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. This should not be confused with the Opposed-piston engine. 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. 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. 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.

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 Preston Tucker in the 1948 Tucker Sedan and Chevrolet in the 1960s Corvair with flat-6 air-cooled engines. The Citroën DS was originally intended to be fitted with an air-cooled flat-6, but this never materialised.

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, like a V6, with an odd number of cylinders on each side of the engine. 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 inline-triple 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.

Aircraft engines

Lycoming developed and currently produces a very successful series of flat-4 and flat-6 aircraft engines, and one flat-8 engine. The configuration is expressed in the engine's model numbers as the letter "O", standing for "opposed" in reference to the cylinder arrangement. Continental Motors is another major manufacturer of flat-4 and 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. Jabiru and Rotax are also reciprocating aircraft engine manufacturers, and both are more popular in the experimental, ultralight and new sport category aircraft. In the 1980s, Porsche adapted their 911 unit to become the Porsche PFM 3200 aero engine, but production ceased in 1991.

Automotive use

Flat-six engines are wide, and would restrict steering lock if placed in the conventional positions for front-engine drivetrain layouts. As a result, most flat-six automobile designs have been rear-engine or rear mid-engine designs, and have been subject to the limitations and drawbacks of these designs. The longest surviving flat-six-engined model, the Porsche 911, is a rear-engined sports car with acceptably compromised interior room and a long history of suspension development to utilize the characteristics of the drivetrain layout.

Subaru, an automobile manufacturer with a history of making aircraft engines, has adapted a front-engine configuration for use with its flat engines. The engine is mounted longitudinally ahead of the front axle and the transmission is 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 layout, Subaru now specializes in all-wheel-drive vehicles.

Subaru's 2003-2009 EZ series EZ30 engine featured a Porsche designed and German manufactured Variable Valve Lift (VVL) system, that further enhances high range power and smoothness at idle. It was discontinued on the successor of the EZ30 engine, the EZ36 engine.

The Chelyabinsk Tractor Plant 6H 358 is a 16.95 L flat-6 multi-fuel engine.

List of automobiles with flat-six engines

  • 1948 Tucker Sedan: 1948 (rear engine, rear drive)
  • Chevrolet Corvair: 1960 - 1969 (rear engine, rear drive, firing order 1-4-5-2-3-6)
  • Porsche
  • 911: 1965 - present (rear engine, rear or four-wheel drive)
  • 914-6: 1970 - 1972 (rear mid engine, rear drive)
  • 959: 1986 - 1989 (rear engine, four-wheel drive)
  • Citroën
  • Citroën DS 1955 using a combined flat six from a Citroën 2cv which had a flat 2 engine.