A Porsche flat-six is a boxer and does not rumble. A Subaru flat-four does, or used to. So the noise is not something the horizontally opposed layout produces on its own — it is the product of a firing order that fires two cylinders on one bank back to back, and an exhaust manifold that made the resulting unevenness worse.
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Two explanations circulate and both are wrong on their own.
“It is because it is a boxer.” Horizontally opposed engines are all over the place — Porsche flat-sixes, aircraft flat-fours, older Volkswagens — and none of them make that noise. The layout is not sufficient.
“It is because the headers are unequal length.” Closer, and the most important single factor, but not the whole thing either. Plenty of engines have imperfect manifolds without acquiring a signature. The unequal headers matter because of what they are acting on: a pulse train that is already lopsided.
Subaru’s flat-four uses the firing order 1-3-2-4, and cylinders 1 and 3 sit on one bank while 2 and 4 sit on the other. A four-cylinder four-stroke fires every 180° of crank rotation, so laid out in sequence that gives:
Follow one bank alone and its exhaust manifold sees two pulses 180° apart, then a 540° wait, then two more. That is a strongly grouped, strongly uneven pattern — a pair of beats followed by a silence, repeating. As with the crossplane V8, an uneven pattern that only repeats over the full 720° cycle generates low-frequency content well below the firing frequency, and low-frequency content is what the ear reads as a rumble rather than a buzz.
Note that the engine as a whole still fires perfectly evenly every 180°. The unevenness only exists from the point of view of one bank’s plumbing — which is exactly the point of view that produces the sound you hear.
On the classic turbocharged EJ engines, the four exhaust runners were not the same length. The turbocharger sits on one side, and the pipes from the far bank have to travel across and around the engine to reach it, while the near-bank pipes take a short route. Differences on the order of tens of centimetres were normal.
A pressure pulse travels down an exhaust runner at roughly the local speed of sound — high, but finite, and slower in a long pipe than the time it takes an engine to turn a few degrees at high rpm. So a pulse leaving a long runner reaches the collector measurably later than an identical pulse leaving a short one. The manifold therefore does not receive pulses at the crank-angle spacing the firing order implies; it receives them displaced by however much each runner delays them.
Stack that on top of a firing pattern that was already grouped, and you get a collector fed by four pressure events at four irregular moments. The energy that would have been concentrated at the firing frequency and its harmonics is instead spread across half-orders and sub-harmonics. That is the burble.
Engine Sim models the exhaust path — runner geometry and pipe resonance — as part of the synthesis rather than as an EQ preset, so swapping the exhaust on a boxer changes the sound the way it changes the numbers. Twenty-one engines, including a boxer twin and flat-four layouts.
Unequal runners are bad engineering, acoustically charming or not. Exhaust tuning relies on each cylinder’s blowdown pulse arriving at the collector at a predictable moment so that the reflected wave helps scavenge the next cylinder. If the runners are different lengths, that timing is different for every cylinder and can only be optimised for one of them. On a turbocharged engine there is a second penalty: the turbine is fed an irregular, smeared pressure signal instead of crisp pulses, which hurts spool-up and response.
So Subaru moved to equal-length headers — first on some markets’ performance models in the late 2000s, and comprehensively with the FA-series engines from the 2015 WRX onward. Every runner became the same length, every pulse arrived at the collector at the crank spacing the firing order dictates, and the exhaust could be tuned properly. Response and mid-range improved.
The sound became an ordinary turbocharged four-cylinder. Not bad, just generic — because the irregularity that made it identifiable had been engineered out on purpose. The owner reaction to this over the following years is one of the better real-world demonstrations that engine sound is a product feature, not a byproduct.
Aftermarket unequal-length headers for otherwise equal-length cars exist precisely to restore the noise, and they work, because the mechanism is exactly as described. They also reintroduce exactly the penalties Subaru removed: less consistent scavenging, a muddier signal to the turbine, and typically a small loss of mid-range torque and response in exchange for the character.
That trade is a clean statement of the whole problem. Everything that makes an engine sound distinctive — an uneven crank, a grouped firing order, a compromised manifold — is a departure from the smooth, even, well-scavenged ideal. The sounds enthusiasts love are, almost without exception, audible engineering compromises.