In the past, we’ve published a couple of articles on flat engines and their history in motorsports and aviation (like this one on flat-eights, and this one on flat-twelves). Almost invariably, someone in the comments section uses the term “Boxer” incorrectly, and then, the fight is on. That confusion motivated me to write this article discussing the differences between the two engines — albeit much later than I had originally planned.
Since I can already feel your teeth starting to grind, and your fingers starting to type, “ACTUALLY…” I’ll just start with the bottom-line up front: all Boxer engines are flat engines, but not all flat engines are Boxer engines. Just like all thumbs are fingers, but not all fingers are thumbs. So, with that out of the way, let’s dive into the specifics.
First, let’s define the term “flat engine”. A “flat” engine is one in with a 180-degree cylinder angle (which often lays flat in the vehicle) and is also correctly referred to as a “horizontally-opposed” engine (abbreviated “H” — as in H6 instead of V6 or I6). It should not be confused with an opposed-piston engine, which we’ve also discussed, but is something completely different, and won’t be mentioned in this article again.
Flat engines fall into one of two categories: Boxer or non-Boxer. The determining factor being the rod journal arrangement of the crankshaft. A Boxer engine will feature an individual journal for each connecting rod, 180 degrees apart from one another. This allows each pair of opposing pistons to mirror each other’s movements.
Both pistons are at top-dead-center at the same time, bottom-dead-center at the same time, and so on. That mirroring makes Boxer engines inherently stable, as there are no unbalanced forces in the engine. This also means that Boxer crankshafts don’t require counterweights, although they are usually incorporated to reduce dynamic stresses and increase the engine’s longevity.
The other category of flat engines is the non-Boxer. The defining characteristic of the non-Boxer flat engine is that the connecting rods of opposing cylinders will share a crank journal, like a traditional “V” engine. That characteristic sometimes leads to a non-Boxer flat engine referred to as a “180-degree V.” With the shared rod journal arrangement, when one of the paired cylinders is at top-dead-center, the other is at bottom-dead-center.
Instead of the Boxer’s mirror-image choreography, or a traditional V-engine’s motion reminiscent of a newborn deer’s gait, the non-Boxer flat engine’s rotating assembly is locked in a constant, perfectly matched, thrust-and-parry dance. This means that, like any other V-engine, be it a 60-, 90-, or 120-degree bank angle, counterweights are required.
You might be asking yourself, “If you weren’t going to utilize the inherent smoothness of the Boxer configuration, why would you bother making a 180-degree V engine?” The answer comes down to vehicle dynamics, rather than engine dynamics. The flat engine offers a lower center of gravity, which vehicle designers can make use of in a performance setting.
While this knowledge isn’t ground-breaking, it is an interesting bit of information and can be an important distinction when discussing flat engines in-depth. You can even be the life of the party by dropping the tidbit from the flat-twelve article linked above about the Ferarri Berlinetta Boxer having a 180-degree V12, not an actual Boxer engine.
This animation of a BMW flat-twin gives you a solid look at how the Boxer principle functions.