How Does a Subaru Boxer Engine Work?

by Richard Rowe

The "flat" or horizontally opposed -- boxer -- engine might seem a little weird in today's world of inline and V-type engines, but it's actually one of the oldest and most prolific designs out there. It's seen a lot of applications over the years, and powered some of the best-known cars there are, but as of 2014 most manufacturers have abandoned the design. Most, except for the two automakers that are literally defined by it: Porsche and Subaru.

What a "Boxer" is

Imagine you had a V-4 engine, with the cylinder banks on either side angled up at 90 degrees like you would find on most V-8 and V-6 engines. Now, slice the block in half at the bottom of the "V," and lay the two sides of the block down flat. That, essentially, is a boxer engine. These engines are known as "horizontally opposed" designs because the engine's cylinders lay right across from each other, with the pistons moving side-to-side instead of up-and-down at an angle. The cylinder heads wind up on either side of the engine block, instead of on top. The terms "boxer" and "flat engine" come from the engine block's shape when viewed from the outside: it's shaped like a box, and flat relative to a "V" or inline engine.

History

This engine design goes all the way back to Karl Benz -- regarded as the "father" of the modern automobile -- who first patented the design back in 1896. Benz's boxer engine was actually the very first practical engine design every installed in an automobile, making his flat engine the grand-daddy of all automobile engines today. For quite a while, air-cooled boxer engines saw heavy use in aircraft, and they're still used in light aircraft applications today. Perhaps most famously, it was the engine that powered millions of Volkswagen Beetles, and their six-cylinder cousins made by Porsche. Several other manufacturers, including Chevrolet, have toyed with the design over the years, but Subaru made its name in America with its first boxer in 1969. The Subaru FF-1 was notable not just because it used a water-cooled boxer engine, but because it was the first front-wheel-drive Japanese car ever exported from Japan. And that was a big part of the reason Subaru used it in the first place.

Engine Features

The V-4 engine had already seen use in a great deal of Lancias, foreign-market Fords and Saabs back when Subaru started experimenting with boxers. The engine was compact, but was inherently unbalanced, shook the vehicle like a herd of buffalo, and was taller and heavier than it needed to be. The boxer engine solved those problems by being light and inherently internally balanced. That made for a smoother, higher-revving, more powerful engine that was still compact -- if about 20 to 30 percent wider -- than its V-configured counterparts. Oddly, one of the true boxer's defining features isn't just its engine block, but its crankshaft. A "real" boxer engine uses a separate crankshaft pin for each connecting rod, instead of putting two rods on the same pin, as is common on V-configured engines. This puts the pistons and cylinder firings in sync, which is what makes the boxer engine so smooth. Without this feature, the boxer would just be a flattened V-configured engine, instead of being truly horizontally opposed.

Benefits

Subaru initially used the boxer design because it recognized some of the inherent problems with front-wheel drive. Front-drive cars are nose-heavy, and put a lot of stress on the front tires; the flat engine's light weight -- about 15 percent less than an equivalent inline-four -- and low center of gravity helped with handling. It was also completely symmetrical side-to-side, which allowed Subaru to build a symmetrical drivetrain that reduced or eliminated the "torque steer" that many front-drive cars would have later. Subaru still uses the boxer for the same reasons today, but the addition of its trademark all-wheel-drive systems later made the symmetry aspect even more important. Subaru's "symmetrical AWD system" is identical on the left and right sides, just like the engine itself. This arrangement made and continues to make Subarus more balanced in terms of weight and power delivery to the wheels than vehicles with non-symmetrical drivetrains.

Considerations

For some time, Subaru used its boxer engine's low center of gravity and it symmetrical drive system to create the nimble handling that utterly dominated the world's rally racing courses. Many years and evolutions later, it's still regarded as one of the best total systems in the game. There are a lot of pros with this design. The only real reason that it's not more common today is that the engine's width makes it more difficult to fit the suspension under the hood. In practical terms, that also somewhat limits the engine's displacement, since a longer crankshaft stroke would mean a wider block. Porsche long ago solved this problem by adding two more cylinders to make a flat six, but Subaru's sticking with four cylinders for now. Additionally, the boxer engine's unique thrumming exhaust note isn't for everyone; but it's the only one for Porsche and Subaru. After all, when a particular engine design is the only design used by two of the world's most dominant names in on- and off-road racing, you have to assume it's got something going for it.

About the Author

Richard Rowe has been writing professionally since 2007, specializing in automotive topics. He has worked as a tractor-trailer driver and mechanic, a rigger at a fire engine factory and as a race-car driver and builder. Rowe studied engineering, philosophy and American literature at Central Florida Community College.

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