The typical firing order for a boxer-four engine is for the left bank of cylinders to ignite one after another, followed by the right bank of cylinders (or vice versa), with the firing interval evenly spaced at 180 degrees. Traditionally, the exhausts from the two cylinders on each bank were merged, with the resulting uneven exhaust pulses causing a characteristic “flat-four burble” exhaust sound.
The other common exhaust configuration (such as used by Subaru since the mid-2000s) is to pair the cylinders with a firing interval offset of 360 degrees, in order to optimise the exhaust pulses. This configuration requires long exhaust manifolds, in order to pair the cylinders on opposite banks, and results in a less distinctive exhaust sound.
