If in multi cylinder engines, various cylinders are made to fire one after the other, interference between adjacent cylinders would occur, since these may have overlapping periods which prevents proper distributions of the fresh charge in the inlet manifold. Likewise, overlapping of the exhaust periods in case of adjacent or nearby cylinders would increase exhaust back-pressure which may prevent the combustion products escaping from the cylinders. Therefore, to obtain best engine performance in multi cylinder engines, the occurring of power strokes in various cylinders is not kept directly one after the other. For example, in 4-cylinder engines it is not kept as 1-2-3-4, but is provided as 1-3-4-2 or 1-4-3-2. Similarly firing order of a 6-cylinder in line engine is 1-5-3-6-2-4 and in a V-8 engine it is 1-5-4-2-6-3-7-8 (where in cylinder nos. 1 and 5 are the front ones). Proper firing order also helps in suppressing certain critical torsion vibrations.
The occurrence of various events in the 4-cylinder engine having firing order as 1-3-4-2 is shown below:
Crank shaft degrees
The driver must know the correct firing order of the engine so that he is able to connect the H.T. wires correctly to various spark plugs.