The boxer engine is firing on all cylinders, thanks to the efforts of Porsche and Subaru
Boxer Shots Photo: Subaru & Porsche Press Pix

Long before inline and vee engines, there were reciprocating internal combustion engines that were “flat”, with the pistons traversing a horizontal plane. One of the earliest automotive pioneers, Karl Benz (of Mercedes-Benz fame), built and patented a flat-twin for several reasons. Firstly, two cylinders were better than one. Secondly, horizontally positioned cylinders on either side of the crankshaft gave geometrical symmetry and a balance of mechanical forces. After all, the power stroke in a 4-stroke engine was about forcing the piston “down” its cylinder, and doing it “sideways” in the engine bay seemed more elegant.

In the early days, engineers struggled to stop their internal combustion projects from shattering to pieces – not from the combustion process, but from the violent vibrations that resulted from the reciprocating motion of the piston. Increasing the number of cylinders was the most basic solution, based on sound technical principles.

The logical consequence of having more cylinders would be the adoption of a flat layout, where the pistons move towards or away from the cylinder head at the same time. With the inertial forces of the pistons acting horizontally in opposing directions, they are effectively cancelled out. Thus was born the “boxer” – the horizontally opposed engine is termed as such because the pistons punch away from the centre in opposite directions at the same time.

Years later, when Ferdinand Porsche set out to design a small engine for the original folks’ wagon, he decided on a pushrod 4-cylinder with its pistons arranged horizontally. The configuration was compact and inherently balanced in terms of combustion forces. This vintage flat-4 is almost as short as an inline-2, and since the cylinders are “split” two on either side, the engine has a low profile. So, a flat-4 provides obvious benefits for the vehicle’s packaging and centre of gravity, the latter a part of the handling equation. 

Today, only Japanese automaker Subaru produces flat-4 engines for passenger cars, but there were a few others in the past. Most notable of these has to be Alfa Romeo. One of the Italian marque’s most memorable boxer engines is the 1186cc unit in the 1971 Alfasud. It produces barely 60bhp, but its exceptional smoothness means drivers frequently revved beyond the 5800rpm redline (there was no ignition cut-out in those days), with only carburettor restrictions and poppet valve problems preventing a literal thrash-to-death.

French brand Citroen, too, used to make a charming flat-4, which was only ever used in the sophisticated GS of the 1970s. Originally displacing a mere 1015cc, the air-cooled boxer is famous for its sweet nature, which masks its lack of power (55bhp).

The two boxer-4s mentioned offer a low centre of gravity, which is one of the main reasons why the Alfasud and Citroen GS possessed a level of handling years ahead of anything else in their category during that era.

Boxer powerplants with eight (Porsche 908 racecar) or 12 cylinders (Ferrari’s BB and Testarossa) exist but remain elusive. More common and well-known worldwide is Porsche’s classic flat-6, introduced in 1964 to replace the Beetle-derived flat-4 in the 356. The then-new flat-6 was ideal for a rear installation, being short and low-slung. It turned out to be perfectly balanced, too, with only the inline-6 and 60-degree V12 able to match its natural equilibrium.

Created for the Porsche 911, the flat-6 in question went on to become one of the most charismatic engines in motoring history. Every single flat-6 used in Porsche 911s since the beginning – no matter what capacity, capability or cooling method – has always been smooth and free-revving.

Besides the modern 911 (see cover story) and its fellow Porsche models, Boxster and Cayman, the only other flat-6 application is Subaru’s EZ36. Employed in the Outback and Tribeca SUVs, this Japanese 3.6-litre is less exotic than its German equivalent but boasts its own boxer character.

Subaru also persists with its range of flat-4 engines. Used in the Impreza, Forester, Legacy, Exiga and, most recently, the BRZ, Subaru’s signature motor works well with 4x4, another cornerstone of the firm’s car-making philosophy.

For the rest of the automotive world, the boxer engine remains an expensive oddity. Mass-producing inline engines continues to be easier and more cost-effective. Therefore, it will still be Subaru and Porsche keeping the boxer dream alive. 


 

This article first appeared in Torque.

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