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Flat-6
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A flat-6 or horizontally-opposed-6 is a flat engine with six cylinders arranged horizontally in two banks of three cylinders on each side of a central crankcase. The pistons are mounted to the crankshaft such that opposing pistons move back and forth in opposite directions at the same time, somewhat like a boxing competitor punching their gloves together before a fight, which has led to it being referred to as a boxer engine.
The configuration results in inherently good balance of the reciprocating parts, a low center of gravity, and a very short engine length.
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Encyclopedia
A flat-6 or horizontally-opposed-6 is a flat engine with six cylinders arranged horizontally in two banks of three cylinders on each side of a central crankcase. The pistons are mounted to the crankshaft such that opposing pistons move back and forth in opposite directions at the same time, somewhat like a boxing competitor punching their gloves together before a fight, which has led to it being referred to as a boxer engine.
The configuration results in inherently good balance of the reciprocating parts, a low center of gravity, and a very short engine length. The layout also lends itself to efficient air cooling. However, it is an intrinsically expensive design to manufacture, and somewhat too wide for compact automobile engine compartments, which makes it more suitable for luxury sports cars, cruising motorcycles, and aircraft than ordinary passenger cars.
The shape of the engine suits it better for rear engine and mid-engine designs, where the low center of gravity is an advantage; in front engine designs the width interferes with the ability of the front wheels to steer. Only a few auto makers, including Porsche and Subaru, currently use horizontally opposed engines. Porsche continues to be the most prominent manufacturer of flat-6 engine luxury sports cars, while Subaru uses it in its all-wheel drive cars, where the difficulties of fitting the engine between the front wheels are offset by the efficiency of adding four-wheel drive to the layout. In the past a number of other manufacturers have used them, notably Tucker in the 1948 Tucker Sedan, Citroën in the GS, and Chevrolet in the 1960s Corvair with flat-6 air-cooled engines.
Balance and smoothness
The movement of the pistons in a horizontal engine is all in the same plane, so it creates less vibration than in a V-configuration engine; particularly one with an odd number of cylinders on each side of the engine, like a V6. Unlike the V6 but like the inline-6, the flat-6 is a fully balanced configuration which is in perfect primary and secondary balance. The three cylinders on each side of the crankcase tend to have an end-to-end rocking motion, like a pair of straight-3 engines, but in the usual boxer engine configuration, the imbalances on each side cancel each other, resulting in a perfectly smooth engine.
The flat-6 is also smoother than the flat-4 or inline-4 because the power strokes of the cylinders overlap in a four-stroke cycle engine. In these four-cylinder configurations, pistons are 180 degrees apart in crankshaft rotation and start their power strokes every 180 degrees, so each piston must come to a complete stop before the next one commences its power stroke. In the flat-6, each power stroke begins 120 degrees after the previous one starts, resulting in 60 degrees of overlap between power strokes and a much smoother delivery of power to the flywheel. This intrinsic smoothness is desirable in cars, motorcycles, and light aircraft which are more powerful and expensive than more common designs.
Corvair
Introduced in 1959, the Corvair was intended to be an economy car. Chevrolet used a rear-engine layout to emulate the Volkswagen Beetle, but made the Chevrolet Corvair engine a flat-6 rather than a flat-4 in an attempt to match the inline-6s in competing American economy cars. This was an unusual choice, if only because a flat-4, although not as smooth, could have been built with the same power and acceptable vibration levels for an economy car. Since a flat-6 is relatively expensive to manufacture, Chevrolet tried to reduce costs elsewhere, with serious consequences. With the majority of the weight behind the center of gravity, a swing axle rear suspension, and no sway bars or any other method of controlling body roll, early Corvairs suffered from terminal oversteer (the rear tires could suddenly lose traction and swing around, causing the car to spin-out or crash). As a result the car could handle dangerously if drivers cornered too aggressively. Chevrolet's recommendations for controlling this included inflating the front tires to a much lower pressure than the rear tires to equalize the traction at the front and rear ends. However many owners overlooked this important requirement, and the resulting alleged rollovers and crashes gave the car a reputation for poor handling. Ralph Nader's 1965 book, Unsafe at Any Speed destroyed the reputation of the Corvair and led to much tighter regulation of automobile safety in the United States. Although Chevrolet corrected the problems in later Corvairs by using design elements and components from the Chevrolet Corvette sports car, giving it much better handling, this made the Corvair more expensive to manufacture than competing economy cars and did little to overcome its bad safety image with the public. The 1964 introduction of the Ford Mustang, which was cheaper to manufacture because it was based on the Ford Falcon economy car chassis, but could be made more powerful because it could accept an optional V8 engine rendered the Corvair uncompetitive, and it was discontinued in 1969. As a side note, the NHTSA in 1972 gave the Corvair a clean bill of health on this safety issue, a fact that was overlooked by the press at the time. Mr. Nader admitted that the reason he went after GM was strictly due to their having "deep pockets" for his newly devised tort scheme .
