There are a large number of non-radial piston engine configurations which have been used since the internal combustion engine was pioneered. More common configurations include inline, where – as the name implies – all the cylinders are arranged vertically in a straight line (slant engines included) and V engines of varying angles, in which two banks of inline cylinders form a V shape.
Not so common are W engines, a name used to describe two distinct configurations. The first more antiquated configuration consisted of three banks of inline cylinders – two angled and one vertical – sharing a common crankshaft. More recently, the “W’ configuration has consisted of two banks of cylinders, each possessing two rows of offset cylinders within the bank of cylinders.
And then there are the “flat” engines. Also known as “Boxer” or “horizontally opposed” engines, they are currently most well known for their widespread use in Subaru and Porsche vehicles in four- and six-cylinder configurations. To determine the actual number of engine configurations possible would require some advanced math that frankly, we don’t want to tax our brains with. Suffice it to say, there have been a large number of variants, both in cylinder positions and number of cylinders throughout the past century.
However, one of the rarest configurations to be seen is the flat-eight. While the video above lists the five examples as the “only” flat eight engines, to exist, it is slightly incorrect, as in the aviation world, there is both a Lycoming IO-720 and a Continental IO-720 engine. Along with the Jabiru 5.1-liter H8 engine, those three comprise the entirety of flat-eight aviation engines, and half of all production H8 engines, with the other half being made up of the three Porsche variants.
Boasting slightly smoother operation than horizontally opposed six-cylinder engines, the main reason the eight-cylinder versions exist were to increase displacement while maintaining more “ideal” cylinder dimensions. However adding that fourth pair of cylinders created packaging size concerns and those concerns are ultimately what has led to the engine’s overall lack of popularity, especially in automotive use.
The Automotive Flat-8: An Extinct Animal
The first successful (in motorsports) flat-eight was the Porsche 804 engine known as the Type 753. Produced in 1962 it was designed for Formula One racing, and spun north of 9,000 rpm while making just under 200 horsepower. Air-cooled, it measured out to 1494cc (91ci) from a 66.0mm (2.598 in.) bore and 54.6mm (2.149 in.) stroke.
From that engine came the powerplant for the Porsche 904/8 – the eight-cylinder racecar version of the 1964-1965 coupe also known as the Carrera GTS. That version of the engine, known as the Type 771, had a larger 1982cc (120 ci) displacement thanks to its 76mm bore – an increase of 10mm over the Type 753. The increase in displacement also bumped the engine’s output significantly, to 240 horsepower. A 2.2-liter variant dubbed the Type 771/1 was also introduced, making 270 horsepower.
While the two engine families were successful in their particular motorsport arenas, unfortunately for both the 753 and the 771 series engines, they were incredibly complex machines, which were insanely labor intensive to assemble, and had a penchant for random high-speed self-disassembly, leading to the end of that engine family.
That led to an all-new 3.0-liter flat-eight engine to be introduced in the Porsche 908 prototype car in 1968. A far simpler and easy-to-produce design, the bump in displacement came about in order to comply with the FIA’s maximum displacement rules. While the new, larger-displacement 3.0-liter didn’t spin as high, it did make significantly more power, turning the rollers to the tune of 350 horsepower.
Unfortunately, while that was an increase over the previous Type 771/1’s output, it wasn’t on par with the rest of the field, as it was still an older, air-cooled, two-valve design. Eventually, Porsche moved to a more compact, lighter-weight flat six design with forced induction, ending the unique configuration’s production run by a major manufacturer.
So while the horizontally opposed configuration currently lives on quite successfully in four- and six-cylinder automotive applications, the automotive flat-eight only exists in specialty historic applications, lovingly kept alive as examples of one of the coolest automotive engine configurations to grace the history books.