The 6 cylinder engine can seem like a great option for your Subaru swap because they are cheap, easy to find, and who wouldn't want more cylinders?! However an inexpensive engine does not mean an inexpensive project. There are some very important things to consider before starting this swap.
The first thing to consider is if you need to pass smog where you plan to register the car. If you need to pass smog, this is not a good swap for you. For most instances, this engine swap WILL NOT pass smog either due to lack of OBD2 functionality (EG33), check engine lights (01-04 EZ30D), or if it's attached to a standalone (recommended for all 6 cylinder applications and required for 05+ DBW models). There is one example that will pass. If the car is a 1999-2004 Automatic Forester, Impreza, or Legacy and you are running a 2001-2004 Automatic 6 Cylinder (EZ30D) then you can run the stock ECU and the car can pass smog. Beyond that, if you put the 6 cylinder into a manual car there will be check engine lights since the ECU will not get the inputs from the automatic transmission it needs.
The second thing to consider is cost. While many 6 cylinder engines are cheap, everything else needed to make the car run will most likely be custom and therefore expensive. There are no off the shelf aftermarket parts, so that means most items will need to be fabricated. Custom items include the exhaust, engine mounts, radiator and fans, fuel system, etc. If you don't have a big budget, this is not a good swap for you.
If you do decide to move forward with this swap, we at iWire suggest using a standalone ECU. This way you can get everything working from an engine stand point, have tune-ability, and have the creature comforts from the factory. In addition if you are looking at forced induction the standalone can accommodate those upgrades. As mentioned before, this setup will not pass smog because standalone ECUs will not communicate with an OBD2 scan tool.
We do acknowledge that people run a 6 cylinder on the stock ECU if the car is older than 2004 and does not use CANBUS. Technically yes, the car will work. We don’t wire this setup because the end result does not work as it should from the factory which means it's something we can't stand behind. If you want to use the factory ECU while acknowledging the potential issues and have the know how or access to someone who does, then go for it.
So overall, do we recommend doing this swap? For the average person who just wants a simple upgrade in power, no. On the other hand if you are up for a challenge and have the resources in money and mechanical know how, then it can be a very fun and rewarding project with excellent potential.
Hear what your 6 swapped Subaru could sound like. This is iWire Owner Brian's GC8 powered by a 3.6 Tribeca engine.