What Is an Engine Firing Order? Why Is It Important?

Engines power every vehicle, and different engines work differently. One thing that separates engines from each other is the firing order.

If the engine is the heart, the firing order is the heartbeat. But what is firing order exactly, and can you tune it to whatever you like?

What is an Engine firing order?

The firing order in an internal combustion engine is the sequence in which the ignition occurs in the engine’s cylinders.

In a spark-ignition or gasoline/petrol engine, the firing order is the order in which the spark plugs fire. The firing order affects the crankshaft load, vibration, noise, and smoothness of power delivery of the engine.

The cylinders in an engine do not fire in the order of 1-2-3-4-5-6 or 1-3-5-2-4-6, etc., since this could bend or break the crankshaft. The firing order is defined as the order in which the engine cylinders fire or produce and transmit power.

The firing order of an engine can also impact the design of the crankshaft significantly. In a Diesel engine, the firing order means the order in which fuel is injected into the cylinders.

Four-stroke engines must also consider the timing of the valves’ openings in relation to the firing order since the valves do not open and close every stroke.

Related: What is an Internal Combustion engine?

Common firing orders

The most common firing orders are listed below. On V engines or flat engines, we will use the left bank as an example. We will say that L1 is the front cylinder of the left bank, R1 is the front cylinder of the right bank, and we will follow that numbering pattern throughout.

• In two-cylinder engines the cylinders may fire at the same time (flat-twin engine) or one after the other (straight-twin).
• In a straight-three engine, there is no practical difference between the possible firing orders of 1-2-3 and 1-3-2.
• In a straight-four engine, the firing order is commonly 1-3-4-2, but there are some British engines that are 1-2-4-3.
• In a flat-four engine, the firing order will typically be R1-R2-L1-L2.
• In a straight-five engine, the firing order will typically be 1-2-4-5-3 to minimize the primary vibration from the rocking couple.
• In a straight-six engine, the firing order will typically be 1-5-3-6-2-4, allowing for perfect primary and secondary balance.
• V6 engines (where the angle is 90 degrees between the two banks), will have had firing orders with either R1-L2-R2-L3-L1-R3 or R1-L3-R3-L2-R2-L1. V6 engines with an angle of 60 degrees had firing orders of R1-L1-R2-L2-R3-L3. 
• Flat-six engines had firing orders which were either R1-L2-R3-L1-R2-L3 or R1-L3-R2-L1-R3-L2. 
• V8 engines have a variety of firing orders, and in fact have different firing orders in engines from the same manufacturer.
• V10 engines use 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 have a variety of firing orders as well.

A radial engine consists of an odd number of cylinders in each bank to maintain a consistent alternate cylinder firing order: for a bank of 7 cylinders, the firing order would be 1-3-5-7-2-4-6.

Furthermore, unless there is an odd number of cylinders, the ring cam around the nose of the engine cannot create the required inlet valve open – exhaust valve open for the four-stroke cycle.

How to determine the firing order of the engine?

Basically, the firing order of any engine is determined by the number of cylinders that the engine has and the additional alignment/offset of each crank-journal of the crankshaft while designing/manufacturing.

The firing order is decided at engine design, to hopefully have the engine run as optimal and smooth as possible. The forces/loads are calculated based on the masses of the pistons, then the calculations are done on the crankshaft.

Then, in all likelihood, the counterweights are determined, and when they are all inserted into the dynamic balancing equations, the firing order will be the one that produces the least vibration.

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?

While making engines with a simple 1-2-3-4-5-6 (and so on) order may seem easier, this type of firing order is less than ideal because it would create uneven twisting forces on the crankshaft. This could damage or even break the crankshaft.

One of the most stressed components of the engine is the crankshaft which rotates between 500 and 7,000 rotations per minute depending on the particular engine and its and its rev range.

The crankshaft could easily bend if the torsional forces were not applied evenly along its length. It is equally critical in the design of the crankshaft to factor in the effect of the firing order on the distribution of the forces being applied by the engine.

Firing phase is physically the strongest part of the whole four-stroke cycle, creating vibrations and mechanical stress.

Engine reliability requires this mechanical stress to be as low as possible, therefore firing should never occur in any adjacent cylinders at the same time this is one reason engines need a proper firing order.

The firing order also has an effect on vibrations, noise, and power produced by the engine including whether the power delivery is even over the RPM range producing uneven power delivery and possibly affecting fuel economy.

Additionally, we need to know the firing order of an engine to make sure we do not switch spark plug wires, which could cause the engine to run poorly and possibly backfire, if it even started at all.

Firing Intervals

It is hard to discuss firing orders without talking about firing intervals. The firing interval is simply the time that elapses between each cylinder firing after firing order.

To achieve a smooth running engine, the engine usually requires the power strokes to be spaced at equal intervals. They all boil down to a math equation that takes into account the number of strokes and the number of cylinders.

Pro Tip: To determine how many degrees of crankshaft rotation are between firings simply divide the 720 degrees of a complete cycle of four strokes by the number of cylinders.

In a 6-cylinder, the interval is 120 degrees between firings. For a 4-cylinder, the interval is 180 degrees; and for an 8 cylinder is 90 degrees.

However, there are engines that do not adhere to equal firing intervals at all. These engines use uneven firing intervals and while this will have a noticeable effect on smoothness, the unequal intervals result in even greater performance.

Firing Order for Different Multi-Cylinder Engines

Every engine has a firing order unique to its number of cylinders, manufacturer, and general design.

For example the GM 3800, a 6-cylinder engine, has a firing order of 1-6-5-4-3-2 and the M104, also a 6-cylinder, from Mercedes, has a firing order of 1-4-2-5-3-6.

Additionally, a Chevy firing order may be different than Ford firing order. Firing order can vary by other considerations as well in a manufacturers line-up; for example, the M272, is also a 6-cylinder engine and has a different firing order than the M104.

From time to time, it can be nice to keep a cheat sheet near for projects big or small. Say you just installed a new cap & rotor kit on your the carbureted V8, and changed out the spark plugs to now install a new set of wires now.

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 Your Engine’s Firing Order

Tuning the firing order of your engine requires an engine crankshaft replacement (or at least a camshaft change), which often impacts reliability.

Nonetheless, you can increase your horsepower if you choose the right crankshaft and camshaft, and you choose the best firing order. Whether it’s worth it or not is entirely up to you.

FAQs.