The firing order is the sequence of sparking of the spark plugs in a reciprocating engine, or the sequence of fuel injection in each cylinder in a Diesel engine. Choosing an appropriate firing order is critical to minimizing vibration and achieving smooth running, for long engine fatigue life and user comfort.
In a straight engine the spark plugs (and cylinders) are numbered, starting with #1, from the front of the engine to the rear. In most cars the front of the engine also points to the front of the car, but some manufacturers (Saab, Citroën) in some models place the engine 'backwards', with #1 towards the firewall.
In a V engine the right bank is numbered first, followed by the left bank.
In a radial engine the cylinders are numbered around the circle, with the #1 cylinder at the top. There are almost always an odd number of cylinders, as this allows for a constant every-other-piston firing order: for example, with a single bank of 7 cylinders, the order would be ...2-4-6-1-3-5-7-2....
In a conventional engine, the correct firing order is obtained by the correct placement of the spark plug wires on the distributor. In a modern engine with an engine management system and direct ignition, the Electronic Control Unit (ECU) takes care of the correct firing sequence.
These are some usual firing orders:
|Number of cylinders||Firing order||Example|
|3||1-3-2||Saab two-stroke engine|
|4||1-3-4-2||Ford Taunus V4 engine|
|[eep CJ 1949-1986|
GM 3800 engine#231 engine
|8||1-8-4-3-6-5-7-2||1988 Chrysler Fifth Avenue|
|12||1-7-5-11-3-9-6-12-2-8-4-10||2001 Ferrari 456M GT|
|16||1-12-8-11-7-14-5-16-4-15-3-10-6-9-2-13||Cadillac V16 engine|