Subaru STi Conversion - J-Spec Model

Adding the limited slip differential in the rear required much more work than what we originally thought. Since the diff bolts right to the factory location, we figured the axles would plug right in. We then realized the STi LSD ran a larger spline and had an extra one-and-a-half inches in width to it. The wider differential made it impossible to use the factory axles. The STi axles were used but then we ran into another problem. The axles' outer splines were also much larger, forcing us to swap out the OEM rear knuckle with an STi unit. With the rear STi assembly bolted up this allowed the six-speed driveshaft to be installed without any modification.

One of the real keys to the STi's success is in the cylinder head. The cylinder head incorporates a variable cam timing system known as AVCS (Active Valve Control System). This seems to be the trend with manufactures today and for good reason; not only does the engine benefit in emissions, it also enables the car to make much more top-end power. With the addition of the AVCS system incorporated, the intake manifold tumble valves were removed. Later in this story the tumble valve wiring will come in handy.

The chassis has only undergone minimal changes over the years and there weren't any changes from our STi engine with our U.S.-spec chassis. As far as pulling the engine out, we were able to do it with the transmission connected. Getting the six speed installed was the tricky part: It's like wedging a round peg into a square hole. It takes some trying but the engine and transmission will go in as one unit. Everything, as far as motor mounts and the cross member, are similar to the U.S.-spec WRX and didn't require any modification.

Back under the engine bay, we had yet another dilemma. When it came to wiring the engine we were stuck on two options. We have the STI engine harness and cabin harness so we could either take the dash out and wire up the whole car or we could figure out what the main difference was between the U.S.- and STi-spec. We ended up taking an extra day figuring out the wiring to simplify the process, and with help from Irvine Subaru and XS Engineering, we were able to narrow down the wiring to eight additional wires over the U.S. spec. Now we were left with using the STi engine harness with a modified U.S.-spec cabin harness. What made life much easier was that the STi doesn't use the tumble valves found on the U.S. spec and the wires that control the valves run in the same plug as the AVCS wiring. Not only do they share the same engine plug, they also share the same ECU plug. Now the wiring work is down to hooking up two cam sensors to the ECU. There are four wires total but the tricky part was that the wires are for magnetic pick up. This also requires putting the necessary shielding around the wires which adds one additional wire to the mix. The only wires that were left from the U.S.-spec harness not needed on the STi, included a tumble valve position sensor and an exhaust temp sensor, but unfortunately neither are shielded nor run in the same harness as the OEM plug for the cam sensors. What was needed was to physically run shielded wire from the engine harness to the ECU. On the driver side of the engine harness, we found that by moving four of the wires on the brown plug up one level the once-tumble-valve wires were now converted to the AVCS solenoids. On the ECU side, four wires needed to be jumped to a different pin location but all on the same ECU plug. Even the cam sensor wires shared the same ECU plug as the AVCS wires. Eight simple wires converted our normal U.S.-spec harness to STi trim, saving us from the headache of pulling the dash apart to wire the car.

It was a very time-consuming process but with these quick notes we are sure any Subaru owner will be able to convert their car as easy as we did. Take our advice, run the extra wires rather than pulling the whole cabin harness out. It took us a day to figure it out but will only take you 30 minutes to wire the sensors up. There's only one thing left to do in order for us to call this a real STi (in wagon trim). We added the badges and light covers and now we can boast true STi Spec.