There's no doubt that you've heard the distinctive noise that is a Subaru in the distance. It's a rumble and you know what it means. A 2.5-liter "boxer" flat-4 engine is heading your way, and it's eager to spit noise into your ear holes. Why does it sound so different than any other engine on the road? That's a question best answered by our friend at Engineering Explained.
The Subaru boxer is of the horizontally-opposed flat design. That's only part of what gives it that unique sound, however, as it's more about the design of the headers. The 2.5-liter Subaru engine makes use of something called unequal length headers, which are exactly what they sound like. The header piping from cylinders one and three are of a different length than those from two and four.
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This means that the exhaust gas coming off the engine has to move different lengths, and the sound produced does as well. The sound is unevenly spaced in the exhaust and it exits the car as such. This is why you don't get a smooth tone, but rather one that rumbles in a manner different from the other vehicles on the road. The newer 2.0-liter engine found in the WRX has equal length headers, and thus it lacks the rumble you'd find in the 2.5-liter-equipped WRX STI.
Why choose one style of header over the other? Click play on the video above and find out.