On the boxer engine, you’ll notice the pairs of pistons move in and out together. This means that the primary forces when the piston reaches the top of the cylinder, as well as when they reach the bottom of the cylinder, are canceled out. On the inline 4 cylinder engine, it’s the same story, the primary forces cancel out as the pairs of pistons reach the top and bottom at the same time.
When we get into secondary forces, however, the engines begin to differ. Secondary forces are created due to the piston traveling faster at the top half of the piston than at the bottom half, something I’ll include a link to in the description for a video that breaks it down in great detail. What you need to know though, is that when the piston reaches the very top of the cylinder, or the very bottom, the secondary force points up or out from the piston. Now with the boxer engine, since the pistons point opposite each other, these forces are balanced out, resulting in a very smooth running engine. For the inline four, all of the forces point in the same direction, and thus do not cancel each other out, causing the engine to vibrate unless balancing shafts are used.
The boxer engine isn’t perfect, however, because the pistons do not perfectly align with each other, it creates a rocking moment which makes the engine want to rotate back and forth along the vertical axis. What’s fascinating, however, is that if you add two cylinders to either of these designs, whether it’s a boxer six or an inline six, you can perfectly eliminate all first and second order forces and moments. You might think the boxer six would have a rocking motion from the cylinder banks of three, but each bank of three cylinders cancels out the rocking motion of the other, unlike in a V6 configuration.
The other biggest advantage of the boxer engine is the low profile, which keeps the center of gravity low and thus reduces the amount of load transfer you have in the car during braking, cornering, or accelerating, which improves grip. With a lower center of gravity, you can also reduce body roll and choose to use softer springs. Additionally, in the event of a collision, it’s easier to position the engine so that it goes underneath the passenger compartment, rather than into the passenger compartment, for improved safety.
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