Subaru's Dilemma： Boxer Engine Advantages, Disadvantages, and Solutions

Today, Subaru and Porsche are the only mass-production automakers manufacturing boxer engines. In 2025, Porsche sold approximately 280,000 vehicles globally, with about 70,000 of them being boxer-powered 911 and 718 models. In other words, while the boxer engine remains an icon for Porsche, it is no longer the main driver of its business. On the other hand, Subaru sold around 900,000 vehicles in the same year, the vast majority of which were equipped with boxer engines. If the inherent structure of the boxer engine were to put it at a disadvantage due to environmental regulations, Subaru would suffer fatal damage. However, Subaru's engineers have worked relentlessly to overcome these structural challenges and will continue to do so. In this video, we will explore the advantages and disadvantages of the boxer engine, and how Subaru has worked to overcome those shortcomings. In this section, we will compare it with an inline-four engine and look at its advantages and disadvantages. One of the main advantages of the boxer engine is its low overall height and short overall length. In particular, the low height significantly lowers the vehicle's center of gravity, which contributes greatly to handling performance. Superior vibration characteristics are another major advantage. Inline-four engines generate secondary vibration. Secondary vibration is vibration that occurs twice per engine revolution. In a reciprocating engine, because the connecting rod tilts, the piston speed differs between the upper and lower halves of the piston stroke. This animation illustrates that behavior, though it is exaggerated to make the difference easier to see. As a result, the inertial forces generated as the pistons move up and down cannot be fully cancelled out, creating a vibration that shakes the engine up and down. In a boxer engine, the opposing pistons cancel out each other's inertial forces, preventing secondary vibration from occurring. High crankshaft mounting rigidity is also a key benefit. In an inline engine, the crankshaft is attached to the cylinder block using bearing caps. To increase mounting rigidity, some engines utilize a bearing cap beam that connects the caps together. In a boxer engine, one half of the cylinder block acts as the bearing cap for the other, providing exceptionally high crankshaft rigidity. In inline engines, heavy crank weights are required to cancel out piston inertia and the imbalance caused by the crank pins. In boxer engines, the opposing pistons act as counterweights, allowing for lighter crank weights and smoother revving. The shape of this crankshaft is sometimes called a knife-edged crank, a term that carries both positive and negative implications. Since this section is about the advantages, we'll treat the knife-edged crank as a benefit for now. Its overall width is one of its disadvantages. Because Subaru mounts its engines in the front, people often assume this negatively affects the front wheel turning angle, suspension type, and geometry. This table compares the wheelbase, turning radius, and front suspension type of the Subaru Forester against its competitors. As you can see, the Forester doesn't lag behind on any point. As shown in this diagram, the front suspension lower arm pivots are actually located beside the transmission. Therefore, the width of the boxer engine has no effect whatsoever on the length of the lower arms. Subaru's engineers have used every bit of ingenuity, redesigning the subframe, routing the exhaust pipes, and optimizing the steering rack placement to offset the inherent disadvantages of the boxer engine. Connecting the exhaust pipes via the shortest route from the widely separated banks results in different lengths on the left and right. This creates an uneven exhaust pulse, producing the iconic boxer sound that many Subaru diehards still love today. For ordinary drivers, however, this was nothing more than a source of unpleasant noise and poor fuel economy. From an engineering perspective, it was a clear disadvantage. To resolve this issue, Subaru's engineers routed the exhaust piping further to make the left and right sides equal in length. Having twice the number of parts around the cylinder block and cylinder head compared to an inline engine is another major drawback. Manufacturing costs, procurement costs, and assembly labor all skyrocket. The ones compensating for this disadvantage aren't the R&D engineers, but the procurement staff and production line engineers. Though, as outsiders, we have no real way of knowing just how successful their efforts truly are. As mentioned earlier, its short overall length is one of the major strengths of the boxer engine. However, there is a catch. One of Subaru's greatest selling points is its exceptional AWD performance, which means the front wheels must be driven as well. Let's take a look at Subaru's powertrain layout. Connected to the rear end of the engine via a torque converter is the CVT unit. The power output from the CVT unit is split by gears. One path sends drive through the propeller shaft to the rear differential. The other path reverses direction and route power forward to the front differential. Because of this layout, the front driveshafts are located right between the engine and the transmission. As a result, the engine mounting position is pushed out in front of the front