This Dictates That - iWire Rules Of Thumb

This Dictates That - iWire Rules Of Thumb

Nov 18, '21
In this blog post we break down our key rules to make sure your project will be a "great success!"
For the following, with very few exceptions, you must follow the rule and not mix and match or else you'll run into trouble.

Heads dictate ECU Type

When picking an engine platform, you must consider how that will affect which ECU you can use. The head configuration is how you would choose an ECU if you don't already have a matching setup (NA to turbo swap scenario). Matching the head type to ECU would also be vital if you replace an engine with something else (my engine blew up or engine upgrade). If the features of the head (cam position sensor type and AVCS) don't match up with what the ECU is designed for, then the ECU would not be able to control the engine. A typical example would involve attempting to run a JDM STi engine in a USDM STi. Although both have AVCS, the JDM STi uses 2 wire cam position sensors. In comparison, the USDM STi uses 3 wire cam position sensors, making them incompatible.
Below are typical head configurations for turbo platforms. The ECU(s) underneath each section are designed to work with that head setup. Although there are many ECUs for each variation, you need to determine which ECU is the most appropriate for your application since each ECU is designed slightly different depending on the chassis. One significant consideration would be that all dual AVCS ECUs are also CANbus equipped, making interchanging these ECUs in other cars extremely difficult and likely not worth it for most situations.
2 wire Cam Position Sensors w/out AVCS
  • JDM V1-6 ECU (these are all non OBD2 so not likely a good option here in North America)
  • USDM WRX ECU (02-05)
2 Wire Cam Position Sensors and Intake Only AVCS
  • JDM WRX ECU (01-06)
  • JDM STi ECU (01-06)
  • USDM WRX ECU (02-05) - NOTE: These can run the engine but the AVCS will not function.
3 Wire Cam Position Sensors and Intake Only AVCS
  • USDM STi ECU (04-07)
  • USDM WRX ECU (06-14)
  • USDM Forester XT ECU (04-13)
  • USDM Legacy GT ECU (05-12)
  • USDM Outback XT ECU (05-09)
  • USDM Baja XT ECU (04-06)
3 Wire Cam Position Sensors and Intake + Exhaust AVCS (Dual AVCS)
  • USDM STi ECU (08-22)
  • JDM STi ECU (07-22) - Also referred to as V10 and newer

ECU dictates intake manifold and throttle type

The ECU configuration can only work with equipment and sensors it is designed for. Once the heads are determined and the appropriate ECU is picked, the ECU choice will determine which intake manifold and harness you can use. Suppose you have heads with a 3 wire cam position sensor plus AVCS and you decide to use a compatible 04 STi ECU. In that case, you must use an 04 STi intake manifold and engine harness because they are designed to work together It would not be possible to use a cable throttle body on this platform because the ECU would not know what to do with a cable throttle. The inverse would also apply to someone trying to install a drive-by wire throttle body while using an ECU (like an 02-05 WRX) designed with a cable throttle in mind.
  • 32 Bit ECU = 3 Wire Cam position sensor = Drive by Wire Throttle
  • 16 Bit ECU = 2 Wire Cam position sensor = Cable Throttle

ECU dictates neutral position switch type

There are two neutral position switch types (on transmission). One defaults to open and the other defaults to closed. The ECU needs this sensor to match so it can adjust accordingly if you are in neutral or in gear.
  • 2004 and Earlier Forester - NPS Defaults to Open
  • 2005 to Current Forester - NPS defaults to closed
  • 2005 and Earlier Impreza - NPS defaults to open
  • 2006 to Current Impreza - NPS defaults to closed
  • 2006 and Earlier Legacy/Baja - NPS defaults to open
  • 2007 to Current Legacy - NPS defaults to closed

Dash dictates body harness

If you want to change the dash, then you must change the harness to go with it. A typical scenario would be an RHD swap in an LHD car. To ensure the dash's interior functions (AC, heat, illumination, etc.) work properly the best solution is to utilize the entire RHD harness and deal with any changes to the ECU to engine wiring if necessary. The sample principles would apply to dash swaps from GC to GD as well.

Cluster dictates speed sensor type

The gauge cluster in the car is what determines which speed sensor to use because the cluster is only designed with one speed sensor type in mind. If your cluster came with a cable driven speedometer then the only solution for a working speedometer is a speedometer cable.
  • Cable Driven (mechanical) = 97 and older
  • 2 wire signal = 98 and some 97s
  • 3 wire signal = 99-07 (Legacy ends in 2005 when it switched to CANbus)
  • CANbus signal from ABS = 2005+ Legacy, 2008+ Impreza, 2009+ Forester
Note: For all types there is an electronic signal sent to the ECU so it know how fast you are going
And a couple bonus mechanical rules because we're nice.

Transmission dictates clutch/flywheel

There are 2 different clutch types, pull and push. The transmission will be set up for one or the other while the engine will be able to work with either so replace the clutch and flywheel based on the transmission not the engine.

Transmission dictates rear differential

The rear differential final drive must match transmission. If they are not from the same car verify your transfer gear ratio to your drive ratio.

Rear differential dictates hubs

  • R160 Rear Diff you can pretty much use any hub
  • R180 Rear Diff you must use the STi rear hubs (most likely 5x114)
If you are changing to 5x114 you should consider the suspension and wheels that will come with this platform

Hubs dictate wheel speed (ABS) sensors

The tone ring in the hub is specific to the sensor that goes into it so the sensor must match the hub. In addition CANbus equipped cars use a different style pick up that wheel speed sensors on non CANbus equipped cars so you'll need to be careful when choosing a hub for your application.

Crossmember Dictates Steering Rack