Engines are the heart of every vehicle, and different engine types operate in different ways. One of the things that differentiates engines from each other is firing order. If the engine is the heart, the firing order is its heartbeat. But what is the firing order exactly, and can you tune it how you like?
What is an Engine firing order?
The firing order of an internal combustion engine is the sequence of ignition for the cylinders. In a spark-ignition (e.g. gasoline/petrol) engine, the firing order corresponds to the order in which the spark plugs are operated. Firing order affects the vibration, sound, and evenness of power output from the engine.
In engines, cylinders don’t fire in the sequence of 1-2-3-4-5-6 and so on as it could cause the crankshaft to deform or break. The order or sequence in which the engine cylinders fire or generate & deliver power is called the engine firing order.
Related: What is an Internal Combustion engine?
The firing order heavily influences crankshaft design. In a Diesel engine, the firing order corresponds to the order in which fuel is injected into each cylinder. Four-stroke engines must also time the valve openings relative to the firing order, as the valves do not open and close on every stroke.
Related: What is Crankshaft?
Common firing orders
Common firing orders are listed below. For V engines and flat engines, the numbering system is L1 for the front cylinder of the left bank, R1 for the front cylinder of the right bank, etc.
- In two-cylinder engines, the cylinders can either fire simultaneously (such as in a flat-twin engine) or one after the other (such as in a straight-twin engine).
- In straight-three engines, there is no effective difference between the possible firing orders of 1-2-3 and 1-3-2.
- Straight-four engines typically use a firing order of 1-3-4-2, however some British engines used a firing order of 1-2-4-3.
- Flat-four engines typically use a firing order of R1-R2-L1-L2.
- Straight-five engines typically use a firing order of 1-2-4-5-3, in order to minimise the primary vibration from the rocking couple.
- Straight-six engines typically use a firing order of 1-5-3-6-2-4, which results in perfect primary and secondary balance.
- V6 engines with an angle of 90 degrees between the cylinder banks have used a firing orders of R1-L2-R2-L3-L1-R3 or R1-L3-R3-L2-R2-L1. Several V6 engines with an angle of 60 degrees have used a firing order of R1-L1-R2-L2-R3-L3.
- Flat-six engines have used firing orders of R1-L2-R3-L1-R2-L3 or R1-L3-R2-L1-R3-L2.
- V8 engines use various different firing orders, even using different firing orders between engines from the same manufacturer.
- V10 engines used firing orders of either R1-L5-R5-L2-R2-L3-R3-L4-R4-L1 or R1-L1-R5-L5-R2-L2-R3-L3-R4-L4.
- V12 engines use various different firing orders.
In a radial engine, there is always an odd number of cylinders in each bank, as this allows for a constant alternate cylinder firing order: for example, with a single bank of 7 cylinders, the order would be 1-3-5-7-2-4-6.
Moreover, unless there is an odd number of cylinders, the ring cam around the nose of the engine would be unable to provide the inlet valve open – exhaust valve open sequence required by the four-stroke cycle.
How to determine the firing order of the engine?
Firing Order is Determined by the Number of Cylinders contained within that engine & Crankshaft Alignment/Offset of each Crank-Journal, during the Design/Manufacturing Process.
The firing order is determined when the engine is DESIGNED, so as to make it run as efficiently and as smoothly as possible. The forces and loads exerted by pistons on the crankshaft are calculated. The required counterweight is calculated. Plugging all these into the dynamic balancing equations, the firing order is determined such that, minimal vibrations are produced.
Designing Parameters of Firing Order:
- Number of Cylinders,
- Torsional vibrations,
- Heat distribution,
- arrangement of cylinders,
- Crankshaft Alignment/Offset of each
- Crank-Journal.
Why Do Engines Need a Firing Order?
It might seem more convenient to make engines with a simple 1-2-3-4-5-6 (and so on) sequence, but this firing order is less than subpar. This is because it would produce uneven twisting forces on the crankshaft and could damage it or destroy it.
The crankshaft is one of the most stressed engine components and performs roughly between 500 and 7,000 rotations per minute based on the specific engine and its rev range.
