Subaguru question #4

[AVIONX]

Wow! You guys are awesome!!! Thanks for all of your help. If my questions get annoying I am sorry. But I have been a Fiat and Small block Ford guy for 25 years. I have a high HP 2003 WRX with stock block, heads, clutch and tranny. Everything else is Top Notch high HP afternarket. I can get my hands on a stock 2007 WRX 2.5L wreck. Both are 4 door sedans. What I want to do is move all of the stock parts [everything in the drivetrain and suspension except block, heads, clutch and tranny] from the 2007 to the 2003. This would leave me with a very sellable, clean, stock 2003 WRX and a very low miles 2007 2.5L drivetrain with tons of awesome mod parts that I assume will bolt right up to the 2007.

Question....Is this doable? Will the intake, Fuel System, intercooler, cold air system, maf, struts, turbo, etc. etc. from a stock 2007 2.5L WRX all bolt onto the stock block,heads of a 2003 2.0L WRX? Will the 2003 need the 2007 ECU? Can the old 2003 ECU run the intake/fuel gear taken from the 2007?

[PhyrraM]

The heads, intake manifold and tract, throttle pedal, wiring harness and ECU must all match. This is because the '03 does not have variable valve timing or electronic throttle.

Other than that, Legos.

[fateo66]

Make sure to swap the rear ends too. The assuming both are manuals the 03 has a 3.545 rear end and the 07 has a 3.7

[BrandonDrums]

Make sure to swap the rear ends too. The assuming both are manuals the 03 has a 3.545 rear end and the 07 has a 3.7

I don't think he's swapping the tranny so he won't need to worry about that.


The heads, intake manifold and tract, throttle pedal, wiring harness and ECU must all match. This is because the '03 does not have variable valve timing or electronic throttle.

Other than that, Legos.

Exactly, but there's more to the ECU story

Essentially, you will basically have to get the 2.5 tuned to match your parts v.s swapping the physical ECU as PhyrraM mentioned, and you will also have to re-flash the 2003 with a factory tune to get that car fully back to stock. You can do this on your own with a Tatrix cable (A USB to OBD II port ~75 bucks) and some free open-source tuning software: RomRaider and ECUFlash All the info you need can be found here www.openecu.org. You'll want the tatrix for the 2.5L anyway so it's worth the investment. A little tuning knowledge and willingness to log your car at least once a month with all those aftermarket components is nearly as important to checking the oil regularly.

Long story long, here's how to do at least put the 03's ECU back to stock with the above cable and software :

To find the stock map, unless you have the read file from yours saved can be found here: http://www.romraider.com/forum/viewtopic.php?f=35&t=7591

I'm not sure if it matters but to find your CAL and therefore ECU id to find which map goes on your car, you'll need to use the tatrix cable and read/diagnostics connectors and ECUflash to read the tune from your car which has both of the ID's imbedded in the map. They will be displayed at the bottom of the ECUflash program window when you have the read image open. I'm pretty confident any 03 USDM Wrx map will work though, same equipment just slightly different tunes but I can't confirm that.

Then you just download and open the OEM tune found on that RomRaider forum page above; USDM 03 WRX with the same CAL ID and flash that to the 03's ECU using ECUflash once again.

If you want to then try to get started tuning the 2.5, here goes...

You can then use some of the tables in the read file (your aftermarket tune) to get some basic figures about your injectors' flow rate(s), injector latentcy, MAF/Engine Load calibration and some of the general targets your 2.0 was running in terms of Air/Fuel ratio, timing, target boost etc. That you will end up close to with the 2.5L if you decide to tune yourself.

If flashing and tuning etc. sounds like a lot (it pretty much is) you'll want to find a reputable tuner who allows you to be part of the tuning process and who will spend the time needed to dyno and road tune the car. If you're comfortable tuning EFI systems then really, you should be able to handle doing a road tune on the 2.5 with the upgraded parts. I

Good luck!

[AVIONX]

Suspension/Steering all interchangeable?

[prematureapex]

Yes, although the rack mounts are different after 2004.

You can still bolt the 05+ rack to the older cross-member, but it requires some minor rigging iirc.

Alternatively, you can swap the cross-member too.