Boxer-4 · turbocharged
Boxer engine sound
The boxer engine sound most people know is the Subaru rumble, a lumpy, off-beat burble that comes not from the boxer layout itself but from unequal-length exhaust headers that make the exhaust pulses arrive unevenly. The flat-four fires at perfectly even intervals; the plumbing is what makes it sound uneven.
Listen: Boxer-4 cars
How a boxer-4 fires
In a boxer engine the cylinders lie flat on either side of the crank, and opposing pistons move in and out at the same time, like a boxer's fists. That cancels the primary shaking forces. A four-stroke engine fires half its cylinders per revolution, so a flat-four fires twice per turn, every 180 degrees, in the order 1-3-2-4.
The firing frequency is rpm / 60 × 2, or 233 Hz at 7,000 rpm, the same as an inline-four. The firing frequency shown on each car page uses the same formula. On frequency alone, a boxer-four and an inline-four are identical; the difference is in what happens downstream.
Where the rumble comes from
On Subaru's older EJ-series turbo engines, the exhaust runners from the two cylinder banks have to wrap around the engine to meet before the turbo, and they are not the same length. A pulse from a cylinder with a short runner reaches the collector sooner than one from a cylinder with a long runner. So although the cylinders fire every 180 degrees, the pulses arrive bunched: two close together, then a longer gap.
That uneven arrival pattern repeats every two revolutions and adds low-frequency content below the firing frequency. The ear hears it as a lope, the same basic effect that gives a cross-plane V8 its burble. It is most obvious at idle and low rpm, before the turbo is working hard.
Equal-length headers and newer Subarus
Equal-length headers give every runner the same length, so the pulses arrive evenly spaced and the rumble mostly disappears. The engine sounds smoother and more like an inline-four, and with a twin-scroll turbo the evenly timed pulses also help it spool. Subaru's later FA-series turbo engines use equal-length layouts, which is a big part of why a current WRX sounds smoother than an older STI.
Plenty of owners deliberately keep or fit unequal-length headers to preserve the rumble, even though equal-length headers generally perform better.
The Impreza WRX STI
The Impreza WRX STI's EJ-series turbocharged flat-four is the textbook rumble engine: a lumpy, off-beat idle, a gravelly burble when cruising, and a smoother, turbo-damped rush at full boost, often with blow-off valve chatter on lift.
Boxer-4 firing frequencies
| Car | Engine | Redline | Hz at redline |
|---|---|---|---|
| Subaru Impreza WRX STI | 2.5 L, Turbocharged | ≈ 6,700 rpm | ≈ 223 |
Frequently asked questions
Why do Subarus sound like that?
Older Subaru turbo engines use unequal-length exhaust headers, so the evenly fired pulses reach the collector at uneven intervals. That bunching creates the rumble.
Do all boxer engines rumble?
No. The boxer layout fires evenly. A flat-four with equal-length headers, or a Porsche flat-six, sounds smooth. The rumble is a product of exhaust design.
Do equal-length headers remove the boxer rumble?
Mostly, yes. They make the pulses arrive evenly, so the engine sounds smoother, and they usually improve turbo response as well.
