburetor, it distributes fuel unevenly.
The other type of fuel injector system is the MFI (multiport fuel injection). In MFI, the fuel is distributed directly to each cylinder with individual fuel injectors. Therefore, the intake manifold is used only for air induction.
Much more efficient in design, the MFI injectors are mounted to aim at the back of the intake valves. Because of this efficiency and streamlining, the engine performance and fuel efficiency are superior, with the multiport fuel injection system.
Even the MFI comes in different varieties depending upon how much you want to spend. Some engines fire all of the injectors at the same time. Others fire in groups. But with SFI, or sequential fuel injection, you get the greatest fuel economy of all, because the fuel is injected only precisely when needed.