Subaru EK20 Hybrid How-To - Engine Swap

What And Why
Why? Maybe you wanted a WRX back when they weren't available here, and now that they are, you can't muster the same excitement. Maybe it's the styling, the dynamics (most agree the old car offered livelier handling), weight, cost, or some neurotic desire to own a fast Forester or Legacy. Don't be ashamed, you aren't alone.

Well, unless you're the Forester guy.

Besides these obvious benefits, the older, naturally aspirated cars also offer shorter gearing, turning the turbocharged EJ20's boost into thrust far more effectively than the long-legged WRX gearbox does.

Picking The Chassis
The Impreza body goes all the way back to 1993, so the options are nearly endless. Here are some guidelines, though.

Start with an all-wheel-drive car. Unless it's free, the cost and effort to convert a front-drive car to all-wheel drive won't be worth it. Unless, that is, you want a front-drive WRX. In that case, knock yourself out.

Cars from 1996 and later are easier (and all of them are AWD) because they share the basic OBDII wiring architecture with the WRX. This swap is possible with earlier cars, but more splicing and connector swapping is necessary.

Most of this Hybrid How To is actually based on a WRX-powered Legacy swap performed by i-Speed USA. These guys do WRX swaps all day long, and they say the procedure is effectively the same for a Legacy as it is for Imprezas or Foresters.

Picking The Engine
If you want your car to be legal in California, you'll need to get a U.S. engine from a car that's the same year or newer than yours. You also might want to shy away from engines newer than 2002 as we've noticed they tend to make a little less power (about 10 hp) than earlier engines.

If California legality isn't an issue, you can also consider a Japanese engine. There have been countless versions of the WRX engine in Japan. If you choose to go this route, though, you can only use this Hybrid How To as a rough guide. Early GC8 chassis (1993 to 2001) Japanese engines had a completely different wiring harness and engine management system than the U.S. cars. The GDB (2002 to current) engines have the same basic architecture, but are missing the evaporative emissions controls and have extra wires for the AVCS variable valve timing.

With either U.S. or Japanese engines, be absolutely sure you get a complete wiring harness and ECU with your engine. For U.S.-spec GDB WRX engines, you'll need the entire vehicle wiring harness, not just the underhood part. For earlier Japanese GC8 engines, you can just get the engine harness.

On its way to the turbo, A WRX's exhaust squeezes between the crossmember and the passenger's side cylinder head. You will need a crossmember from a WRX or 1990-to-1994 Legacy Turbo to provide clearance.

Picking The Transmission
Aside from the gear ratios, we couldn't find any definitive info about differences between the WRX and lesser Subarus. They all look the same, and the iSpeed boys know only that it seems to take about the same amount of time to break a regular Impreza transmission as it does to break a WRX tranny. As with any transmission, durability depends on how you treat it. i-Speed's Adam Levy, whose brain was the source for most of this information, has been running a 2.5 RS transmission behind his 260-whp WRX engine for 30,000 miles without problems. He knows others, however, who with only slightly more power, go through a transmission every other month. Your mileage may vary.

If you can treat the gearbox with respect, you want anything but a WRX transmission. WRX gears are way too tall, and the ratios in the naturally aspirated cars are much shorter. If you do switch to a WRX box, be sure to switch to a WRX differential as well, otherwise the front and rear wheels will go different speeds and you'll break the gearbox before you get out of the driveway.

The other option, of course, is an STi six speed. If you do this, remember that you'll need an STi diff, all four axles, and all four hubs. Everything STi is substantially beefier.

Whichever you choose, be sure to use a clutch designed for your transmission. Naturally aspirated cars use a push-style clutch, while WRXs use a pull-style; the two are not interchangeable. You'll need a stronger aftermarket clutch, of course, if you use the naturally aspirated transmission.

Subaru engines are most easily removed, sans transmission, through the top.	Holding the transmission in place is easy with a quick trip to the hardware store. The transmission is supported on the bracket that would otherwise hold the engine pitching rod that braces the transmission to the firewall.	A WRX crossmember (front) is notched to make room for the exhaust's path toward the turbo. You'll need this crossmember, obviously.
The WRX control arm bushing (on the right) is slightly wider than that on earlier generation cars, so using the earlier control arm means getting a few washers to fill the gap between the small control arm bushing and the larger mounting slot in the WRX crossmember.	The upper radiator hose can be made from two 90-degree hoses and a short pipe to connect them. Don't use the plastic hose splices commonly available at auto-parts stores. They don't last.	The WRX's pressure cap is in the small coolant tank on the engine. To properly integrate this tank into the system, you'll need to break out the pressure relief valve from the radiator cap so coolant can flow between the filler neck and the coolant tank.
Two of the three engine-compartment branches of the WRX harness are on the left, under the brake lines, and on the right, behind the intercooler.	The critical engine-related parts of the harness are integrated into the main chassis harness. You'll have to find and free them.	The WRX passenger's side firewall grommet contains wires for the MAF, boost control solenoid, EGT probe, front O2 sensor, and passenger's-side tumble generator valves. You should be able to strip this portion of the harness down until it's one clean bundle from these connectors to the ECU.