Subaru EK20 Hybrid How-To - Engine Swap

Prepping The WRX Harness
The WRX doesn't have separate engine and chassis wiring harnesses like most cars. The two are integral, with the critical engine-related portions entering the engine compartment in three different places. To do this swap properly, i-Speed starts with a complete WRX harness, from the engine all the way back to the fuel pump.

If you can, buy a harness that's already out of the car. If you have to remove it yourself, the main branch of the harness is under the dash along the firewall. Pull the dash, the dash bar, the heater core, and then pull the extremities of the harness in toward this section. If nothing else, it'll be good practice for when you do the same thing to integrate the WRX harness into your car.

First, you should identify and label every engine-related part of both the WRX harness and the one in your car. On the WRX harness, there are two firewall grommets. The passenger's-side grommet contains wires for the MAF, boost control solenoid, EGT probe, front O2 sensor, and passenger's-side tumble generator valves. Keeping the engine compartment portion intact, you should be able to strip this portion of the harness down until it's one clean bundle from those connectors to the ECU.

The grommet behind the intercooler has the wires for the rear O2 sensor, gear position sensors, and speed sensors. You'll want to keep the O2 sensor plug, but the plugs on the transmission tend to be different between the WRX and the earlier cars, so it's simpler to cut these wires and splice the new engine harness to the car's transmission harness.

A third branch of the harness runs down the left front fender and terminates in two large connectors, one brown and one black. For simplicity, we've dubbed this branch of the harness the Very Important Branch (VIB), and the brown and black connectors the Very Important Connectors (VICs). The VICs house wires for the oil pressure switch, power-steering oil pressure sensor, driver's-side tumble generator valves, idle air-control valve, coolant temp sensors, Throttle Position Sensor, Manifold Air Pressure sensor, the ignition coils, engine ground, purge control solenoid, and fuel injectors. This VIB also carries power from the main power relay. All of the VIB, except the main power wires, go to the Super Multiple Junction or SMJ, (Subaru's phrase, not ours) a large connector under the left side of the dash that allows the VIB to be separated from the rest of the harness fairly easily.

There should be a fourth branch of the harness going back to the fuel pump and evaporative controls. You'll need this whole branch, the WRX fuel pump, and the fuel pump controller. The controller varies the fuel pump voltage, giving more pressure when the ECU asks for it. Naturally aspirated Subarus run the same fuel pump voltage all the time.

With these four branches isolated, there should still be several wires scattered throughout the harness that you need to identify and free from the harness. Double check with that wiring diagram you got, but you should be looking for wires for the defroster, illumination, OBDII connector, the ECU reflash plug, and the check connector used for Subaru's diagnostic tool. For many of these, it's easiest to cut the wire and splice it to the existing wire in the new car.

Installing The Harness In The New Car
Most of the work happens inside the car, so remove the dash, dash bar and heater core. Now that you're good and pissed off, take a break before tackling the wires.

There are two basic branches of the ECU harness; one goes to the engine compartment and the other scatters throughout the chassis. First, you'll want to pull the engine compartment wires into the interior, identify them, and determine which need to be replaced with wires from the new engine harness.

Next, mount the new ECU and connect the new ECU connector to it. You can feed the engine compartment wires through the firewall one at a time, routing them neatly to their final destinations. As you do this, keep them bundled neatly; you'll eventually wrap these and they'll become your new harness.

Finally, one by one, you'll need to identify all the chassis wires and splice them to the chassis harness. As you do this, you'll be trimming these wires from the car's original ECU harness. After somewhere between 30 and 50 splices, you'll find the only thing left connected to the old ECU connector are the engine compartment connectors you already replaced.

You did it. You should test start the car before wrapping up the harness and re-installing the interior. By now you'll be dying to crank up the boost and go break off some Camaros, but have patience. Be sure to bolt the steering shaft back in properly and have some wrench-savvy friends check over your hack job before you crank the boost. And one last thing: Shift gently.

Swap Basics
*Chassis: First-Generation (GC8) Subaru Impreza, Outback Sport, or Forester, or 1995 or newer LegacyThough not as rigid as the current model, the first Impreza was significantly lighter. Depending on trim, the difference can be as much as 400 pounds. Other than that, the two cars are substantially the same, with virtually the same suspension, brake and steering systems. The Outback Sport is just an Impreza with two-tone paint; the Forester an Impreza with a taller, boxier body on it; and the Legacy basically an XL Impreza. Mechanically, they're all brothers.

*Engine: EK20, Pulling the engine from a U.S.-spec WRX and doing the conversion thoroughly will allow you to register the car legally, even under the strict eye of the California smog man. If that's not an issue, pulling the engine from a Japanese model can get you substantially more power, but be warned; the wiring can be very different on Japanese cars.

*Concerns: Mechanically, this is as simple as swaps get. Electrically, though, it's a mess. You don't have to do anything fancy to put a WRX wiring harness in your Impreza, but you do have to fearlessly disassemble both your harness and the donor harness. Be sure you can read a wiring diagram.