Flat-four engines: What’s the big deal?

The advent of the internal combustion engine in the late 1800s saw the launch of many different types of engine configurations; from the inexpensive inline variety, to exotic rotary type, including V-type configurations and the flat-type as well. These layouts of the engine are based on how the cylinders are placed. As the names imply, inlines have their cylinder in a single line (and are usually vertically mounted), Vs have two banks of cylinders in a V layout (the angle between the cylinders are variable 45 degrees and 180 degrees), while flat engines are horizontally opposed. The rotary engine is somewhat in a league of its own since it doesn’t feature cylinders and pistons in the conventional sense. It will feature in its own article coming soon.

Inline, V, flat, and rotary type engine configurations
Clockwise from the top: The BMW M2’s inline 6-cylinder engine, The Rolls-Royce Ghost’s V12 motor, an old Porsche 911’s air-cooled, flat-six engine, & Mazda’s old rotary engine. (Image source: BMW & Rolls-Royce press sites, & Wikipedia Commons)

We don’t really think about this when talking about cars (or bikes for that matter), but internal combustion engines create miniature explosions that are harnessed to provide propulsion. Today’s engines use a four-stroke cycle, colloquially: suck, squeeze, bang, and blow, which causes all sorts of motion and vibrations. The number of cylinders and their configurations help to minimize (or worsen) these vibrations. This is why each engine layout has its pros and cons:

The inline engine is relatively cheaper to build yet tend to be inherently unbalanced and create vibrations, except for inline-six engines. Inline engines require just one cylinder head and a single set of camshafts (that move the valves during the suck and blow stages). The engine notes on inline engine cars aren’t all that exciting, but five-cylinder and six-cylinder inline engines do have a distinct sound. BMW has championed the inline-six engine for a while now in its smaller M cars that boast brilliant performance with a unique sound track. Recently Jaguar (thanks to their new Ingenium modular engines) and Mercedes-Benz have announced that they will phase out their V6 engines in favour of inline-sixes.

With its iconic exhaust note and characterful performance, the inline six-cylinder engine in the BMW M2 is brilliant. (Image source: BMW press site)

V12s (essentially two inline-six engines mated to a single crankshaft) are said to be perfectly balanced as each cylinder bank negates the movement of the other. V8 engines usually have an angle of 90 degrees between the banks while V6 engines have 60 degree angles to minimize the vibration. However, neither is perfect as they require balancing shafts and harmonisers to balance the engine’s movements. The upside to this type of layout is that they are compact when compared to other types of configurations, especially with an increase in the number of cylinders. You can also find some unusual V-type configurations like the Audi R8’s V10 and Bugatti Veyron’s (and Chiron) W16. The iconic bass-y rumble of an exhaust note that V8 engines and other V configurations make is one of its most attractive features.

Bugatti Veyron Super Sport and its W16 engine
The W16 engine (inset) is unique as it consists of two ‘offset double-row’ banks of eight cylinders in V configuration. (Image source: Bugatti website)

Flat-engines, incorrectly referred to as Boxer engines (due to each pair of pistons moving in and out together like the gloves of a boxer), have been around since Karl Benz made one in 1897. The main difference between flat engines and Boxer engines is that boxer engines have one crankpin per cylinder, the flat (horizontal V) engine uses one crankpin per two horizontally opposed cylinders (refer to the image below). In the mid-70s, Ferrari’s Berlinetta Boxer and its successor, the Testatossa caused some confusion with their engine. Although termed as a 12-cylinder flat-engine, it is technically a V12 engine with a 180 degree angle between the banks.

This type of engine has advantages in the form of its low centre of gravity (aiding handling and minimising body roll) and a comparative lack of extra vibrations as the opposing cylinders balance each other better than inline and V configurations. This is particularly true for flat-six engines. The downsides to this engine are that it is more complex to build needing more parts, and more difficult to maintain (changing spark plugs is usually mentioned in this context).

The 180 degree V12 engine in the Ferrari 356 GT4 Berlinetta Boxer (left) has a single crankpin for two cylinders as you can see from the movement of the pistons, while the boxer-six in the 911 Targa (right) has individual crankpins for each one. (Image source: Wikipedia Commons)

Subaru has been making Boxer engines since 1966. The only Japanese manufacturer to offer cars with this engine layout (barring Toyota’s GT86, jointly developed by Toyota and Subaru), Subaru has indeed made a name of itself. The Impreza’s World rally Championship exploits have been greatly aided by this engine configuration. The road-going 2015 WRX is also a noted as a proper enthusiast’s car thanks to this feature. A significant detail is the unique warbling exhaust sound that accompanied some of the older iterations of the Japanese car maker’s models. Distinctive in its own right from Porsche’s Boxer engines thanks to unequal length exhaust headers which cause the more pronounced sound.

Subaru boxer diesel engine, Porsche 718 Boxster, Toyota GT86
Subaru’s diesel boxer motor is one-of-a-kind. The boxer engine configuration is found in the Porsche Boxster, Caymasn, various Subaru models, and the Toyota GT86. (Image source: Wikipedia Commons, Porsche press site & Toyota press site)

While the resurgence of the flat-four engine has made news recently due to Porsche’s downsizing efforts, the German manufacturer has a history of making these unique engines, beginning in 1938 with the first Volkswagen Beetle (incidentally designed by Ferdinand Porsche) and continuing until 1972 with the Porsche 914. Naturally, there was a humongous outcry from purists and Porsche fan boys about the loss of the inherently perfect flat-six engines that powered earlier Boxsters and Caymans. Sure there is a noticeable difference in the exhaust note (though a large part of that is down to the turbocharger) and the free-revving nature of the engine, in my opinion, Porsche hasn’t ‘sold out’ or ‘bowed to environmental regulations’. The company has simply returned to its roots.


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