Why Engine Smoothness Matters: A Deep Dive into Inline Cylinder Firing Orders

When it comes to internal combustion engines, performance is often judged by power, torque, or even sound. But there's a more subtle—and often overlooked—trait that defines a great engine: smoothness. Engine smoothness isn't just about comfort; it's about efficiency, longevity, and the harmony of mechanical motion. One of the biggest contributors to smooth operation is the firing order and how evenly power strokes are distributed throughout the 720-degree engine cycle.

In this blog, we’ll explore the firing order and crankshaft design of inline engines with 2, 3, 4, 6, 8, and 9 cylinders, culminating with the marvel of the straight-9 Rolls-Royce engine. We'll examine how different configurations handle the balance of forces and torque, and why some engines feel smoother than others.

The Basics: 720 Degrees and Four Strokes

All four-stroke engines go through the same sequence: intake, compression, power (labour), and exhaust. This sequence spans 720 degrees of crankshaft rotation—two full revolutions. Each cylinder only contributes power once every 720 degrees.

Therefore, to maintain continuous torque output, multiple cylinders must be timed to fire at evenly spaced intervals. The interval between power strokes is calculated as:

720° / number of cylinders

The smoother the interval between these power strokes, the more balanced the engine will feel.

Inline-2 (Straight-Twin)

Common Firing Orders: 1-2 (360° crank) or 1-2 (180° crank)

Straight-twin engines are common in motorcycles and compact cars. They typically come in two flavors:

• 360° crank: Both pistons move up and down together, but fire alternately. This gives an even firing interval, but poor balance.

• 180° crank: One piston is at top dead center while the other is at bottom dead center. This gives uneven firing (0°, then 720°) but better balance.

Smoothness: Poor. Inline-2s are inherently unbalanced and can be quite rough unless counterbalanced or rubber-mounted.

Inline-3 (Triple)

Common Firing Order: 1-3-2

The inline-3 engine offers a compact design with more power and smoother delivery than a twin.

Each cylinder fires every 240 degrees (720° / 3), which is spaced evenly over the cycle. The three pistons are spaced at 120° intervals on the crankshaft.

Smoothness: Moderate. The even firing helps, but the primary and secondary balance is imperfect. Balancer shafts are often used.

Inline-4 (Straight-Four)

Common Firing Order: 1-3-4-2

One of the most ubiquitous engine types, the inline-4 powers everything from economy cars to motorcycles. It fires every 180 degrees.

• Cylinders 1 and 4 move together, as do 2 and 3.

• The typical firing order (1-3-4-2) gives good exhaust scavenging and relatively smooth operation.

Smoothness: Good in low to mid-range, but secondary imbalance creates vibration at high RPMs. Many modern inline-4s use balance shafts.

Inline-6 (Straight-Six)

Common Firing Order: 1-5-3-6-2-4

This is often considered the "sweet spot" for inline engines. Each cylinder fires every 120 degrees, and the primary and secondary forces are perfectly balanced.

• Pistons are arranged in three pairs that move in opposition to each other.

• Power delivery is silky smooth, which is why inline-6s are favored in luxury cars and classic performance engines.

Smoothness: Excellent. Inline-6s are naturally balanced and deliver seamless power.

Inline-8 (Straight-Eight)

Common Firing Order: 1-6-2-5-8-3-7-4

The straight-eight was popular in the early 20th century, particularly in American luxury cars like the Duesenberg and Packard.

With a firing interval of 90 degrees, it offers very smooth torque delivery. However, the long crankshaft can suffer from torsional vibrations.

Smoothness: Excellent at low RPMs, but can become less stable at high speeds due to crankshaft flex.

Inline-9 (Straight-Nine) – The Rolls-Royce Special

Common Firing Order: 1-3-5-7-9-2-4-6-8

Rare and fascinating, the inline-9 is best known from engines like the Rolls-Royce Falcon. In this configuration:

• A power stroke occurs every 80 degrees (720° / 9)

• The crank angle difference between pistons that fire successively is also 80°

• This even spacing ensures near-continuous torque and excellent smoothness

Labour strokes follow one another precisely every 80°, and the engine maintains momentum with minimal vibration or dead spots.

Smoothness: Outstanding. The straight-9 is among the smoothest inline configurations ever made, thanks to its high cylinder count and evenly spaced ignition.

Final Thoughts: More Cylinders, More Smoothness (Usually)

As a rule of thumb, more cylinders firing at shorter intervals results in smoother operation. However, there are trade-offs:

• Packaging constraints (especially for long engines like the straight-8 or straight-9)

• Increased complexity and cost

• Torsional vibration in long crankshafts

That’s why modern cars favor V-configurations or inline-4s, but for vintage machines, these straight engines still hold a mechanical charm that’s hard to beat.

Understanding crank angle diagrams and firing orders not only helps you appreciate the engineering behind these engines—it lets you feel the rhythm of combustion, cylinder by cylinder.

Bonus: Wear the Rhythm – T-Shirt Feature

If you love the math and mechanics of internal combustion as much as we do, check out our exclusive "She Fires on All Nines" T-shirt. Featuring a 1940s-style pin-up girl proudly presenting a crank angle and labour diagram of the iconic Rolls-Royce straight-9, it's a wearable tribute to one of the smoothest engine configurations ever built.

👉 Shop the T-shirt now and wear your engineering passion with pride!