A firing order diagram consists of a schematic illustration of an engine and its cylinders, for which each cylinder is numbered to correspond with a numeric firing order indicating which spark plug is triggered during engine operation. Some firing order diagrams also include a small circle indicating the distributor cap of the vehicle, and each port for a spark plug wire is numbered according to the cylinder that the port is intended to power.
Firing orders for automobile engines are often written in a form such as 1-4-6-2-5-3, for which each number corresponds to the cylinder numbers given in a firing order diagram. This varies from engine to engine due to many different design characteristics that are influenced by the firing order, including the amount of noise and vibration produced by the engine and the smoothness of the engine's power delivery. Wiring an engine's cylinders to the distributor cap in the incorrect order can severely reduce performance, prevent the engine from running or even seriously damage the engine, so it is important to know the firing order for a given engine when repairing or tuning that engine.
The numbering sequence for engine cylinders is not standardized and varies between different engine types and manufacturers, so it is not safe to assume that the numbering of cylinders is consistent even between different engines from the same manufacturer. For example, some manufacturers number the cylinders in banks of a V-style engine sequentially such that the one through four cylinders in a V-8 engine are all on the same side of the engine. However, other manufacturers number their cylinders in a staggered fashion between the two cylinder banks in these types of engines.