When Subaru talk about their famous ‘boxer’ engine, what do they mean? And what is the difference between a boxer engine and a conventional in-line engine or a V8?
Firstly, it’s worth noting that Subaru weren’t the first manufacturer to produce a boxer engine – Volkswagen and Porsche have been using a similar engine layout for many years, but Subaru could probably be credited for bringing more boxer-engined cars onto the market.
Subaru first adopted the boxer engine format when it released its Subaru 1000 in May 1965. In that period after the Second World War, Japan was just starting to get back onto its feet after the destruction caused to its manufacturing plants by the Allied Forces.
So, it was time for new car companies to shake off the memories of war and get on with production.
The Subaru 1000 was a new, family sized car, which was to use an engine that has been used in every production Subaru since that time – the boxer.
On its release, the Subaru 1000 was not only the first Japanese car to use that type of engine, it was also Japan’s first mass-produced front-wheel drive sedan. How things have moved on in the past 55 years.
The origins of the engine go back further than that, though.
Subaru were linked to the Nakajima company, which built Zero fighter boxer aircraft engines in Japan during World War 2, and which later became Fuji Heavy Industries, the current name of the Subaru parent company. The engines were chosen for much the same reason that the car engines are today – compact design combined with low weight.
But back to our description of boxer engines. In the majority of engines produced these days, the pistons are connected to the crankshaft via conrods which force the pistons up and down vertically (or in the case of “Vee” engines, almost vertically) in the block.
Subaru STi turbo engine cut-away.
With a boxer engine, the pistons and rods are horizontally-opposed to each other. There are two pistons on either side of the central crankshaft which operate horizontally in the block, rather than vertically.
Their resultant ‘punching’ motion gave rise to the term ‘boxer’ because the designers believed that the action of the pistons and the conrods suggested a boxer in a fight, fists punching backwards and forwards on successive strokes.
Incidentally, that first boxer engine from Subaru was rated at 977cc and developed a mere 55bhp, a far cry from today’s sophisticated, powerful engines.
One of the beauties of a boxer engine is its balance. Because it’s a symmetrical layout with a north-south crankshaft, the driveline is able to extend from the front of the engine right through to the rear wheels. In addition, the weight distribution either side of that straight line is symmetrical, a feature that enables the engine to benefit from this balance effect.
Subaru's 2.0-litre turbo engine.
Over the past 65 years, Subaru have had numerous opportunities to change to a more conventional engine layout design, but have stuck with the boxer all this time. One of the principal reasons is the engine’s balance and light weight. It’s also compact, is mechanically simple, strong and rigid.
If you look at a Subaru engine you’ll notice that the crankshaft webs are very thin because the flat-four is so well balanced. Compare a Subaru engine to a conventional in-line crankshaft and you’ll notice the large cast-iron balancing webs found on other engines.
The flat-four engine benefits from better volumetric efficiency. There’s equal-length intake manifolds and a crossflow cylinder head design which helps the engine’s breathing.
An incidental but important feature of the design is that it allows the engine to sit low in the engine bay, offering optimum handling, as opposed to other sorts of engines which sit very high in the chassis. Chassis balance is also a plus-point as the engine doesn’t overhang the front axle.
The boxer engine sound has made rally Subarus some of the sport's most popular cars.
It’s strong, too. Subaru engines commonly go for 200,000 kilometres or more without being touched, thanks to the initial design.
Structurally, the boxer engine has a rigid cylinder block where the crankshaft is sandwiched between the left and right crankcases with their own cylinder head on each bank.
As far as the cylinder heads are concerned, there are four valves per cylinder in a crossflow format, inclined at 30 degrees. This helps to keep the combustion chambers a compact size; there is also a single camshaft and roller rockers.
Turbocharged or non-turbocharged, Subaru’s now-familiar boxer engine has developed an enviable record right around the world for power, reliability and strength. On top of that, there’s that distinctive boxer beat from the exhaust that never fails to send a shiver of excitement down the spine.
For road use or competition, Subaru’s famous engine looks like punching on for a long time to come.
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