An increasingly popular upgrade is installing a Version 8, 2.5 liter STI engine into the WRX. The increased displacement and stroke really goes a long way to making a tolerable 400+ WHP daily driver but this conversion requires a few changes in order to use the WRX ecu.
The following information is accurate to the best of our knowledge and should only be used as a guide. Always consult a professional or the Subaru service manual when questions arise. Use this information at your own risk.
Why install the entire engine instead of just the Version 8, 2.5 STI short block with the WRX heads? The V8, 2.5 engine’s demand for air is greater due to the larger bore and higher piston speeds found in the longer stroke engine. Subaru’s V8 STI heads have larger valves, higher lift and longer duration cams, as compared to the WRX. The stock WRX heads will bolt up but they don’t work very well unless better cams are installed and or you have a reputable engine builder perform some head work. The other reason is the option to run the variable cam timing system of the STI.
Cam Position Sensor
The Version 8 STI engine uses two cam position sensors located on the back end of the heads. These sensors are mounted in the heads and are used to monitor both intake cams for the AVCS system. The Version 7 WRX engine however uses one cam position sensor that is mounted externally on the front of the head and uses the pick up points found on the driver’s side intake cam sprocket.
2 cam sensors
Cam sensors have 3 wires
Monitors both intake cams
1 cam sensor
Cam sensor has 2 wires
Monitors driver’s side cam sprocket
The US Version 7 WRX engine does not have the AVCS intake cam system of the Version 8 STI engine. Luckily the V8 engine has the same cam sensor pick up points located on the driver’s side intake cam sprocket as the WRX. While the V8 STI engine does not use these cam position sensor pick up points, the V7 WRX engine does so you will need to utilize them.
Mounting Cam Position Sensor
The mounting boss for the Version 7 WRX cam sensor is present on the V8 STI engine but the boss needs to be drilled and tapped to secure the WRX cam position sensor. Do not uninstall the oil feed line for the AVCS system even if you will not be using it!!!! This must remain to supply oil to the cams.
The V7 WRX cam position sensor gets mounted as shown.
Since the V8 2.5 STI engine has a longer stroke and a larger bore, getting complete combustion is more difficult and the risk of detonation is higher. Subaru specified a longer spark plug that sits in the heads deeper, aiding in the combustion process, and reducing the chances of detonation. The spark plug protrudes farther into the bore, possibly so the flame front does not have to travel as far since it sits closer to the center of the combustion process.
Intake Manifold Assembly
The V8, 2.5 STI engine has a drive-by-wire throttle system that is not compatible with the WRX ecu. Since the STI manifold is specific to the drive-by-wire system, you must use your WRX intake manifold and throttle body.
Remove the entire intake manifold, tumble generator valve housings, fuel rail system, and engine wire harness in one assembly. There are 8 bolts securing the assembly to the heads but make sure to unplug all electrical connectors before removing.
The orange connector is for the STI knock sensor but it will not plug into the WRX harness. You can change the connector or swap the sensor for the WRX version. Now is a good time to do this.
The V8 intake manifold is for a drive by wire throttle system and will not work with the standard WRX throttle body as is does not have the air bypass port for the idle control valve. The picture shows the WRX throttle body gasket on the V8 intake manifold so you can see there is no bypass port. If you do not have a WRX manifold you may be able to cut a bypass port into the V8 manifold.
We decided to use the V8 fuel system with side feed injectors but if you choose to use your stock WRX fuel system all you need to do is install your whole intake manifold assembly and engine harness in one piece.
The V8 STI fuel system is attached to the TGV bodies in one piece so all you need to do is mount the WRX intake manifold to the V8 TGV assembly. You must then install the WRX engine harness to the intake manifold assembly.
The V8 STI injector connectors are different so you must solder the V8 connectors to your V7 engine harness. There are 8 wires to solder. You can bolt this entire assembly to the heads now.
The V8 fuel pressure regulator is underneath the manifold. Don’t forget to hook up the boost pressure source to your fuel pressure regulator or say good bye to your new engine. The V8 fuel pressure regulators boost fitting is next to the throttle body.
Turbocharger Oil Line
The turbocharger’s oil feed line is different from the WRX as it is shared with the AVCS oil feed line. You can purchase the STI turbocharger oil feed line for your WRX or you can have a custom oil line made that feeds both the turbocharger and the AVCS system.
We opted for a custom oil line so a larger turbocharger could be fitted. The picture only shows one oil feed line for the turbocharger but a second line is needed along with a banjo fitting that has two outlets. The two line banjo fitting allows a second line to be run to power up the AVCS system and lubricate the cams.
Accessories and Miscellaneous
The V8 STI AC compressor and power steering pump has different fittings than the AC compressor on the V7 WRX. We swapped over the WRX AC compressor, alternator, power steering pump and the mounting brackets. The upper radiator hose is also a bit different so you will need to purchase the STI radiator hose.
The rest is no different than installing a stock WRX engine. Just be sure to have your ecu tuned for your new engine since the injector pulse width and ignition timing is based on 25% less displacement and slower piston speeds.