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Expectations for the 6.7 Cummins Diesel were high. It was next in line following in the footsteps of the very popular 5.9 Cummins, which was known for its high mileage and durability. Along with its reputation for solid performance, the 5.9 also had a wide range of aftermarket support. With the 5.9 being such a popular engine for Dodge Ram diesel trucks, people were hoping the 6.7 would be even greater.
The 6.7 Cummins first came on the scene in 2007. When it arrived, it brought over 40 percent of the 5.9 Cummins engine’s internal parts with it. So far, things were looking good. But would the 6.7 Cummins be able to stand up against its 5.9 predecessor? We’ll break down the main differences between the 5.9 and 6.7 engines.
2006 Dodge Ram 3500 SLT Mega Cab Dually 5.9L Cummins 4×4 – Image courtesy of and copyright of Bring a Trailer
2011 Ram 3500 ST Crew Cab 6.7L Cummins 4×4 6-Speed – Image courtesy of and copyright of Bring a Trailer
One of the primary features of the 6.7 Cummins was that it came equipped with a full range of emission control parts. Most of these parts were unique to the 6.7 engine, including an EGR (exhaust gas recirculation) system, a DOC (diesel oxidation catalyst), and a DPF (diesel particulate filter). Beginning in 2013, an SCR (selective catalytic reduction) became standard in 6.7L engines.
The 6.7 came out during stricter emissions requirements from the EPA, restrictions that weren’t in place when the 5.9 Cummins was made. The restrictions had an impact on the 6.7 engine’s performance as well. Parts like the EGR prevents the engine from reaching hotter combustion temperatures, so the fuel doesn’t burn as completely in the 6.7.
This produces more NOx (Nitrogen Oxide), which causes excessive carbon buildup that lowers the quality of your engine oil and coolant. The 5.9 engine did not have an EGR, and the lack of an EGR probably helped it maintain its reputation for durability at high mileage.
Compared to the 5.9, the 6.7 Cummins offers more torque at low RPM. This is thanks to the 6.7’s increased stroke: 4.88 inches vs the 5.9 Cummins’ 4.72 inches.
Trucks with better low-end torque have greater towing capabilities, so this is a clear benefit for the 6.7 Cummins. This comes with drawbacks as well –it also causes more cylinder pressure, which can cause head gasket failure. Head gasket failure is a common issue with the 6.7 Cummins, while the 5.9 Cummins head gaskets rarely had such a problem.
If you regularly use a 6.7 Cummins for towing, you’re more likely to have head gasket issues by the time your truck hits 200,000 miles. An engine modified with performance upgrades will have these problems even sooner. Head gasket problems can also be exacerbated by the 6.7 Cummins’ reduced sealing space between its water jackets and cylinders.
The 6.7 Cummins features a variable geometry turbo, which is different from the fixed geometry turbo on the 5.9. The fixed geometry turbo is a simple system, which consists of a 58mm compressor wheel, an internal wastegate, and a 58mm turbine wheel. This system had a reputation for being durable and long-lasting.
The turbo’s fixed geometry design resulted in low RPM turbo lag. Unlike the fixed geometry turbo, the 6.7 Cummins turbo system doesn’t have a wastegate, but it does have a 60mm compressor wheel. The system responds faster at lower speeds since the exhaust flow varies over the system’s turbine wheel. That means you won’t get any turbo lag, but the system still performs at a high RPM comparable to a bigger turbo unit.
Simply put, the 5.9 fixed geometry turbo is more reliable, but the more advanced design of the 6.7 variable geometry turbo will give you better performance.
Another feature that boosts the towing capabilities of the 6.7 Cummins is its exhaust brake. This device causes significant backpressure of the truck’s exhaust system to slow its engine speed and provide extra braking. A sticking turbo is also a common issue with Cummins engines which the exhaust brake helps alleviate.
Thanks to its variable geometry design, the 6.7 turbo has excellent braking capabilities. This helps save your Dodge truck a lot of wear and tear. Towing capacity is only part of a truck’s towing capabilities.
A truck needs to have total control of its extra weight, so stopping the large load is important as well. Because of its aggressive exhaust braking abilities, the 6.7 Cummins-powered Rams are some of the greatest tow trucks on the market.
Both the 5.9 and 6.7 Cummins engines use a high-pressure common-rail fuel injection system created by Bosch. The difference between these systems is that the injectors in the 6.7 Cummins have been specifically designed to handle high pressures and operate at a lower voltage. With higher-pressure injectors, you’re capable of generating more torque and more power. This is because the 6.7 engine’s higher pressure injectors force fuel into your engine to help you reach your maximum power faster.
You can swap cylinders on the 5.9 Cummins, but the 6.7 has been programmed so that you can’t switch into different cylinders.
If you’re looking for higher performance, then the 6.7 Cummins offers better performance over the 5.9 Cummins in almost every category. The 6.7 also had the added benefit of being less harmful to the environment. However, the simplicity of the 5.9 Cummins means that even if it doesn’t perform as well, it still has unbeatable reliability, including at high mileage.
We’ve tried giving you a thorough understanding of the main differences between the 5.9 and 6.7 Cummins engines. You might want the newer technology, or you might want to stick with an older, but reliable engine. Whatever choice you make for your engine, ProSource Diesel will carry the diesel truck parts you need. We’ll help you maximize the performance and longevity of any Dodge Ram truck with the highest quality Cummins parts.
