Like the W formation engine discussed recently, the flat engine has been around since the very early years, in fact Karl Benz used a boxer engine in his first car! There is actually a subtle difference between a flat engine and a boxer engine: both are flat however a boxer engine uses horizontally opposed cylinders where the corresponding pistons on each side reach top dead centre at the same time. On a normal engine the pistons reach top dead centre at different times.
There are other differences too. The timing on a boxer engine means that it is much smoother than other engines and as such doesn’t require any balancing shafts – but they do require bigger flywheels due to larger torsional vibrations, a by-product of the firing order.
Strangely, flat non-boxer engines are sometimes known as 180º V’s because they are essentially V formation motors that have been sat on and flattened out!
Flat engines are typically used in rear or mid mounted configurations because they are wider that a normal motor. This presented a problem for some early front engine designs as the motor interfered with the steering components. Subaru fixed this by moving the motor back in the chassis which removed the problem and also provided a better weight distribution in the car.
Probably the most well known flat motors were found in related cars: the VW Beetle and early Porsches notably the 356 and 911. The Beetle used a flat 4 and like the early Porsches they were air-cooled. It was only later in the 911’s life that it became water cooled as the power output had increased substantially.
Citroen were another exponent of the flat engine in their 2CV, Dyan and GS series cars. Most of the early small flat engines like the VW and Citroen were inspired by BMW. They had developed a flat boxer motorcycle engine back in the 1920s – a design that was in production for many decades!
It is noticeable that most flat engines came out of Europe – the design of the Chevy Corvair was inspired by the Beetle and Porsche 356 and used a 2.3 litre flat motor which was completely different to all the other motors built by the US arm of General Motors. Japan typically produced them for motorcycles – Honda use them in the GoldWing in 4 and later 6 cylinder configurations. Subaru were the only Japanese manufacturer to develop flat or boxer engines and these can be found in the Forester, Legacy and Tribeca models.
In racing and high performance cars, the notable users have been Porsche and Ferrari – remember the Berlinetta Boxer? This used a 180º V12 rather than a true boxer motor. Porsche also developed a flat 12 non-boxer motor for its racing campaigns in the late 1960s and early 1970s.
Who knows, the flat motor may become very popular as manufacturers look for lightweight and small designs that can be fitted to smaller cars for the future. If they can power motorcycles, why can’t they power small cars – could BMW develop the motorcycle engine to go into smaller city cars?
This article first appeared on the Motoring Weekly Patreon page.