Once it was synonymous with Volkswagen and its sporting kin Porsche, but these days people are more likely to associate Japan's Subaru with the horizontally opposed engine.
It's also known as a boxer and as a “flat” engine.
In a world where pistons mostly go up and down, a few carmakers have chosen motors in which the pistons go “sideways”, although it's the configuration of preference in piston-engined light aircraft.
So what makes them want to march to the tune of a different exhaust note? And if the flat layout is so good why doesn't every company use it?
On the plus side, a horizontally opposed layout offers a low centre of gravity, short length and good balance. And its pistons counteract each other to cancel vibration.
The layout is also excellent for air cooling, a key reason why Volkswagen chose it in the 1930s because it did away with a radiator and its associated parts. Air cooling is why the aviation industry likes flat motors.
On the other hand they cost more to make and this is often the spoiler when designs are costed. Also, designers may not like the width.
However, some companies wouldn't be without it, like Subaru and Porsche. Subaru now makes more horizontally opposed engines than anyone else. Its engineers like the relatively light weight and the ability to mount it lower in the chassis. These attributes help provide the excellent handling and dynamics for which the brand has become known.
Subaru has also introduced the world's first horizontally opposed diesel engine, the boxer turbo diesel. The company chose to stay flat for its diesel partly because the crankcase construction is strong enough to take the high combustion pressure.
There's no reason to avoid a flat engine when you're looking for a car, other than they may use a little more fuel than equivalent “vertical” configurations. However, fuel economy issues may be attributable to the sporty nature of the motor that entices a driver to drive enthusiastically.
It's cool to own a vehicle with a flat motor. They're supposed to suggest that the owner thinks out of the square and enjoys the driving experience. And they lack the negative connotations that may come with a V8.
After all, one of the world's finest sports cars uses them and hip people in the 1960s drove vans with them. James Dean had one. You can't really go wrong.
