How many engines can you identify with your eyes closed? Aside from one on its last legs. The flat or boxer engine found in most Subarus is an easy one to catch by its sound. It's part throaty bass - like the original VW Beetle - with a distinctive high-end note. The name comes from a unique configuration designed by Karl Benz in 1896; pairs of pistons pushing horizontally, much like the double glove touch of boxers before a bout. The classic boxer sound derives from both the piston configuration and the exhaust manifold design which consists, in a four-cylinder, of two long and two short pipes which help differentiate from some engines which are flat in configuration but don’t enjoy the unique crankshaft configuration which defines a boxer.
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The flat or boxer engine pushes pistons horizontally in opposition, each connected to a single crankshaft by its own crankpin, which makes it different from V-type flat engines which may be nearly 180 degrees in separation, but share crank pins, and don't share the unique boxer motion where pistons punch out on one side and then the other in pairs. The original boxer was a two-cylinder model, but engines up to twelve cylinders have been produced. Not surprisingly Subaru's implementation of the boxer configuration in four cylinders has been highly successful in passenger vehicles.
Karl Benz introduced an engine design in 1896 which quadrupled power from 5 horsepower to 20 horsepower and met the increasing demands for performance in both racing and passenger cars of the time. The basic single-crankshaft design which balanced two cylinders in horizontal opposition for an even, balanced motion. With intake and exhaust cycles of the cylinders in 180-degree opposition, the engine not only provided a platform for increased performance as vehicles evolved but offered smoother, more reliable operation in commercial vehicles and large-scale transit vehicles.
Count smaller car companies which didn’t survive and international car makers who don’t sell in the U.S. and you can come up with nearly two hundred companies which put a version of the boxer in their vehicles, but names Americans will recognize include Subaru, Porsche, BMW, but also early Fords, Toyotas, Saab, Lancia, Volkswagen, and Citroën. The boxer has been a staple in military vehicles, especially Volkswagen's early models, and has powered a range from Citroën's compact 2CV to Ferrari's Testarossa and Berlinetta Boxer.
Subaru has kept the boxer as its mainstay, even in an unsuccessful but elegant F1 twelve-cylinder version. It’s hard to imagine, but some cars were previously boxer-based and now use perhaps less elegant power plants, or evolved to V-configuration engines such as the early Ford lines which included boxers in their 1903-04 Model A, the model C, Model F, and Quadricycle. The air-cooled version of the Porsche 911 was a long-time boxer engine vehicle which ended as the 993 in 1998 and its historical relative the air-cooled VW Beetle ended production in 1997 after nearly 70 years of boxer power.
Subaru cars from as far back as the 1970s can be identified by the same low-toned drumbeat of the boxer engine when Subaru took advantage of the flat-style engine to locate the spare tire in the engine compartment. Along with Porsche, Subaru is one of the few car companies which has kept with the boxer engine for racing performance. The Toyota Scion FR-S and 86 produced from 2012 to the present and the Subaru WRX share a common boxer lineage for performance results, switching in 2005 to a new FB/FA engines which incorporate increased durability and new engine technology including, in 2019, direct injection with a large selection of aftermarket performance equipment also available.
From the list of companies that have put boxer engines to work it’s clear that there are solid reasons to use the opposing piston design including benefits which are found in more complex engines such as the Wankel rotary engine, with number one on everybody’s list the balanced operation, something also found in the rotary engine, which reduces vibration and the related costs and weight involved in getting the shakes out of traditional engine configurations. Performance and fuel consumption make this engine configuration a great mass market car powerhouse which served in the original Porsche 911 from 1963 through 1989, but they have also have endeared the boxer to motorcycle manufacturers and made it a staple of small, single-engine airplane manufacturers who require lightweight power.
The low-vibration boxer engine has served motorcycle manufacturers like BMW and small plane engine manufacturers such as Lycoming whose O-360 flat-four engine was used in the most-produced aircraft of all time, the Cessna 172, starting in 1968 and in many other small planes and rotorcraft. Especially in the air, the elimination of vibration-reduction components which aren't needed allowed the development of engines which consumed a minimum of the plane's precious lift allowing greater plane capacity and range. Motorcyclists enjoy the comfort of a flat opposed-two engine running smoothly on their ride, and BMW revived the use of the boxer in their 2007 HP2 which earned acclaim for sportiness, light weight, and performance beyond other models.
With a somewhat different engine cycle supporting the pairs of pistons and simplified valve operation, designers had to think a bit about performance enhancements. Turbos, superchargers and compression changes brought Subaru's four-cylinder engines over 600 horsepower and other boxer designs have reached up to F1 competitive performance numbers on the flat-12 while WRX owners, in particular, have experimented with air-oil separation, custom exhausts, intake mods and custom tuning to the point where performance-oriented suspension adjustments are a must. In the increasingly popular hybrid car class, the 2019 Forester is available with an "e-boxer" system which combines battery power with classic flat-4 performance.
Can-Am competition has been for many car manufacturers a proving ground for engine designs and for Porsche the rules offered room to test designs like the winning 1972 850 horsepower Porsche 917 turbo, one of Porsche's flat-12 and experimental flat-16 917 models developed starting in 1969 which became an almost unbeatable part of Can-Am racing history. Porsche used the smooth performance, graceful power, and deep rumble of boxer-powered high-end cars in powering consumer performance car models such as the Boxster and original 911.
Performance cars from the usual elite brands like Porsche's 1969 917 and Ferrari's 1964 Model 1512 brought the boxer engine into the elite 12-cylinder class but not enough to make much headway in the consumer car market, but producing results which improved the flat-6 engine, the largest flat-engine cylinder count found in widespread production in automobiles and aircraft. The flat-12 boxer expanded designs such as Porsche's 917 beyond 8-cylinder powerplants to a well-balanced six-and-six array of pistons in a flat configuration and even, experimentally, in a Porsche flat-16 which didn't outperform the 12-cylinder enough to earn its keep on the race circuit. Flat-12 engines did find productive uses in heavy equipment such as the 1954 Panhard armored car and the WWII British Covenanter tank.