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5.9 vs. 6.7 Cummins: Which Diesel Reigns Supreme?

Determining whether the 5.9 Cummins or the 6.7 Cummins is “better” isn’t a simple black-and-white answer. It depends entirely on your priorities and intended use. The 5.9L generally boasts a reputation for being more mechanically simple and easier to work on, making it attractive for those who prioritize reliability and DIY maintenance. On the other hand, the 6.7L offers increased power, especially low-end torque, and often features more modern technology, appealing to those who need serious towing capability and a more refined driving experience.

Diving Deep: 5.9L Cummins

The 5.9L Cummins engine holds a special place in the hearts of diesel enthusiasts. Produced from 1989 to 2007 (with variations), it’s become synonymous with durability and straightforward design. Let’s examine the pros and cons:

5.9L Cummins: The Good

Simplicity and Reliability: The 5.9L Cummins is renowned for its simple mechanical design. This translates to fewer components that can fail and easier troubleshooting when problems do arise. Particularly the 12-valve versions (’89-’98) are known for their robust construction.
Aftermarket Support: Due to its popularity and long production run, the aftermarket support for the 5.9L is vast. Parts are readily available and often more affordable than those for the 6.7L.
P-Pumped Power Potential: Specifically, the 1994-1998 5.9L Cummins equipped with the Bosch P7100 injection pump (the “P-Pump”) is highly sought after. This mechanical injection pump is incredibly robust and can support massive horsepower figures with modifications. It’s a favorite for performance enthusiasts.
Fewer Emissions Controls (Early Models): Older 5.9L Cummins engines, particularly pre-2004 models, often have minimal emissions equipment beyond a catalytic converter. This lack of complex emissions systems can contribute to improved reliability in the long run.
Durability: With proper maintenance, the 5.9 Cummins engine can easily last over 600,000 miles.

5.9L Cummins: The Not-So-Good

Lower Factory Horsepower & Torque (Compared to 6.7L): In stock form, the 5.9L Cummins generally produces less horsepower and torque than the 6.7L. While this can be remedied with modifications, it’s something to consider if you need maximum power straight from the factory.
Older Technology: Being an older engine, the 5.9L lacks some of the modern conveniences and refinements found in the 6.7L. This includes things like advanced engine management systems and smoother operation.
Variable Geometry Turbo (Later Models): Later 5.9L engines (particularly the 2004.5-2007 models) incorporated a variable geometry turbo (VGT). While offering improved performance, the VGT is often cited as a potential source of problems compared to the fixed geometry turbos found on earlier models.
Head Gasket Concerns: While generally reliable, some versions of the 5.9L, especially when heavily modified for power, can be susceptible to head gasket failures due to higher cylinder pressures.

Leveled Up: 6.7L Cummins

The 6.7L Cummins arrived on the scene in 2007.5, offering increased displacement and a host of technological advancements. While it shares the Cummins legacy of durability, it presents a different set of strengths and weaknesses.

6.7L Cummins: The Good

Increased Torque: The 6.7L boasts significantly more low-end torque than the 5.9L, thanks to its longer stroke. This makes it a superior choice for heavy towing and hauling.
Higher Factory Horsepower: The 6.7L Cummins produces significantly more horsepower in its stock configuration than the 5.9L did. Later versions can even reach the 400 horsepower mark.
More Refined Driving Experience: The 6.7L often features more advanced engine management systems and refined fuel injection, resulting in a smoother, quieter, and more responsive driving experience.
Modern Technology: The 6.7L benefits from advancements in diesel engine technology, including features like improved fuel economy and cleaner emissions (although this can also be a downside, as detailed below).
Longer Expected Life: The 6.7 Cummins engine has a longer expected life than the 5.9L, with many engines lasting up to 300,000 miles with proper maintenance.

