​​Detroit Vs. Cummins Diesel Engines: What's The Difference Between Them?

Quality is the biggest factor in picking a diesel engine for a large truck. With most commercial drivers putting around 100,000 miles on their trucks every year, it's important to get the right diesel engine for the job. For the most part, it boils down to three options — and more realistically, only two in the modern age. After Caterpillar stopped building on-highway engines in 2009, the main fight is between Detroit and Cummins when it comes to the best commercial diesel engines on the market currently.

Both companies have a proven track record that has kept them in business for the better part of a century. As a subsidiary of General Motors, Detroit Diesel began manufacturing diesel engines suited for military and construction applications including tanks, power generators, and aquatic landing crafts in 1938. Detroit built their reputation on two-stroke diesels before venturing into more efficient four-stroke designs in 1987. Cummins began independently in 1919, first developing marine four-stroke diesel engines before moving on to automotive engines. Cummins is now an international company known for providing engines for applications ranging from buses to construction equipment.

While both companies are known for a rich history in the diesel engine space — gaining a reputation for reliability, quality, and durability — Detroit Diesel and Cummins approached designing diesel engines very differently, especially in their early days. Their differences in design philosophy led to both companies developing a dedicated fan base in the trucking community that has lasted into the present day.

Detroit Diesel and Cummins built their legacy based on two different approaches to the diesel engine. While Detroit Diesel started with and continued offering two-stroke diesel engines until the start of the 21st century, Cummins has always been known for their four-stroke design. There are pros and cons to each type of engine, which garnered each company to develop loyal followings based on their respective design philosophy.

Detroit's initial Series 71 diesels were two-stroke engines, meaning that the engines completed an entire combustion event into only two piston cycles. Instead of separating intake, compression, power, and exhaust strokes into four piston cycles, two-stroke diesels — like many DD engines until 1998 — combine intake and ignition into a single stroke and compression and exhaust into a stroke. Two-strokes generally have higher power density and weigh significantly less than four-stroke diesels, at the cost of increased emissions and less overall efficiency. Despite Detroit Diesel building their reputation with two-stroke diesels, some of their later four-stroke designs are their most popular models, selling more than a million of their Series 60 four-stroke engines.

In contrast, Cummins has always been famous for their four-stroke diesels. Four-stroke diesel engines have dedicated piston strokes for intake, compression, power, and exhaust processes. While two-stroke engines can generally outperform four-stroke diesels high in the rev range, four-stroke diesels produce better low-end torque, which is better for towing. Four-stroke engines are also more efficient, producing fewer emissions. They also tend to be more reliable, which is a reputation that Cummins has retained since even their earliest Model U and Model K engines. Cummins' four-stroke design helped them take the lead in the on-highway diesel market after the federal government made it harder for two-stroke diesel engines to meet increasingly strict emissions requirements in the mid-2000s.

The common consensus in the trucking industry is that the choice between Detroit Diesel and Cummins hinges on personal preference and meeting individual needs for specific applications. Both companies have an extensive track record of building exceptionally dependable diesel engines that can put in many miles of strenuous work. For a lot of truckers, the choice between the two is dominated by positive experiences with one brand or the other in the past. That's especially the case when it comes to heavy truckers who have experience with both Detroit Diesel two-stroke and Cummins four-stroke engines.

Many seasoned truckers prefer two-stroke Detroit Diesel engines because Series 53 and 71 engines were common in large trucks in the 1960s and 1970s, have a widespread service network, are reliable, and have good high-RPM power. However, others didn't like that the two-stroke Detroits had such a narrow power band, like many other two-stroke engines, and the fact that they would often leak oil. When the Series 60 was introduced in 1987, it quickly became popular amongst many truckers, as it was more fuel-efficient than many of the earlier two-stroke engines and was a versatile engine series, suiting many applications.

Cummins has a fanbase just as big, if not larger, in the trucking community due to the reliability, long service intervals, and solid all-around performance of their diesel engines. Cummins engines also have the reputation of being quieter engines than most in the DD line, especially the two-strokes. Cummins engines, especially the newer ones like the ISX, generally have a wider powerband than DD15 engines, allowing them to operate better at a range of operating speeds. The ISX line of Cummins engines is also the more fuel-efficient option in the modern age.