iwire subaru harness merging

Best Subaru Engine Swap Choice for your Chassis

May 17, '21

March 2023 Update:
We can no longer wire up engines from cars older than the chassis they are being installed in. The problem is many states are taking the stance, like California, that for any engine change, it must be from the same year or newer, or the car can't pass emissions testing. We've already gotten reports of customers who have done swaps in the past that can no longer pass emissions testing because the engine is older than the car in states like Utah. So, for example, if you have a 2005 Impreza, you would need to swap a 2005+ turbo engine into your vehicle to ensure that even if you don't have an issue today, you'll protect yourself against being unable to register your car in the future.

This blog post reviews our recommendations for a turbo Subaru engine swap based on the non-turbo chassis type. If you have a turbo car, you are looking to swap; we generally recommend replacing the long block and leaving the rest of the car intact. More about turbo to turbo or NA to NA swaps here - My Engine Blew Up, What Should I Replace It With?


A question we get asked often is, “What engine do you recommend for my swap/upgrade?”. And our response is always, “What chassis are you putting the engine into?” Not all cars are created equal, so the chassis type has a lot of effect on what engines can be swapped into the car. Below we have categorized different chassis types with our recommendation for the best engine swap.

Please note all of these swaps take time, money, and effort. There will be mechanical changes as well as wiring changes. Click here to read our Subaru Dos and Don'ts tips for your project. Click here to learn How to do a WRX swap into your RS.

Some definitions:


Group 1: Non-Turbo, Non-CANbus, Cable Throttle Chassis:


93-04 Impreza

98-04 Forester

95-04 Legacy

96-04 Outback

02-06 Baja

This chassis group has the most options for an engine swap because you can stick with cable throttle or upgrade to a drive-by-wire platform.

The engines we recommend for this group include the following:

JDM 2.0 Engines like the EJ205 or EJ207 (not the EJ20X/Y (learn more here)), USDM 02-05 WRX. Another set of good options would be DBW 2.5 liter models like USDM 06-07 WRX, USDM 04-07 STi, or USDM 04-08 Forester XT. If emissions are a concern in your area, you'll want to stick with a US engine for your swap.

Check out this blog with our full write-up on “easy” engine swaps for these chassis.

Special case for USDM 2002-2005 WRX chassis. The JDM 02-05 WRX and STi models are simple plug-and-play options. More about these here. The USDM 04-07 STi, USDM 06-07 WRX, and USDM 04-08 FXT are great options but require a harness merge to run the upgraded 32-bit ECU and drive by wire.

Group 2: Non-Turbo, Non-CANbus, Drive by Wire Chassis:


04-08 Forester

05-07 Impreza

For this group of chassis, we recommend sticking with a DBW engine. You could swap in a cable throttle engine, but you’ll lose some features (more about that here), and it will not pass emissions if you do this.

The engines we recommend for this group include the following:

USDM 04-07 STsimilar-year8 Forester XT, or USDM 06-07 WRX. Most DBW JDM motors need to either be converted to cable throttle (JDM Forester) or their ECUs are CANbus equipped, making them difficult to swap (JDM EJ20X/Y), so we don't really recommend them for any non-turbo to turbo application.

Group 3: Non-Turbo, CANbus Equipped, Drive by Wire Chassis:


05+ Legacy

05+ Outback

08+ Impreza

09+ Forester 

Regarding CANbus-equipped chassis, it is challenging to swap in a new engine. There are just too many inputs/outputs on the CAN line that the car is generally unhappy when installing a new engine/ECU. For these cars, we recommend finding a similar year turbo donor car (for example, a 2010 WRX for a 2010 Impreza or a 2012 Forester XT for a 2012 Forester) and putting all of the wiring and engine components from the donor car into the NA chassis. This would include rear harness, ABS pump, dash, HVAC, etc. Every component from the turbo car must be put into the NA chassis to work correctly. Since you basically need an entire car for this to work, we generally recommend just purchasing a turbo car and adding some mods to that. However, some chassis have sentimental value, so we understand why keeping them alive may be more appealing than buying a new one. Please click here for more about the CANbus complexities.

Swapping the long block and keeping the wiring and ECU intact is the other option to keep a chassis alive. More about this option here - My Engine Blew Up, What Should I Replace It With?

Non-EJ Engine Choices:

Some may have noticed that we do not recommend any FA engines (2015+ WRX or 2014+ Forester) or any of the 6-cylinder engines (EG33, EZ30, or EZ36). We do not generally suggest these engines for the average DIY project because of the difficulty and expense of swapping them. We've written in-depth blog posts about both engine types so you can learn more about the pros and cons to see if it's a good engine for your project.

FA Engines: https://www.iwireservices.com/post/fa20-engine-swap

6 Cylinder Engines: https://www.iwireservices.com/post/2018/11/27/6-cylinder-subaru-swap 

Some more information about 6 Cylinder projects:

6 Cylinder vs. STi Comparison: https://www.iwireservices.com/post/subaru-engine-comparison-6-cylinder-vs-sti

iWire H6RS Build Specs: https://www.iwireservices.com/post/iwireh6rs-build-specs

A note about V1-4 JDM WRX and STi engines:
We do not wire these engines up at this time because they are non-OBD2, making smog/emission impossible and tuning very difficult. When you factor that in with the lack of replacement parts, it becomes hard to build these engines and maintain them. A better solution would be to use an engine with OBD2 compatibility to make your project simple.
An additional note about V5-6 JDM WRX and STi engines:
Similar to the above situation, the ECUs that came with these engines are not OBD2 compliant in North America. The upside is that you can convert them to a USDM WRX ECU. Please click here to see the steps so we can wire up this application for you.