6.7L Cummins: The Not-So-Good

Complex Emissions Systems: The 6.7L is equipped with more complex emissions control systems, including a diesel particulate filter (DPF), exhaust gas recirculation (EGR), and selective catalytic reduction (SCR). These systems can be prone to failure and expensive to repair.
Variable Geometry Turbo (VGT): The 6.7L utilizes a variable geometry turbo (VGT), which, while offering good performance, is often cited as a potential point of failure.
Higher Cylinder Pressures: Due to its increased torque output, the 6.7L Cummins experiences higher cylinder pressures than the 5.9L. This can increase the risk of head gasket failures, especially when the engine is tuned for increased power.
More Expensive Parts: Parts for the 6.7L are generally more expensive than those for the 5.9L, due to its more complex design and newer technology.
Turbo Lag: Some drivers report noticeable turbo lag, especially on the earlier models.

The Verdict: It Depends On You

Ultimately, the “better” engine depends on your specific needs and priorities.

Choose the 5.9L Cummins if: You prioritize simplicity, reliability, affordability, and a strong aftermarket. You value a more “raw” driving experience and are comfortable working on your own truck. The 1994-1998 P-pumped 5.9L is particularly desirable for performance enthusiasts.
Choose the 6.7L Cummins if: You need maximum towing power, prioritize a more refined driving experience, and appreciate modern technology. You are willing to deal with the potential complexities and costs associated with more advanced emissions systems.

Before making a decision, consider:

Your intended use for the truck: Will you be towing heavy loads frequently?
Your budget: Can you afford the potentially higher maintenance and repair costs of the 6.7L?
Your mechanical aptitude: Are you comfortable working on your own truck, or will you be relying on a mechanic?
Your tolerance for emissions equipment: Are you willing to deal with the potential issues associated with complex emissions systems?

Frequently Asked Questions (FAQs)

1. Which Cummins engine is considered the most reliable?

Many consider the early second-generation 5.9L 12-valve Cummins (1994-1998) to be the most reliable due to its simple mechanical design and lack of complex emissions controls.

2. What year Cummins engine should I avoid?

The article you provided recommends avoiding 1991, 1992, 1993, 1999, 2001, 2003, 2006, 2007, 2008, and 2011 models due to potential issues with transmissions, charging systems, and electrical components. However, specific issues can vary, and a thorough inspection is always recommended.

3. How many miles can a 5.9L Cummins last?

With proper maintenance, a 5.9L Cummins can easily last over 600,000 miles.

4. How many miles can a 6.7L Cummins last?

A 6.7L Cummins can often last up to 300,000 miles with proper maintenance.

5. Which Cummins is better for towing?

The 6.7L Cummins is generally better for towing due to its higher torque output, particularly at low RPM.

6. What is the “holy grail” of Cummins engines?

The Bosch P7100 injection pump (the “P-pump”) found on the 1994-1998 5.9L Cummins is considered the “holy grail” by many performance enthusiasts due to its robust design and ability to support massive horsepower.

7. Is the 5.9L Cummins a 12-valve or 24-valve?

The 5.9L Cummins was produced in both 12-valve (1989-1998) and 24-valve (1998.5-2007) configurations.

8. What are the common problems with the 5.9L Cummins?

Common problems can include hard starting, lack of power (often related to the overflow valve on P-pumped versions), and issues with the variable geometry turbo on later models.

9. What are the common problems with the 6.7L Cummins?

Common problems often relate to the complex emissions systems (DPF, EGR, SCR) and the variable geometry turbo.

10. Which is better, Duramax or Cummins?

The choice between Duramax and Cummins depends on personal preference and intended use. Cummins is known for torque, while Duramax can offer more horsepower.

11. How much horsepower can a 5.9L Cummins block handle?

The forged-steel rods in early 5.9L engines can handle up to 1,500 hp with proper conditioning and quality components.

12. Is a 12V or 24V Cummins better?

The 24V Cummins generally offers better throttle response, drivability, airflow, and coolant flow compared to the 12V. However, the 12V is simpler and often considered more reliable by some.

13. What is considered high mileage for a Cummins engine?

Anything over 300,000 miles is generally considered high mileage for a Cummins engine.

14. Which Cummins engine is the best for an engine swap?

The 5.9L Cummins, particularly those from 1989-2007, are often favored for engine swaps due to their reliability, power, fuel economy, and relatively simple design.

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Is 5.9 or 6.7 Cummins better?