This means the crankshaft could be easily bent if the torsional forces weren’t distributed evenly across the length of it. The firing order largely affects the force distribution and has to factor into the crankshaft design.
The firing phase is physically the strongest of the entire four-stroke cycle, creating vibrations and overall mechanical stress.
To maintain engine reliability, the mechanical stress should be kept at the lowest possible level, so firing should not occur at any adjacent cylinders at the same time, this is one of the reasons why engines need a specific firing order.
The firing sequence also affects the engine’s level of vibrations, noise, and generated power, including the evenness of power delivery across the RPM range. An improper firing order can then also negatively impact fuel economy.
In a more literal sense, we need to know the firing order of an engine to not mix up the spark plug wires, which could cause the engine to run poorly and backfire if it started at all.
Firing Intervals
It’s difficult to talk about firing orders without also talking about firing intervals. The firing interval is the amount of time that passes between each cylinder’s ignition following the firing order.
The engine’s power strokes usually need to be spaced at equal firing intervals for the engine to run smoothly. These intervals rely on a specific formula based on the number of strokes and the number of cylinders.
Pro Tip: Determine the number of degrees of crankshaft rotation between firing events by dividing the 720 degrees of a complete four-stroke cycle by the number of cylinders. A 6-cylinder has 120 degrees between firing events. A 4-cylinder has 180. An 8-cylinder has 90 degrees between firing events.
Some engines forgo the equal firing intervals entirely though. These engines use uneven firing intervals instead, which affects their smoothness but can be worth the tradeoff for higher performance.
Firing Order for Different Multi-Cylinder Engines
Each engine has a specific firing order, which varies not only based on the number of cylinders but also the manufacturer and general engine design. For example, the 6-cylinder GM 3800 engine has a 1-6-5-4-3-2 sequence, while a 6-cylinder from Mercedes (M104) has a 1-4-2-5-3-6 sequence.
Similarly, a Chevy firing order may be different from a firing order for Ford. Moreover, firing sequences can also vary within the manufacturer’s lineup (i.e., the 6-cylinder Mercedes M272 has a different firing order from the 6-cylinder M104).
Sometimes it’s helpful to have a cheat sheet in hand to help you navigate projects–big or small. For example, let’s say you’ve just installed a new cap and rotor kit on your carbureted V8, changed the spark plugs, and are ready to install a set of new wires.
But which wire goes where?
We’ve put together this V8 engine firing order and rotation (where applicable) cheat sheet for just such an occasion:
- AMC (most V8 engines): Clockwise 1-8-4-3-6-5-7-2
- Buick (most V8 except HEI): Clockwise 1-8-4-3-6-5-7-2
- Chevrolet: Clockwise 1-8-4-3-6-5-7-2
- GM LS: 1-8-7-2-6-5-4-3
- Small Chrysler: Clockwise 1-8-4-3-6-5-7-2
- Big Chrysler and Hemi: Counter-clockwise 1-8-4-3-6-5-7-2
- Most Ford V8: Counter-clockwise 1-5-4-2-6-3-7-8
- Ford (5.0L HO, 351W, 351M, 351C, 400): Counter-clockwise 1-3-7-2-6-5-4-8
- Most Ford modular (4.6/5.4L): 1-3-7-2-6-5-4-8
- Ford 5.0L Coyote: 1-5-4-8-6-3-7-2
- Oldsmobile (1967 and up): Counter-clockwise 1-8-4-3-6-5-7-2
- Pontiac (most 1955-81 V8 engines): Counter-clockwise 1-8-4-3-6-5-7-2 (Note: 307 Pontiac V8 engine rotates clockwise)
It’s important to remember that the numbering conventions for engine cylinders differ by manufacturer.
Tuning (Changing) Your Engine’s Firing Order
Tuning your engine’s firing order is no easy feat. It requires you to replace the engine’s crankshaft (or at least the camshaft), which often impacts your vehicle’s reliability. However, you can increase your horsepower if you pick the right kind of crankshaft and camshaft and choose the optimum firing order. Whether it’s worth it is entirely up to you.
