Why is a boxer engine actually called a boxer engine?

The Porsche 356 already had a four-cylinder boxer engine, and since 1963, the six-cylinder boxer engine has defined the Porsche 911. No other engine is associated with one brand as much as the boxer engine with Porsche. But what exactly makes the boxer engine so special? How does it differ from a V-engine? And why did Porsche develop a flat-12 engine – which was not a boxer engine – specifically for the legendary 917 Le Mans race car? We’ll shed some light on the matter!

How does a boxer engine work, and how did it get its name?

The four-stroke internal combustion engine with a reciprocating piston operates on a clear principle. Simply put, a piston moves back and forth inside a cylinder. In doing so, it goes through the four strokes: intake, compression, power, and exhaust. During the intake stroke, fresh fuel-air mixture enters the cylinder; during the compression stroke, the contents of the cylinder are compressed. The combustion of the fuel-air mixture generates pressure, which drives the piston during the power stroke. These exhaust gases are then expelled. Linear motion from the piston is then transmitted to the crankshaft via a connecting rod and converted into a rotational motion, meaning the engine runs. The number of crankshaft revolutions per minute then indicates the engine speed.

Throughout its history, Porsche has used inline engines – for example, in the 924, 944, and 968 – V-engines, such as in the 928 and even VR-engines in the Panamera. Boxer engines were to be found in the 356, 911, and numerous mid-engine sports cars.

Engine designs vary depending on the arrangement of the cylinders. In an inline engine, all cylinders are arranged one behind the other in a single row. This makes it relatively narrow, though it becomes longer as the number of cylinders increases. In a V-engine, by contrast, the cylinders are distributed across two so-called cylinder banks, which are arranged at an angle to one another. This allows for the construction of shorter engines with the same number of cylinders. The connecting rods of two opposing cylinders typically engage a common crankpin on the crankshaft. How the two pistons move relative to each other depends on the angle between the cylinder banks.

A boxer engine also has two cylinder banks. The cylinder bank angle is 180 degrees. The cylinders are thus arranged flat on opposite sides of the crankshaft. In addition, each piston has its own connecting rod journal. The connecting rod journals of an opposing pair of pistons are offset by 180 degrees relative to each other. As a result, both pistons move evenly away from and back toward the crankshaft. If you look at a horizontally mounted engine in which both pistons are moving outward at the same time, the motion resembles the fists of two boxers. That is where the name “boxer engine” comes from.

Why Porsche Relies on the Boxer Engine

Porsche’s connection to the boxer engine dates back to the 1930s. At that time, Ferdinand Porsche’s design office developed a small car with a rear-mounted boxer engine for NSU. Porsche subsequently developed the design for what would later become the VW Beetle, including its air-cooled four-cylinder boxer engine. When Ferry Porsche and his team developed the 356 after World War II, he drew on this engine.

Porsche therefore remained true to this fundamental principle with the successor to the 356 as well. The 911 featured two additional cylinders. The six-cylinder boxer engine developed by Hans Mezger offered not only greater power potential but also particularly good mass balance and, consequently, exceptionally smooth operation. Over the decades, Porsche modified the displacement, cooling system, and fuel injection, but retained the cylinder layout. As a result, the boxer engine became one of the defining technical features of the entire brand.

This design offered several advantages for the early Porsche sports car. Its horizontally opposed cylinders allowed for a low-profile design, and compared to an inline engine with the same number of cylinders, the engine was shorter. This made it easier to mount it under the rearward-sloping body. The boxer engine also offered ideal conditions for the air cooling used at the time: its cylinders, distributed on two sides, could be easily supplied with sufficient cooling air.

Not every flat engine with two cylinder banks is a boxer engine – not even at Porsche!

While Porsche opted for a six-cylinder boxer engine in the 911, a flat engine with a completely different internal layout was developed for motorsports. The Porsche 917, introduced in 1969, had twelve cylinders that were also arranged opposite each other on either side of the crankshaft. Despite this familiar layout, however, its engine was not a boxer engine, as was still the case with the 908.

In a boxer engine, each connecting rod has its own pin on the crankshaft, and the pistons move in opposite directions. In a 180-degree V-engine, two opposing pistons share a crankpin and always move in the same direction. Ferrari also demonstrates that this distinction is not entirely straightforward. Its 512 and 356 GT/4 models were nicknamed “Berlinetta Boxer,” even though they were equipped with 180-degree V12 engines.

Richard Lindhorst, Elferspot

The key difference lay in the connection between the connecting rods and the crankshaft. In the boxer engine, each connecting rod has its own crankpin. The opposing pistons therefore move toward or away from the crankshaft simultaneously. In the 917’s twelve-cylinder engine, by contrast, two connecting rods from opposing cylinders shared a single crankpin. As a result, the two pistons moved in the same spatial direction: while one approached the crankshaft, the other moved away from it. When one reached top dead center, its counterpart was at bottom dead center.

That is why it was not a boxer engine, but a 180° V12 engine. This design offered tangible advantages for a twelve-cylinder engine. Instead of the twelve crankpins found in a comparable boxer engine, the 917’s crankshaft required only six. This made it possible, on the one hand, to build the engine in a more compact longitudinal configuration and, on the other hand, to use a lighter crankshaft.

Porsche Carrera- Rennmotor Bausatz

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A boxer engine also has its drawbacks

The fact that Porsche already deviated from the boxer engine design with the 917 clearly shows that the design advantages of the boxer engine are offset by disadvantages. First, there is the increased width required. This is because a 180° cylinder bank angle also results in the widest possible layout. Depending on the installation position, this presents challenges. If the exhaust system runs below the engine, the engine itself must be raised. This, in turn, partially negates the advantage of the low center of gravity. Access to spark plugs and attached components can also be more difficult. Compared to a classic V-engine, the boxer engine is also longer and heavier. After all, it requires twice as many crankpins.

Furthermore, a boxer engine also presents aerodynamic disadvantages in racing. What may seem counterintuitive at first becomes clear upon closer inspection. This is because the flat-six engine takes up a significant amount of width deep within the vehicle. As a result, the downforce-generating Venturi channels on the underbody cannot be positioned as freely as they would be with a V- or inline engine. Furthermore, they tend to have to be smaller, which compromises the diffuser’s efficiency. For this reason, most 911-based race cars have very tall and large rear wings to compensate for the diffuser’s downforce deficit.

The Porsche 911 RSR from the 2017 season demonstrated just how much the engine’s mounting position affects aerodynamics. Porsche reversed the layout of the engine and transmission, placing the six-cylinder boxer engine in front of the rear axle. This transformed the RSR into a mid-engine race car and freed up space for a larger rear diffuser. For the race prototypes that followed the 911 GT1, Porsche therefore opted for V-engines, which offered greater design freedom for the entire vehicle. The RS Spyder was equipped with a 3.4-liter naturally aspirated V8 engine. The 919 Hybrid, with which Porsche secured three consecutive overall victories at Le Mans from 2015 to 2017, was powered by a 2.0-liter V4, while the 963 was powered by a 4.6-liter V8.

The trio of “Porsche, 911, boxer engine” is inseparable – probably until the end of the internal combustion engine

Porsche’s engineers are often accused of being somewhat stubborn. While all other sports car manufacturers have switched to mid-engine layouts, the team in Zuffenhausen is still refining the rear-engine design. Even after more than 60 years, they continue to stick to the 911’s silhouette. When you think of Porsche, you think of the 911. And the 911 has a flat-six engine in the rear. That will likely remain the case until the very end of the internal combustion engine.

© Cover photo: Porsche