Understanding Why Six-Cylinder Engines Can Have More Power Than Four-Cylinder Engines

When it comes to engine performance, many people assume that the number of cylinders directly determines the power output. However, this assumption is not always accurate. While the number of cylinders can affect power, several other factors, such as engine displacement, RPM (Revolutions Per Minute), and compression ratio, play a significant role in determining the actual power output.

History of Engine Power: A Ride Through the Early 1950s

Historically, the power output of engines was marked by notable differences between engines with different cylinder configurations. For instance, in the early 1950s, the Hudson Hornet came with a 5-liter inline 6-cylinder, side-valve engine that produced a remarkable 145 horsepower. Meanwhile, a side-valve 4-cylinder Ford flathead V8, with a 3.9-liter displacement, delivered 110 horsepower. The Oldsmobile "Rocket" V8 from 1949, sporting the same 5-liter displacement, managed to produce 135 horsepower. These examples illustrate that achieving higher power does not always require more cylinders.

Addressing Common Misunderstandings

Misunderstandings about engine power often stem from the belief that multiple cylinders fire simultaneously or the firing sequence is irrelevant. However, in most modern engines, each cylinder fires in sequence, and no cylinders fire at the exact same time. Let's delve deeper into this concept.

Misunderstanding 1: Simultaneous Firing of Pairs of Cylinders

When observing that the pistons of pairs of cylinders reach TDC (Top Dead Center) simultaneously, it's crucial to remember that the 4-stroke combustion cycle requires two full crankshaft revolutions, totaling 720 degrees of rotation. In this cycle, the cylinders fire alternately, 360 degrees apart. In a two-cylinder engine, both pistons can reach TDC simultaneously, but only one is ready to fire while the other is at TDC on the exhaust stroke.

Misunderstanding 2: Firing Sequence in 4- and 6-Cylinder Engines

A 4-cylinder, 4-stroke engine will fire once every 180 degrees, or twice per crankshaft revolution. In contrast, a 6-cylinder, 4-stroke engine will fire every 120 degrees, or three times per crankshaft revolution. This difference in firing intervals can lead to a perception of increased power in engines with more cylinders.

Misunderstanding 3: Power Output and Cylinder Count

Engine power output is not directly linked to the number of cylinders or the firing sequence. It can vary greatly depending on other design factors. Some engines with fewer cylinders can produce more power than those with more cylinders, and vice versa. This flexibility in design allows for optimization of power output, torque, and smoothness.

A More Basic Approach to Understanding Engine Power

To provide a simpler understanding, consider that the transition from a 4-cylinder to a 6-cylinder engine increases power output by approximately 50%. This is because a 6-cylinder engine provides three surges of power for every revolution, compared to two surges with a 4-cylinder engine. This increase in power is due to the 120-degree overlap in firing sequence, allowing for continuous power delivery and efficiency.

Specifically, a 6-cylinder engine has a 90-degree overlap, meaning that 1.5 cylinders are firing at any given time per revolution. This is a significant increase in power for the same displacement and design. In comparison, a 4-cylinder engine has a power stroke every 180 degrees, while a 6-cylinder engine has a power stroke every 120 degrees, providing 50% more power for the same displacement and design.

Engine Torque Diagrams

Figure 2.11 below provides a visual representation of multi-cylinder engine torque diagrams, showing how the power delivery differs between a twin-cylinder engine, a three-cylinder engine, a four-cylinder engine, a five-cylinder engine, a six-cylinder engine, and an eight-cylinder engine. These diagrams illustrate the distinct torque delivery patterns of these engines.

Conclusion

While the number of cylinders can influence engine performance, it is not the only factor. Engine displacement, RPM, and compression ratio all play critical roles. Understanding the firing sequence, power delivery, and torque distribution can help clarify why a six-cylinder engine might produce more power than a four-cylinder engine, even when considering their firing sequence. The key takeaway is that optimizing these design elements can lead to enhanced performance, regardless of the number of cylinders.

Figures

Fig. 2.11. Multi-cylinder engine torque diagrams. A. Twin-cylinder engine B. Three-cylinder engine. C. Four-cylinder engine. D. Five-cylinder engine. E. Six-cylinder engine. F. Eight-cylinder engine.