Small planes use boxer engines because they're lightweight, run smoothly, and have better cooling — all of which are crucial ...

When compared to inline-4s which only require a single cylinder head, horizontally opposed four-cylinder engines require two separate cylinder heads, which adds complexity.

When compared to inline-4s which only require a single cylinder head, horizontally opposed four-cylinder engines require two separate cylinder heads, which adds complexity.

H4 engines are horizontally opposed four-cylinder engines also commonly known by names like flat-four and boxer. For comparison, most four-cylinder automobile engines are upright inline designs.