In this post, we discuss the similarities and differences between the 2004-2008 Forester XT and the 2004-2007 STi models since many Forester owners are looking to add STi features. Our general recommendation is to retain as many Forester parts as possible to eliminate the wiring changes that would otherwise be required.
Wiring Harness
This is the most common misconception amongst all Forester and Impreza lines. Although they have a similar chassis and dash, the harnesses are completely different. We DO NOT recommend putting in a WRX or STi dash in any Forester since it will create more problems than it would solve.
Engine
The Forester XT and STi use the same basic setup for the engine. Both are drive-by-wire, use 3-wire camshaft position sensors, and have intake-only AVCS. This means that if you want to run an STi engine, all you need to do is bring over the long block and retain the original Forester XT intake manifold and harness. This post below is a great resource for this compatibility.
- https://iwireusa.com/blogs/iwire-university/my-subaru-engine-blew-up-what-should-i-replace-it-with?_pos=1&_sid=1beb728a6&_ss=r
Transmission
If the Forester XT came as an auto. In that case, the auto-to-manual swap is best done by either finding the same-year Forester XT manual harness or modifying the original auto harness for the manual setup. We have a service to handle this. Please click the link below for more information.
- https://iwireusa.com/pages/merge-services?discontinued=true
In situations where the Forester XT came with a manual transmission, it's just a matter of our DCCD transmission adapter and adding in a DCCD controller. More about that can be found here.
- https://iwireusa.com/blogs/iwire-university/subaru-dccd-controller-how-does-the-controller-work
ECU/Tuning
The ECUs can be interchangeable depending on the year of the Forester and STi, but we still suggest sticking with the Forester XT ECU if possible. We've included some notes below about compatibility if considering the STi ECU in your Forester.
- 2004 FXT = 2004 STi
- 2005 FXT = 2005/2006 STi
- Requires either immobilizer reprogramming or swapping out the ECU, immobilizer box, and ignition cylinder with key
- 2006 - 2008 FXT = 2007 STi (kind of)
- Be careful here because the 2007 STi engine has a slightly different air pump and valve setup and purge valve configuration that may cause check engine lights.
Gauge Cluster
The Forester and STi have very different wiring harnesses for the gauge cluster, so the Forester cluster needs to be modified for the STi cluster if you decide to make that switch. We can build a custom adapter for this, so please send us an email to info@iwireusa.com if you are interested.
Overall, these cars seem the same, but there is more to the story. There are plenty of interchangeable parts, but wherever you can use Forester parts to avoid making wiring harness modifications, we suggest you do so since the Forester XT ECUs have the same capabilities as the STi.
