GROUP FIRED :
This injection system generally triggers a group of the injectors simultaneously. The ignition system or crank trigger provides the signal to the electronic control unit (ECU) for speed input and thus injector triggering.

The good thing about the group fired system is it's low cost. The problem is that while some injectors will be close to properly timed, others will not. If an injector is triggered at the wrong crank angle, fuel may collect in areas of the intake port or cling to the runner walls. When the intake valve opens, only a portion of the fuel injected will be in suspension when the air enters the cylinder, the remainder may be running down the port wall as liquid. This can cause erratic mixture conditions to exist in that cylinder, especially during low speed engine operation, due to the small amount of air movement in the intake runner.

Since EFI injectors turn on and off, the interrupted flow causes pressure waves to bounce around in the fuel rails, especially at wide open throttle where the flows are the highest. In a group fired system you often trigger two or more injectors at a time in a given fuel rail. The simultaneous pluses can reinforce each other at some rpm to give unusually high pressure pulses, sometimes causing poor fuel distribution. It often helps to run a larger diameter fuel rail. Kinsler extruded aluminum fuel rails are .680 inch inside diameter. A large diameter smoothes out the pulses quite well when compared to smaller sizes. Running higher overall system pressure also helps. We often run about 72 psi instead of the more normal 36 to 43. The amplitude of the pressure spikes will remain about the same, but will be relatively smaller based on the percent change in injector flow. We also like the better atomization achieved with the higher pressure and it sometimes gives better power and economy. Be careful about running the fuel pressure too high, as some ECUs don't have enough current to lift the disc or pintle in the injector consistently off the seat against this added pressure, which will result in poor fuel distribution. Be sure your fuel pump is capable of supplying the engine with the volume required at a higher fuel pressure.

SEQUENTIAL FIRED :
This system triggers each injector at a precise crank angle on every cylinder, usually near top dead center overlap (intake valve opening). This improves idle quality, low speed engine smoothness, and fuel economy. Some systems can even be programmed for different injection phasing for each speed site in the fuel map.

Triggering a sequential system is more complicated than a group fired system, as it requires a separate triggering signal to reference the start of the injector firing sequence. This signal is typically generated once every two crankshaft revolutions on a 4 cycle engine and is most commonly referred to as the " CAM" or "SYNC" signal. The sequential system also requires a "Crank" signal, generated at a specific crankshaft angle on each cylinder. This signal is used to calculate engine rpm and crank angle position for injector firing and ignition triggering from the ECU. "Crank" and " CAM" signal requirements will vary with each manufacturers ECU. Most manufacturers required either a sine wave signal, typically generated by magnetic sensor, or a square wave signal, typically generated by a Hall effect sensor.