For much of the world, the day after Christmas is celebrated as “Boxing Day,” so in the spirit of the occasion, today we’re looking at box-er engines.
What Is a Boxer Engine?
A “boxer” engine is a nickname for a type of engine that uses horizontally opposed cylinders.* The engine architecture means that the pistons all line up in a flat plane, which is why you might see these motors referred to as “flat fours” or “flat twins” in reference to the number of cylinders.
In engineering literature, you’ll often hear these engines called “horizontally opposed” as well or, if you’re really short on time, “H4” or “H6” depending on the cylinder count.
These engines got the name “boxer” because if you look at the way the engine is laid out, the opposed pistons appear to fight each other—moving in and out simultaneously as the engine rotates through a combustion cycle, touching “gloves” together, like the ceremonial start of a boxing match.
* All opposed cylinder engines are not automatically considered “boxer” engines, because by definition, the nickname implies that the pistons mirror each other, mimicking the aforementioned boxing match. Though rare, some opposed-cylinder engines do not use a symmetrical back-and-forth crank movement.
It’s an interesting engine design with a few distinct advantages and, although not as common as a Vee or inline configuration, is still popular with several automobile manufactures to this day, notably Subaru and Porsche.
Why Automakers Use Boxer Engines
Admittedly, we could spend a few thousand words describing the engineering nuances behind some advantages of the boxer engine design, so we’ll just stick with some broad strokes here.
For four cylinder engines, boxer engines are better inherently balanced that their inline-four counterparts. Again, without going into a dissertation on vehicle engineering, the opposing piston movement in a boxer four can cancel out vibration, making for a smoother running engine. (This advantage goes away however when you compare in a inline six to a H6, though.)
As an added bonus, thanks to the amount of exposed surface area on the opposed cylinder jugs, boxers are often able to shed heat better than a traditional inline or Vee engine—which explains, in part, why so many early air-cooled engines used a boxer configuration. The lack of a complex water cooling system adds significantly to the engine’s overall simplicity and, some would argue, it’s durability.
The obvious drawback with that increased surface area is, of course, physical engine size.
Boxers tend to be relatively flat, with much of their mass spread out on a horizontal plane, again in contrast to a more vertically-oriented inline or Vee engine. With the bulk of the engine lower in the body, the vehicle’s center of gravity is lower too, which can have big benefits to its overall handling characteristics.
Conversely, putting the engine lower in the car means that engineers often have to work around clearance issues with steering and suspension geometry. Those issues can be mitigated in large part by putting the engine in the back of the vehicle, like what you’ll see on a Porsche 911 or air-cooled Volkswagen.
Some drawbacks include serviceability—and if you’ve ever had to do spark plugs on one, you know what we’re getting at here.
Because the boxer’s cylinder heads are located on the side of the engine, it means any work to the “top” of the cylinder is actually done at the side, often requiring your to work in the tight space between the head and, say, a fender or steering/suspension components.
And remember, since boxer engines tend to be physically wider than a similar displacement inline or Vee design, that space is usually pretty tight.
What Cars Use a Boxer Engine?
We alluded to the inherent cooling benefits of the boxer design, so you may not be surprised to hear that the most well-known air-cooled engine of all time is a boxer—the ubiquitous Volkswagen flat four.
Four- and six-cylinder boxer engines are the de-facto powerplants for a wide range of Porsche automobiles as well—though they’re no longer air-cooled.
One of the reasons why automakers moved away from air cooling was the challenge of meeting ever-tightening emission requirements. Since air-cooled engines operate at a wide temperature range, it’s difficult to get accurate, repeatable vehicle emissions without the aid of water cooling to stabilize engine temperatures and create uniform emissions.
Fun Fact: The first era of water-cooled flat fours from VW were nicknamed “Wasser” boxers—or Water boxers auf Englisch.
