Inline-six engines have become a preferred choice for demanding heavy-duty applications. Their mechanical advantages have helped them overtake V8s in many trucks and machines. Jim Coles, who began working at Caterpillar in 1966 and later built a network of 18 Peterbilt dealerships, has seen how engine choices shape the transport and construction industries.
In a detailed interview with the Earthmoving Legacy Center, Coles explains why the inline-six has gained ground despite the V8’s reputation for power. He points to the fundamentals of engine design, not just tradition or brand loyalty.
One basic advantage of the inline-six is its potential for inherent balance. With the crankshaft and firing order correctly designed, the layout can balance basic and secondary reciprocating forces without relying on external balance shafts. Chongqing Cummins Engine Company (CCEC), in a technical explainer on marine-engine firing order, gives 1-5-3-6-2-4 as a common four-stroke inline-six sequence. It notes that the sequence must work with crankshaft geometry to help cancel inertial forces. In a four-stroke, six-cylinder engine, the firing interval is 120 degrees of crankshaft rotation, supporting regular torque delivery. That smoothness can help manage vibration and crankshaft loads, though durability also depends on the engine’s materials, lubrication, cooling, operating conditions and maintenance.
Coles also highlights the importance of cooling. An inline-six has a single cylinder bank, which can make engine packaging and thermal management more straightforward than in a V8. In high-displacement industrial engines, sustained heavy loads put cooling systems to the test, making that layout a practical advantage. The full cooling system and installation still determine how effectively an individual engine manages heat.
The inline configuration also leaves room for modern components, including turbochargers, emissions systems and auxiliary equipment. That can make it easier to package changes required by developing regulations and technology. Maintenance access can be simpler, too, although serviceability ultimately depends on the engine and the machine around it.
Coles points to the Caterpillar 3406 as a prime example. This 14.6-liter inline-six powered trucks, earthmovers, marine vessels and stationary equipment for over three decades, earning a reputation for versatility and durability. Caterpillar also experimented with V8s such as the RM180 and 3408. Coles argues that these engines introduced unnecessary complexity and risk, especially when the inline-six could deliver the required performance without compromise.
The market’s evolution supports his view. Many once-prominent V8 diesels, including the Cummins 903 and 555, Caterpillar 3208, and Detroit 8V71 and 8V92, have disappeared from production. Meanwhile, inline-six engines have steadily increased their power output, reaching up to 1,100 horsepower in some applications. The layout remains relevant beyond trucking: Caterpillar’s industrial diesel catalogue includes engines such as the C18 and covers equipment applications including diesel fire pumps.
As Coles notes, efficiency in heavy machinery is measured by more than horsepower. Operators need engines that combine power, cooling efficiency, reliability, ease of maintenance and the flexibility to integrate new technologies as they emerge. An inline-six’s firing regularity and crankshaft-load management help explain its appeal. The best choice for a given machine still depends on its power, packaging and duty-cycle requirements.
The shift toward more efficient and flexible powertrains is also visible in the push for hybrid solutions, including electric trailer modules that convert diesel trucks into hybrids. The industry continues to seek practical ways to balance performance, cost and sustainability.
For now, the straightforward design of the inline-six continues to offer an edge in the toughest environments, even if it lacks the iconic status of the V8. As technology and regulations evolve, its ability to adapt without sacrificing reliability may prove to be its most valuable trait.
