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Comparing Subaru's EJ and FA Engines: A Detailed Guide

Posted by IAG Performance on Mar 1st 2024

Comparing Subaru's EJ and FA Engines: A Detailed Guide

Subaru has been using the boxer engine in their cars for over 50 years, and any Subaru enthusiast can tell you that there is a lot to love about the flat engine layout. Boxers offer a lower center of gravity, which equates to a better handling ride. Additionally, the horizontal design and balance of the boxer style engines means fewer engine vibrations, while providing a quieter, smoother ride. While the flat, or horizontally-opposed, engine is the common thread between all the Subaru models, there are big differences between the various engine lines. Whether you’re a Subie newbie and want to expand upon your Subaru knowledge, or if you just want to brush up on your EJs and FAs, we’ve got you covered. In this article, we will break down the differences between the long-reigning Subaru EJ series engine and its modern successor, the FA series engine.

Photo of Subaru EJ20 engine, not installed.


Subaru’s EJ series engine was first introduced in 1989, and became the most commonly used engine in the Legacy and Impreza in the early ‘90s. The EJ remained a staple all the way until recent times, where it showed up in the final 2021 model year WRX STI. The EJ has long been a favorite among Subaru enthusiasts because there were endless aftermarket options to maintain or upgrade every last engine component that you could think of. With decades of aftermarket parts and support, the EJ was able to make the transition from daily driver to high horsepower racer with relative ease.

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Some of the main characteristics that discern the EJ from the FA is that the EJ uses a port injection fuel system, as opposed to the direct injection fuel system on the FA. While port injection systems spray fuel into the intake ports, the fuel injectors in the direct injection system are located in the cylinder head and spray fuel directly into the combustion chamber. The port injection system allowed for a steady and stable combustion rate, whereas the switch to direct injection improved upon the combustion efficiency, both increasing fuel economy and lowering emissions output. FA series engines used in the BRZ and FRS feature a combination of port and direct injection to help with emissions.

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Photo of port injection fuel system for EJ Subaru engine.
Photo of EJ timing belt for Subaru EJ engine.


Another difference between the EJ and the FA is the timing system. The EJ series engine incorporated the use of an external timing belt system that required replacement of the belt, rollers and other components every 100,000 miles, or so. Since the EJ was an interference engine, that meant it had the potential to self-destruct if you pushed beyond the service interval for the timing service. In contrast, the FA series engines utilize an internal dual timing chain system to ensure mechanical timing is kept in check. The FA engines also incorporate the engine oil pump into the timing cover assembly. One of the biggest differences between the EJ and FA engines is the shape of the connecting rod. While the EJ series uses a standard connecting rod shape, the FA engines utilize a new asymmetrical design.

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The EJ series engine proved to stand the test of time, and was used in the following- 2002-2012 Subaru Impreza (including WRX and STi), 1998-2012 Subaru Legacy/Outback (including GT and XT), 2003-2006 Subaru Baja (including Turbo), 1998-2011 Subaru Forester (including XT), 2012-2014 Subaru Impreza WRX, and 2014-2021 Subaru WRX STI. While the EJ had a lot of upgrades throughout the years to keep it competitive and relevant, it was eventually phased out in favor of its modernized successor, the FA series engine.

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Photo of Subaru GC8 on track.
Photo of FA series engine, uninstalled.


The FA series engine was introduced in 2012, making its debut in the Forester and in 2015 with the WRX. In addition to the upgraded injection system, the FA series incorporated the use of cast aluminum pistons with a unique dome shape that allowed for the optimization of fuel delivery through the direct injection system. This not only helped with the aforementioned emissions output levels, but also led to improvements in the overall output as well as fuel efficiency. Another upgrade in the FA series engine was the addition of a dual timing chain system. While it is still necessary to replace the thermostat and water pump around the same intervals as the EJ timing service, the FA timing chain is basically a no-maintenance system, or at least that’s what Subaru says.

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The cylinder head design is completely new for the FA engines. The switch to dual timing chains allowed the valves to be placed at a narrower angle, promoting better airflow. The valvetrain also switched to a rocker arm design from the modern shimless bucket EJ design. Additionally, the asymmetrical connecting rods employed in the FA series engines allow for an increased stroke length without a drastic change in overall block dimensions. The offset design was meant to allow for easier assembly, but some of our own specialists have remarked that the time to assemble an FA long block is about double of what it takes for an EJ long block, while the short blocks are more closely matched for time.

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Photo of Subaru timing chain, uninstalled.
Photo of Blouch turbo, uninstalled.


While the FA series engine is currently only available as a 2.0L or 2.4L capacity, they have a lot of potential for power upgrades. The FA20DIT, for example, is easier to modify to up to 350WHP than an EJ25. The addition of basic bolt ons will get you there quickly, but making the jump to over 500WHP is a little more complicated than the EJ, requiring a built engine, secondary fuel system, and a larger turbocharger.

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Since the FA engine is still in its infancy in comparison to the EJ, there aren’t as many aftermarket performance options available, and there is still a lot to learn about the way these engines handle modification and power. For instance, the FA24 engine is brand new and has only shown up in the Ascent, Legacy XT, Outback XT, and 2022+ WRX. It hasn’t taken much time for FA24 owners to start pushing for more power, and early reports indicate that they’re seeing around 350-400WHP with pretty basic modifications and that the engines are responding positively to the additional power.

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Photo of Subaru FA complete engine.
Timing belt kit for


The FA series engines can be found in the following models- 2012+ Subaru BRZ, 2012-2016 Scion FR-S, 2016-2020 Toyota 86, 2015-2021 Subaru WRX, 2014-2018 Forester XT, 2022+ Toyota GR86, 2018+ Subaru Ascent, 2020+ Subaru Legacy XT, 2020+ Subaru Outback XT, and 2022+ Subaru WRX. While the jury is still out on some of the later models, the FA seems to be promising as far as their adaptability to power, though it may take a bit more effort than the EJ to obtain those higher horsepower numbers, we’ve had our own success in building engines that are rated up to 800 crankshaft HP for FA20DIT and FA20.

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The transition from the long-revered EJ series to the innovative FA series showcases Subaru's commitment to advancement. The EJ, with its robust aftermarket support, made its way into the hearts of enthusiasts, while the FA introduced modern enhancements like direct injection and a maintenance-friendly timing chain system, promising manageable service intervals, greater efficiency and reduced emissions. While the FA engines are still unlocking their full aftermarket potential, they're proving their adaptability and responsiveness to modifications, hinting at a promising future for Subaru enthusiasts. Of course, if you have any questions or need some advice in regards to your own Subaru, IAG Performance has the expertise you need. Feel free to give us a call at 1(410)840-3555 or shoot us an email at

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