Porsche
Porsche is the most prominent manufacturer of cars with flat-6 engines. In 1963 it introduced the Porsche 911 sports car with a rear mounted, air-cooled, 2.0 L overhead cam flat-6 engine as a more powerful alternative to the pushrod flat-4 powered Porsche 356. Early 911s had problems with terminal oversteer due to the heavy rearward weight distribution of the rear engine. In high-speed cornering, this caused the rear wheels to run wider than the front wheels, requiring the driver to reverse the steering input to prevent the car from turning too quickly and potentially skidding off the road facing backwards. Racing drivers liked it because the cars turned faster than the ponderously understeering front-engined cars of the time, but it often tricked inexperienced drivers - if they panicked and lifted off the throttle or applied the brakes in a high-speed corner, the 911 was likely to spin out of control and crash. However, 40 years of suspension system development have largely eliminated the oversteering, and modern stability-management systems will compensate for slow driver reflexes, making modern 911s much safer for novices to drive.
The 911 is one of the longest-lived car lines in history and has continued on with variations on the 911 body to the present day. The original engine turned out to have much more capability for enlargement than its designers originally intended and has grown considerably in displacement and power over the years. The 2007 versions in the Porsche 997, the latest member of the 911 family, have nearly twice the displacement and up to four times the power of the original 1963 engine. Although Porsche has produced flat-4, straight-4 and V8 powered sports cars over the years, all current Porsche sports cars use flat-6 engines. The Porsche Boxster and Porsche Cayman use them in a mid-engine rather than rear-engine layout.
Subaru
Subaru offers flat-6s as optional or standard engines on its larger models, starting with the ER-27 in the Subaru XT6, the EG-33 in the SVX, and the EZ series currently used in the Legacy, Outback, and B9 Tribeca SUV. The use of a flat engine results from Subaru's history as a manufacturer of aircraft engines, and the flat-six was relatively easy to derive from its existing flat-4 since, unlike V engines, flat engines can be expanded by simply adding more cylinder pairs to the end of an engine without the complications of changing a V layout. In contrast to Porsche, Subaru uses it in a front-engine configuration with the engine mounted longitudinally ahead of the front axle and the transmission mounted longitudinally behind the front axle. Although this layout is intrinsically more expensive to manufacture, less compact, and less space efficient for front wheel drive than a transverse V6, it allows the addition of four-wheel drive by taking power off both the front and back ends of the transmission, since the transmission is located between the front and rear axles. As a result of this unique layout, Subaru now specializes in all-wheel drive vehicles.
Aircraft engines
Lycoming developed a very successful series of flat-6 aircraft engines, as used in many Cessna aircraft. Continental Motors is another major manufacturer of flat-6 aircraft engines. Flat engines largely replaced the historically more popular radial engines in small aircraft after World War II because the radials, although they had good cooling, added large frontal area which caused too much drag. It was relatively easy to derive flat-6 and flat-8 engines from a flat-4 design by simply adding more cylinder pairs, and the engines could use many of the same components. The only problem is that in an air-cooled engine there can be cooling problems with the middle cylinder pairs. The flat-6 is more powerful and smoother than the flat-4 aircraft engine, although also more expensive.
Other engines
Flat-6 engines are also used in Honda Gold Wing and Honda Valkyrie motorcycles.
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