Subaru is a brand that's known to do things a bit differently than other mainstream car manufacturers. All but one of its cars come with standard all-wheel-drive, it has a historic commitment to occupant protection, and all of its internal combustion-engined cars feature a unique horizontally opposed engine layout. These horizontally opposed engines are called "boxer engines," and they are oriented differently than every other engine configuration on the market.
You probably already know that V8 engines are V-shaped, and inline-6 engines are oriented in a straight line, but what about a flat-4, horizontally opposed boxer engine? Boxer engines earn their namesake because their pistons are laid out flat, and when they move as the engine runs, their motion mimics a boxer punching their fists together. Subaru is the only mass-market automaker that uses boxer engines, though the Subaru/ Toyota partnership that resulted in the Toyota GR86 and Subaru BRZ means Toyota technically has one boxer-engined car too, but it is primarily produced at Subaru's Gunma, Japan manufacturing facility. Porsche is the only other carmaker that currently uses boxer engines, but it only uses them in its 911 and 718 sports cars at the moment.
Boxer engines have several advantages over V-shaped and inline engine designs, especially in four-cylinder applications. These include less inherent vibration during operation, a lower center of gravity, and crash safety.
Four-cylinder engines are not traditionally known for their operational smoothness or refinement; inline-4s often suffer from excessive vibration since the cyclical vertical motion of the pistons create excessive upward forces called secondary forces. When the pistons are horizontally opposed as they are in boxer engines, vibration is minimized because there are equal forces occurring on either side of the engine that effectively cancel each other out. Inline-4 engines are frequently fitted with counter-rotating balance shafts that combat the vibration, but boxer engines are inherently better balanced than inline-4s.
Boxer engines are wide and relatively flat since their pistons move side-to-side, parallel to the ground, where inline and V-shaped engines tend to be narrower but taller. The flatter shape of boxer engines lends itself to a lower center of gravity since the heaviest components can be mounted lower under the hood, and their low profile keeps that weight lower than other engine configurations. A lower center of gravity reduces the amount of load transfer that occurs during cornering, braking, and accelerating, which increases vehicle agility.
The flat, low-mounted engine can allow for better occupant safety in the event of a frontal crash, since the engine can be designed to move underneath the vehicle and away from the vehicle occupants, rather than getting pushed straight back into the interior, where humans sit.
Boxer engine drawbacks
That sounds great, but boxer engines are not without drawbacks. The nature of a horizontally opposed engine necessitates some additional components that make them more complicated than inline engines, and their increased width limits access to components, as well as limiting space for suspension, and steering angle at full lock. When compared to inline-4s which only require a single cylinder head, horizontally opposed four-cylinder engines require two separate cylinder heads, which adds complexity. Not only do boxer engines require an extra cylinder head, the low mounting point which offers a lower center of gravity than other engine orientations also means it's exceedingly difficult to access those cylinder heads for maintenance and repairs. A task that's relatively simple on an inline-4 like changing spark plugs is much more challenging for the DIY wrencher to accomplish on a car with a boxer engine.
The flat boxer engine also takes up a significantly wider space in a car's chassis than a narrower inline or V-oriented engine. This can cause packaging issues for car designers and engineers who need to design suspension components to make the car ride, steer, and handle properly. When you're trying to make a tight turn, the front wheels must have space to move without contacting any components that could damage the tire, and the splayed-out design of boxer engines can eat into that space. Since Subaru has been using boxer engines for about 60 years now, its engineers are familiar with what it takes to safely and efficiently house a boxer engine while minimizing the impact of its inherent drawbacks.