JDM EJ205 24 PSI on Stock 2.5RS ECU

MY2000 Impreza EJ251 ECU Running a Turbocharged EJ205

The Subaru Impreza 2.5RS sedan and coupe have been a popular platform for turbo swaps for a while now. Typically the donor turbo engine was not compatible with the factory EJ251 ECU and required a complicated wiring harness merge to get the car running on a swapped-in turbo ECU. Here you can see how it can be done without the wiring hassle by keeping the EJ251 ECU and using our projectLAMBDA Tuner to get it to play nice with the turbo engine. With the engine harness connectors being nearly the same, the swap becomes almost a plug and play affair.

For this build, an EJ205 from a JDM Forester SF5 was swapped in to replace the original EJ251. Other suitable candidates are the EJ207 and EJ205 from the Impreza GC8 and GF8 WRX/STI. A lot of the sensors and wiring are shared between these 2 engines despite the fact that one is turbocharged and the other is not. Both come with a 32-bit ECU although the JDM runs on a mass airflow sensor, has different harness connectors and does not support OBD2 protocol. The USDM NA ECU uses a speed density fuel calculation instead of MAF and also has a Bosch LSU 4.2 wideband front O2 (lambda) sensor from the factory so that needs to be taken into account when doing the swap. The USDM ECU also does not have boost control but we are going to change that in the future. For now a manual boost controller was installed to keep the turbo under control.

The original EJ251 intake manifold has a Bosch 1.0 bar TMAP installed in the plenum to measure manifold pressure and air temperature. Since the EJ205 does not have any provision for this, a GM intake air temperature sensor was installed to support the ECU’s speed-density fueling. A 3.0 bar MAP sensor measures boost off one of the intake manifold vacuum ports. Another cleaner option would be to modify the manifold to fit a Bosch Motorsport 3.0 bar TMAP. This gives minimal measurement delay on the MAP signal versus a remotely mounted MAP sensor.

The owner stepped up from the original TD04HL to a modified VF39, then finally an FP Blue along with supporting mods to reach his power goals. In its current form, the engine is tuned for up to 24 psi of boost on 93 octane pump gas, but 22 psi seemed to be the sweet spot for power and torque. This could be a limitation of the unmodified EJ205 heads and cams. Not bad at all considering the JDM EJ205 tune has a boost target of only 10.6 psi. The end result is a cool-looking daily driver that can back up its looks with performance.

Build Info

  • MY2000 Impreza 2.5RS sedan chassis
  • JDM EJ205, stock block and heads, 9.0:1 CR
  • FP Blue Turbo with EWG, 1.0 bar wastegate spring
  • DW1000 Injectors
  • VA STI top mount intercooler
  • Header, downpipe, exhaust

Posted in EJ251, Tuning.