The term "firing order" refers to the specific sequence in which the cylinders of an internal combustion engine are ignited. This sequence is designed to result in the balanced operation of an engine, so it is not necessarily a linear progression. Cylinder number one is often ignited first, though after that the sequence is dictated by the engine design. The order that the cylinders are fired is important for diagnostic purposes, and it is also essential information if spark plug wires are to be arranged properly. Wiring a distributor cap sequentially instead of according to firing order will typically result in an engine that misfires badly or does not run at all.
When an internal combustion engine has multiple cylinders, each one is referred to by a number. The numbering sequence differs from one manufacturer to another, though cylinder number one is typically at the front of an engine or the end that has the crank pulley. In straight engines, the numbering sequence is linear. Cylinder number one in V type engines and other designs with two cylinder heads is usually also located closest to the front end of the engine. Certain engines have all even numbers in one head and all odd numbers in the other, though some are numbered in a linear manner.
The firing order refers to the sequence in which the cylinders are ignited rather than their numerical values. Cylinder number one is typically also number one in the firing sequence, though the second one in the series could be any other cylinder. The order that the cylinders are fired in is determined when the engine is designed, and any attempt to change it will result in a misfire to some degree. This is due to the fact that an incorrect firing order results in spark plugs not receiving spark on the power stroke.
Unless a diagram is available, both the cylinder numbers and the firing order must be known to properly install spark plug wires. The firing sequence must also be known in order to perform many types of diagnostics. An automotive oscilloscope that reveals the firing pattern for each cylinder will sometimes show them in the order that they are fired rather than sequentially, so it is important to be able to translate such a display into useful information. Without knowing the firing order, it would be hard to discern which cylinder corresponded to a bad pattern on the scope readout.