The Edelbrock is the only Multi-Port Injection (MPI) system, which has a injector for each cylinder. The Holley, and the other units, are a Throttle-Bottle Injection (TBI). Which has 4 (I think) injectors in the main throttle body. It works somewhat like a carb, where as the MPI throttle body is only used as an air intake and doesn't have fuel flowing through it (unlike the TBI). The MPI injects the mixture right before the head on the intake manifold side. The TBI injects the fuel through the throttle body.
