To most people stateside, converting to STi spec is merely adding the light covers and OEM badges. To be considered a fully-converted STi means physically pulling out the U.S.-spec five-speed transmission and 227-hp engine and replacing it with the 271-horsepower, J-spec model. Yeah, yeah we know, the U.S.-spec, 2.5 STi is 300 horsepower, but we chose the J-spec model. In any case it's still better than the standard engine. Since there are very few people that have attempted the swap, we decided to give it a shot. Before we got our feet wet we first had to get our hands on the setup, and just as luck would have it, a gentleman by the name of Ben Lake happened to call me and tell me he could get a low-mileage Subaru engine through Japan. After a quick meeting with him it turned out they are able to get used engines directly from Subaru at very low prices. We went through a distributor located in Canada, Four Star Motorsports, and an engine setup was on its way. At the time of the swap we only expected the key components needed to complete it but we ended up with a complete transplant to convert not only the engine to STi, but the whole vehicle to an STi. This included brakes, tranny, wiring harness and ECU.
Physically looking at the two setups side by side the only major difference is the red intake, but if you go beyond appearances, any true Subaru owner will know what it takes to boast the badges.
When it comes to the STi engine's basic engine layout, it looks exactly like the U.S.-spec, 227-hp model but its main difference is its red intake manifold. What counts is what's inside. To start with, the bottom ends of the two engines are completely different. While the U.S. model comes with an open deck water jacket, the STi comes with a semi-closed deck for added strength. Other components that are beefed up to withstand turbo boost abuse include forged pistons and forged high-carbon steel connecting rods. Rods have been beefed up on the rod journal side and if any of you have thrown one of these out of the block, you know the most vulnerable point on the rod is closest to the rod journal. Adding more strength to the bottom end is a reinforced steel crankshaft, far superior to the U.S. spec. With just this bottom end alone, we have seen EJ20s from down under hit 450-and-up horsepower on the stock bottom end, even when they are daily drivers.
Not found on the U.S.-Spec, 227-horsepower engine is the AVCS system. The AVCS allows cam timing to be altered through the power band and the key to its success is the combination of the cam sensors and AVCS solenoid.
The key to putting the STi horsepower to the ground requires a transmission that can handle the power. For this we decided to stick with the STi six-speed transmission since it has been known to hold plenty of power. When comparing the five- and six-speed transmissions side by side the shear size of the six speed shows that it is more than an extra gear installed. In fact, other than the EJ20 engine's bolt pattern, the six speed doesn't share any similarities in gear ratio or design with the five speed. With a much longer tail shaft, we knew if we were to use this tranny we would have to swap out to a shorter driveshaft as well as the rear differential to match the front diff's gear ratio.
Since there are very few people that have attempted the swap, we decided to give it a shot.
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.
The key to putting the STi horsepower to the ground requires a transmission that can handle the power.
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.
As you can see, the size difference went from large to much larger. Trying to fit the six speed in between the cross member and firewall was extremely tricky.
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.
Getting the six speed installed was the tricky part: It's like wedging a round peg into a square hole. It takes some elbow grease but the engine and transmission will go in as one unit.
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.
The size difference with the differentials required everything in the rear driveline to be swapped out. Since the housing is much larger, the STi axles are shorter. The shorter axles also had a much larger spline in the diff as well as the hub. This meant we had to swap complete knuckles as well. | 
On the driver's-side harness, the black harness contains the AVCS solenoid wires. By jumping the wires up one row we were able to use the tumble valve wiring for the solenoid wiring. Keep in mind, we also had to jump the ECU wiring around |  |
For the cam sensor wiring, we were forced to add four conductor shielded wires. These wires connected to the same ECU plug as the AVCS wires. | 
Running the four cam sensor wires and jumping the now-AVCS wires was all done on the same ECU plug. Since the AVCS took over the U.S.-spec tumble valve wires, all that was needed was a quick pin jump. | 
A: Passenger's side cam sensor; B: Passenger's side AVCS solenoid; C: Driver's side cam sensor; D: Shielding for cam sensor wires; E: Driver's side AVCS solenoid. |
Since the six-speed tranny is much longer than the five speed, it was necessary to use the shorter driveshaft supplied with the STi unit. | 
The STi differential bolts right into the OEM location but the complete drivetrain must be switched to STi status. |  |
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Looking throughout the U.S. and STi diffs you can tell one is a spider-gear type while the STi LSD runs a clutch-pack type. | 
Pulling the five speed and engine was very easy and also done as a whole. |