The EVO 8 uses a dual voltage fuel pump. The stock pump is what we would call a 160 LPH pump. As we found out a long time ago, if you increase the voltage to the fuel pump with better and more direct wiring, it will flow more. Mitsubishi is doing the same thing here. When demand is low, they run the fuel pump voltage through a resistor. When fuel demand is higher, the ECU triggers a relay to run voltage straight to the pump instead of through the relay.
This same circuitry allows the use of much larger upgrade pumps with out adversely affecting idle and part throttle air fuel ratios. On DSMs, when guys would run a Supra or Walbro 255 lph pump on, they would run too rich at low fuel demand times. They would have to run an aftermarket higher flowing fuel pressure regulator. EVOs seem to get away with out needing this and this additional circuitry seems to be why.
Here is how Mitsubishi officially describes it:
FUEL PUMP CONTROL •
•
A control function has been added to change the fuel discharge volume of the fuel pump in two stages in accordance with the engine load. When the load on the engine is low if the fuel consumption volume is also low, this control actuates the fuel pump at a lower speed. As a result, the fuel return volume decreases, reducing the discharge of fuel vapors.
DESCRIPTION OF FUEL DISCHARGE VOLUME CONTROL When the load on the engine is low, the engine control module (ECM)
turns the power transistor ON,
When the load on the engine is higher than the prescribed value, the ECM turns OFF the power transistor, which causes the contacts in fuel pump relay 3 to close. This eliminates the voltage drop by the resistor. As a result, the pump speed increases, thus increasing the fuel discharge volume of the fuel pump.