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Although Subaru didn’t invent the boxer engine, many would say they perfected it. With a long and cherished motorsport history, the Subaru EJ has collected millions of fans worldwide. Today we’re taking a deep dive to see what’s so special about the Subaru EJ platform.



The late EJ series is considered by many to be the pinnacle of Japanese boxer technology, but Subaru’s strange obsession with flat-four engines started way back in the 1960s.
Released in 1965, Subaru’s first boxer was a 900cc, water-cooled engine known as the EA-52.  It made just 55hp which was actually not too bad given that it was fitted to a car that weighed only 550 kilograms.



Subaru continued to develop the boxer engine investing a considerable amount of resources into a platform that would eventually lead to the EJ. The EJ series made its debut in 1989 in the Subaru Liberty and the Subaru Legacy in the North American market. It remained in production until 2021, making it one of the longest-running production engine platforms of the modern era and was the mainstay of Subaru’s engine line. 




It was the EJ that gave Subaru international clout powering its World Rally Championship cars, and of course we can’t talk about the EJ without mentioning the famous car it was fitted into – the mighty WRX. 
In 1992 Subaru partnered up with British engineering firm ProDrive to develop a new compact chassis. Based on the existing Impreza it was dubbed WRX which stands for “World Rally Experimental” and featured a new all-wheel-drive system and a powerful, turbocharged EJ20. The engine was rated at 250hp at all four wheels although rumour has it, it actually made closer to 300hp. In the capable hands of Colin McRae, the famous 555  WRX STi went on to win the WRC constructors title for three consecutive years in ‘95, ‘96 and ‘97. 


Due to their reputation as a compact, “reliable” engine platform, the EJ can be found in numerous amateur and professional motorsport series. They’ve also been a popular conversion for Volkswagen guys as a modern flat-four successor to the aging air-cooled engines, and, along the same lines are a popular engine for kit cars based on the older Porsche models. 



Boxer vs Flat Four

Before we go any further let’s get the definitions of a flat and a boxer engine right – because they’re not interchangeable. A boxer is a flat engine with each pair of pistons connected to a different crankpin – mirroring each other.
But not all flat engines are “boxers” because a flat engine can also have piston pairs sharing the same crankpin – working in the same fashion as any other V configuration engine.
So while all boxer engines are actually flat engines – not all flat engines are boxers.



The EJ engine has been produced in over 20 variants since 1989, which means two things. There are plenty of options to suit your requirements, and depending on the model they can be plentiful and affordable. You can split most EJ engines into two groups, pre-1998 and post-1998. Both groups include naturally aspirated, and turbocharged variants. 
An easy way to tell if it’s an early or later model EJ is by looking at the engine code. The earlier models end in a letter (Like EJ20G for example) while the later ones end in a number (Like EJ205). For performance builds, the most desirable EJ would probably be the EJ207. These were found in STis made in 1999 and then from 2001 to 2005. They come with a forged rotating assembly that tends to enjoy higher boost levels than the other variants.
Of course it is possible to turbocharge naturally aspirated EJs or increase the boost on turbocharged ones, but keep in mind that the cast OE pistons found in most EJs are considered a weak link and would need to be upgraded.




When it comes to modding an EJ series engine, thanks in part to the standard architecture, it’s common to mix and match blocks, cylinder heads, and rotating assemblies depending on your build. There are plenty of setups making 300 to 350hp on a stock engine, with basic bolt-on mods; but, if your horsepower aspirations are in the 400-plus region you’ll need to look at an even bigger turbo, an intercooler,  upgraded fuel system, and serious engine internals upgrades including piston and conrods. While you are there a set of head studs and a quality head gasket wouldn’t hurt!
In order to control your fresh build, there are a few engine management options. Depending on the year model the Stock ECU is very “tuneable” and can work well; however, once you start to push an EJ my preference is always to move to a standalone ECU to take advantage of the engine protection strategies and to know exactly what you are adjusting in order to get a consistent and guaranteed result.

If you’re looking at a stand-alone ECU upgrade, Haltech offers a full range of “plug and play” and “wire-in” ECUs that’ll provide you with a completely programmable engine management solution. The Plug’n’Play kits come with this neat adapter box that plugs into your factory harness on one end, then, into your Haltech ECU on the other. Load the supplied base map (which is supplied with the kit) and you’re ready to hit the dyno.
Our plug-in kits cover  WRXs from 1993 through to 2010; check the model compatibility list here.






Just like with any engine there are some caveats. Factory turbocharged engines have a reputation for burning oil, sticking ring lands, and if abused or mistreated;  cracking pistons or worse. Remember that the WRX and STi models were sold as performance vehicles and it’s safe to assume that every one of them has had a pretty hard life. 
Even the non-turbo engines will need a good check over, as the cramped engine bay of a Double overhead cam boxer makes them difficult and expensive to maintain – Doing a set of spark plugs on some of these models is an ordeal and often skipped over until there is a problem.
As mentioned earlier the factory pistons and connecting rods are considered a weak point and are commonly upgraded in performance builds, but due to the age of these engines – if it’s coming apart for any reason, it’s worth looking at a set of pistons and rods to ensure reliable operation moving forward. Some believe the factory oiling system isn’t adequate, but consult your engine builder before pulling the trigger on a high-volume oil pump as there are differing opinions here.


The Head Gasket Issue

There’s enough solid data out there to say the factory head gaskets are prone to failure at 150K to 200K kilometers.
Once again, because of its flat-four layout, replacing head gaskets on an EJ is an expensive job.
But there’s some good news! Firstly, not all EJs are heavily affected. It seems that the models most prone to head gasket failure are the EJ25s produced between 1996 and 2004. Interestingly, the problem seems to be more prevalent in the naturally aspirated and single overhead variants.



So, should all that stop you from getting an EJ? Let’s look at it this way – if you’re planning to put that engine in your race car then you’re probably going to upgrade the internals anyway so changing head gaskets will be a part of the job. Knowing the common issues and the limitations of what mods or fixes can be performed without removing the engine will help in planning your build. Aside from the common problems (and let’s face it, every engine has some) the EJ is a snappy and great-sounding engine with a low center of gravity. 





It’s also a relatively reliable engine to use as an upgrade in your existing Subaru, an engine swap into a VW or kit car, or as a fully built monster for your dedicated race car. 

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