Fuel System HarnessIt's no surprise that OE engineers designed fuel systems to match the set parameters and performance of a vehicle as it comes straight from the factory. As we tune our cars and extract the power outside of the stock box, the amount of fuel the engine needs to run safely and efficiently goes up. More often than not, the stock fuel pump has to be replaced to handle the sufficient volume of fuel needed. The misconception here is that you need more fuel pressure when actually it's the volume of fuel that needs to be increased when you bump up the duty cycle on the injector or change to larger-sized injectors.

Fuel pumps are electric motors that are voltage dependant. The more voltage you add, the harder it works (in a safe range of voltage, of course). In the case of newer model vehicles-like the Mitsubishi Evolution-the pump runs on a staged voltage, increasing as more throttle and load is added. On that premise, ensure that the pump is running the maximum amount of voltage under wide-open throttle.

Unfortunately for Evos and other vehicles using the staged voltage setup, the pump doesn't always get the maximum amount of voltage. The fluctuation in pump capacity is due to the voltage having to travel through several hoops (i.e. the fuse box and ECU, before it finally reaches the main pump). Along the way, there's a drop in the voltage signal-often seen on the dyno when attempting to tune at a higher rpm. To remedy this problem, we found a simple yet effective solution that can be done in the comforts of your own backyard by using a over-the-counter relay harness to safely divert battery voltage from the battery straight to the pump.

Here are some solutions for a relay harness on two cars that are common candidates for fuel pump upgrades. For the first vehicle, an Evo VIII CT9A, we went with an Auto Produce Boss LAP pump relay harness, which is a popular upgrade among Evo owners overseas. The AP Boss harness is premade with factory-type Molex connectors and high-grade wire shielding. The harness is a plug-and-play unit that will work on the Evo 7, 8 and 9, including the MR. We also opted to swap the fuel pump with an AP Boss 240lph LAP fuel pump, as the owner had plans to increase horsepower in the near future. The second vehicle we put under the knife was a '91 240SX S13. The 240SX with aftermarket turbocharged SR20DET swaps in the states demands more fuel for obvious reasons. To remedy the issue of pump voltage here's a step-by-step process on how to build your own harness from scratch.

Tools and supplies you'll need: 1/4-inch ratchet, 10mm shallow socket, 10mm deep socket, 8mm socket, 3/8-inch ratchet, 14mm deep socket, 10mm open-end wrench, Phillips head screwdriver, flat head screwdriver, panel popper, needle-nose pliers, dikes, pick set, crimpers, soldering iron, razor blade, cable ties and rags. Note: Make sure you're working in a well-ventilated area with access to a fire extinguisher.

Step 1
Remove the negative terminal from the battery. Make sure you have the radio code or you'll have to go to the dealer. Loosen the fuel cap to relieve fuel pressure.

Step 2
Locate the tab under the rear seat. Pull the tab while pulling up on the rear seat to remove it. To give access to the carpet, remove the step covers by the door. Remove the two clips on the far side of the carpet and pull back to expose the fuel system harness. On the passenger side, locate the pin eight connector D-12 and pin one connector D-30. To provide easier access to the connectors, remove the two screws holding the harness down.

Step 3
Connect the relay harness inline and find a suitable place for the relay that's not in the way of passengers' feet.