As the firing order on an ordinary flat-four boxer engine [2] on 'L'eft and 'R'ight banks are LLRR or RRLL with each ignition (and thus exhaust) being 180 degrees apart. By counting two characters (2 x 180 =360) to the right of each 'L' or 'R', the cylinders that fire with 360 degree crankshaft rotational angle offset are shown to be located on opposite banks. If the exhaust manifold is designed to merge two exhaust ports on a bank into one exit as was common in the designs in the past, the pulse spacing becomes irregular, which causes uneven filling of intake into cylinders and the characteristic 'burble' of exhaust note on older flat-four engines.