axle. In longitudinally mounted inline or V engines, as shown in this diagram, the front differential sits alongside the cylinder block, with the driveshaft passing straight through the oil pan. The engine, which is the heaviest single component in the vehicle, is mounted far from the center of gravity, requiring a much larger lateral force when cornering. That said, don't misunderstand this point. This does not mean, however, that you should jump to the conclusion that Subaru vehicles have heavy understeer. Subaru's engineers have been desperately trying to suppress this behavior through suspension design and power distribution. We will examine the common belief that a boxer engine's low profile and lower center of gravity directly benefit handling. For simplicity, assume the cylinder block, cylinder head, and crankshaft have the dimensions and weights shown in this diagram. The center of gravity for the inline engine is about 47 centimeters, compared to 30 centimeters for the boxer engine, a significant difference of 17 centimeters. There is no question that this has a major positive impact on a vehicle's roll characteristics. However, things are not quite that simple. With a boxer engine, the exhaust pipes have to run underneath the engine. On top of that, modern engines need to have turbochargers and catalytic converters mounted as close to the If the exhaust system takes up 15 centimeters of height, that 17 centimeters gap shrinks to just 2 centimeters. If it takes up 20 centimeters, the center of gravity advantage actually reverses. That said, even if the center of gravity height ends up being the same, a lower engine overall still has a lower moment of inertia around the roll axis. So while the effect might not be as dramatic as commonly claimed, it's fair to say that a boxer engine still benefits the vehicle's characteristics. Subaru markets its signature tagline, Symmetrical AWD, around the world. They proudly emphasize that the entire powertrain, from the engine to the wheels, is arranged in a perfectly symmetrical layout. As seen in this diagram, it certainly looks impeccably balanced, conveying a strong impression of high dynamic performance. Indeed, Symmetrical AWD is a layout that could never exist without a boxer engine. However, Subaru conveniently forgets one crucial fact. And of course, they do so intentionally. Needless to say, a car does not drive itself. It requires a driver. The heaviest individual component in a vehicle is the engine, followed by the transmission. When an adult male sits in the driver's seat, the driver effectively becomes the second or third heaviest mass in the vehicle. To make matters worse, this substantial mass is not rigidly bolted to the chassis. It sways and shifts every time g-forces act upon the vehicle. With such a massive weight biased entirely to one side, meticulously arranging the mechanical hardware in a symmetrical layout loses all practical significance. In a sense, the statically beautiful balance of Symmetrical AWD is nothing more than a narrative crafted by the PR department. Granted, it is also true that a compelling narrative is essential for building a brand. There was a time when poor fuel economy was simply laughed off with a sigh. It's a Subaru, what can you do? Because it had a unique charm that far outweighed any of that. But the world has changed. Under today's cafe regulations, poor fuel efficiency directly incurs heavy financial penalties. While there are countless ways to improve fuel economy, Subaru first had to tackle the inherent drawbacks of the boxer engine. The path they chose was narrowing the valve angle and switching to a long-stroke configuration. Both are highly effective for fuel efficiency, but both inherently increase the width of a boxer engine. To prevent interference with the front suspension and steering functions, making the engine any wider was simply out of the question. Since a boxer engine requires us to swap height and width in our minds, let's look at this issue through the lens of an inline engine to make it more intuitive. They switched the valvetrain from a direct acting bucket setup to a swingarm design, narrowing the valve angle without increasing cylinder head height. This modification also reduced valvetrain friction losses. To achieve a longer stroke, you have to push the crankpin further from the crank journal. Increasing the crank radius. Subaru's engineers achieved a longer stroke without increasing the cylinder block height by shortening the connecting rods. Through these two methods, they improve fuel efficiency without expanding the overall width of the boxer engine. However, as the crank journal and crankpin move further apart, the rigidity of the crankshaft decreases. In an inline engine, rigidity is secured by thickening the crank webs. Now, recall what makes Subaru's crankshaft unique, the knife-edge crank. Without making the crank webs thicker, a longer stroke inevitably reduces crankshaft rigidity. While this might not be the only reason, the latest generation long-stroke FA engine still couldn't surpass the peak output of the previous generation short-stroke EJ engine. Until a world of nothing but battery EVs arrives, Subaru's engineers must continue to wrestle with this lovable, troublesome engine. Their struggle goes on. Thank you for watching. If you enjoyed this video, please consider subscribing to the channel. We'll see you in the next video.