In Japan, Subaru began producing opposed cylinder engines for its cars back in 1960s and has since become synonymous with boxers. In fact, the signature burble of a nice, tuned Subaru exhaust can be credited to the unequal length exhaust headers—a necessary design constraint resulting from the horizontally opposed engine layout.
As we learned during our MagnaFlow podcast interview, it’s an exhaust note that simply can’t be replicated by an inline or Vee cylinder configuration. .
When it comes to boxers Stateside, one vehicle immediately comes to mind: the Corvair.
It’s no secret that the Corvair itself was created, in part, as response to the European import market lead by Volkswagen, so the fact that Chevy opted for a flat air-cooled six wasn’t that shocking.
And much like VW, the flat six found itself in a family of Corvair-derived trucks, vans, coupes, convertibles, and sedans. While most gearheads are familiar with the Corvair’s saga and ultimate demise, it’s worth mentioning that variations of its air-cooled boxer six have soldiered on in dune buggies, aircraft (and air boats), race cars, and hot rods.
Perhaps the most famous…or infamous…use of a horizontally opposed engine in an automobile was the ill-fated Tucker 48.
It’s 100% true that the 48’s flat six engine was originally intended for a helicopter, but Tucker engineers significantly altered the engine (notably converting it from air to water cooling) in order to adapt it to an automobile. After the bugs were worked out, the engine made around 160 horsepower and, like many of the boxers on the list, it was carried in the back
Other Places You’ll Find Boxer Engines
From a smooth, naturally balanced design to its unique footprint, there’s a lot to like about horizontally opposed engines. As a result, boxers have wound up in a lot of places.
We’ve already talked about their use in motorcycles, particularly with BMWs, but plenty other bike manufacturers used flat engine architecture too—and we’re not just talking about the BMW-derived Urals or Chang Jiangs, either.
The most noteworthy non-BMW example is perhaps the Honda Goldwing. The low center of gravity, low vibration, and linear driveline geometry makes the opposed-cylinder engine an attractive choice for a shaft-driven motorcycle and, as such, Honda engineers chose a boxer design for their groundbreaking touring bike.
Another interesting boxer motorcycle came from Douglas over in the U.K. early in the 20th century.
Instead of mounting the engine in a transverse orientation like BMW and Honda did, Douglas placed the boxer longitudinally, inline with the motorcycle. The configuration allowed the engine to be placed really low in the frame, and contributed to Douglas’ success in early motorcycle racing.
The configuration proved popular for a while, and several other motorcycle manufacturers imitated Douglas’ inline boxer approach—including both Indian and Harley-Davidson at one point.
Sadly, Douglas and its inline boxer layout faded away during the 1950s.
As alluded to earlier, boxer engines are relatively simple designs and are capable of respectable power, which means that opposed-cylinder engines are common in lightweight aircraft as well, ranging from purpose-built aviation engines, to Corvair- and VW-derived airplane powerplants.
The engine’s characteristic layout opened up unique mounting possibilities, allowing the aircraft designers to put the engine where it would offer the best weight balance in the nose of the airplane.
It’s also important to mention the role of the air-cooled VW boxer four in the aftermarket kit car industry. Thanks to their abundance, compact size, and lack of coolant plumbing, VW-sourced boxers have been the go-to choice for kit car builders for over a half-century now.
From Mercedes Gazelles to GT-40 clones, the humble VW four-banger has powered them all.
In the same vein, the Meyers Manx and the many copycats it inspired are typically powered by a flat four boxer engine poached from VW.* Again, the engine’s lightweight, compact footprint and the lack of cooling plumbing means these boxer powerplants are practically tailor-made for swaps—particularly in lightweight kit cars, light aircraft, and trike conversions.
* Another Fun Fact: The dune buggy famously driven by Steve McQueen in The Thomas Crown Affair was actually powered by a boxer six poached from a Corvair.
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All told, horizontally-opposed engines are an integral part of automotive history—and if Subaru, VW, and Porsche enthusiasts have anything to say about it, we’ll certainly be seeing the boxer design evolve well into the future.
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