What's the difference between flat-4 and inline-4 engines?


The 4-cylinder engine has become a standard across the automotive industry thanks to its compact size and efficiency. But, one variant of 4-cylinder engine doesn't rule. There are two popular variations of the 4-cylinder engine: inline-4 and flat-4.

Each comes with benefits and downfalls, which is something Jason Fenske of Engineering Explained tackles in his latest video.

Foremost, both an inline-4 and flat-4 are based on the same four strokes: intake, compression, power, and exhaust. However, the differences are apparent right away. The flat-4's firing order is 1, 3, 2, 4, while the inline-4 fires at 1, 3, 4, 2. This firing order brings us to the first main difference.

The primary forces in each engine are identical, but the secondary forces do not mirror one another in the flat-4 and inline-4. In the inline-4 engine, when the pistons reach the top or bottom of the cylinder, the secondary forces point outward. In the flat-4 engine, the pistons point opposite of each other, which means the forces balance out.

This is why a flat-4 is smoother than an inline-4; the inline-4 engine has no way to balance the forces out, which causes vibrations and less balance. The flat-4 doesn't get away with perfection, though. Since the pistons are not aligned perfectly, the engine wants to rotate along its vertical axis with rocking motions.

A flat engine, often referred to as a "boxer" because of its horizontally opposed pistons resembling two boxers going at it, has another advantage with its low profile. The lower overall profile helps keep the center of gravity low and reduces the amount of load transfer during braking and cornering. But, the inline-4 engine swings back with its overall smaller size, which leaves room for suspension geometry and allows for a better steering angle.

Now, the answer to a question asked far too often: why does the flat-4 engine have a distinct burble? In the case of Subaru, it's been come to be known as the "boxer burble." This distinct tone actually comes from its unequal-length headers. The exhaust design often pairs well with the packaging needed for a flat-4 engine, and also happens to create the sound so many people know and love.

There aren't any quizzes on this information, but it's certainly fun to know and impress your friends with. Dive deeper into the knowledge in the video.

 
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