The 6.7 Cummins diesel replaced the 5.9 Cummins diesel when the 6.7 was first put in the 2007 line of Dodge Ram diesel trucks. The expectations were high for the 6.7 as the 5.9 engine was well-known for its durable performance and ability to withstand high mileage. If the 5.9 was great, the 6.7 had to be greater right?
The 6.7 Cummins came on the scene with over 40 percent of the 5.9’s internal parts carried over, meaning it was off to a good start because The 5.9 Cummins had a reputation for solid performance and a wide array of aftermarket support with parts and service.
So, what are the main differences between the 5.9 and 6.7? ProSource Diesel is here to help you with a breakdown of the variations between the two engines.
The 6.7 Cummins comes equipped with a full range of emissions control parts, most of which are not found on the 5.9 Cummins. These parts include an EGR (exhaust gas recirculation) system, a DOC (diesel oxidation catalyst), a NAC (NOx absorption catalyst), a DPF (diesel particulate filter) and beginning in 2013, an SCR (selective catalytic reduction). After that, the SCR became standard in Cummins 6.7L engines.
Emissions control systems are necessary for modern diesel engines like the 6.7 due to strict requirements from the EPA. Unfortunately, they do interfere somewhat with performance.
Without EGR, the engine produces a hotter combustion temperature, which allows it to burn fuel more completely. This results in less particulate matter but leads to more production of NOx (Nitrogen Oxide). The problem with producing more NOx is that it results in excessive carbon buildup and that degrades engine oil and coolant. As opposed to the 6.7, the 5.9 did not have an EGR system and that likely helped its reputation for durability even with very high mileage on the engine.
The 6.7 Cummins offers a lot of torque at low RPM compared to the 5.9. This is due to the increased stroke, which is 4.72 inches in the 5.9 Cummins vs. 4.88 inches in the 6.7 Cummins.
Increased low-end torque means more towing ability, so you can easily see why this is a benefit. However, it also means more pressure in the cylinders. More cylinder pressure can lead to head gasket failure which is a common problem on the 6.7 Cummins. Conversely, the 5.9 Cummins rarely had head gasket issues.
A 6.7 Cummins that is regularly used to tow is likely to have head gasket issues by 200,000 miles. Modified engines with performance upgrades can have head gasket issues even sooner. Another aspect to head gasket problems on the 6.7 Cummins is the reduced sealing space found between the water jackets and cylinders thanks to the larger bore on the 6.7.
Fixed Geometry Turbo vs. Variable Geometry
The 5.9 Cummins offers a fixed geometry turbo, which differs from the 6.7 Cummins variable geometry. The fixed geometry unit on the 5.9 is a simple system with a 58mm compressor wheel, internal wastegate and a 58mm turbine wheel. It has the reputation of being extremely durable and long-lasting provided it does not get 45 psi or more of boost on a consistent basis.
However, the fixed geometry design means that you will likely experience low RPM turbo lag. The 6.7 Cummins turbo system does not have a wastegate and it has a 60mm compressor wheel. The exhaust flow varies over the turbine wheel, which allows the system to respond more quickly at lower speeds. This means no turbo lag and it still performs at high RPM like a bigger turbo unit.
To sum up, the more simplistic 5.9 fixed geometry turbo means that while it tends to be more reliable, the more advanced 6.7 variable geometry turbo gives better performance.
Exhaust (Turbo) Brake
The Exhaust Brake is a big part of why the 6.7 Cummins is such a powerful towing machine. The exhaust brake is a device that creates a restriction in the exhaust system which causes significant backpressure to slow the engine speed and offers some extra braking. Using the exhaust brake at all times on this engine also helps prevent the sticking turbo issue, which is common on these engines.
The variable geometry design of the 6.7 turbo system allows for powerful braking ability. This saves quite a lot of wear on the regular brakes of a truck. A big aspect of towing ability is also braking ability.
Being able to pull a large load is only part of the equation. You need to be capable of stopping the large load as well. The aggressive exhaust braking ability of the variable geometry design allows for effective and powerful deceleration. This makes 6.7 Cummins-powered Rams one of the frontrunners in the towing category.
Both the 5.9 and 6.7 Cummins use a Bosch high-pressure common-rail fuel injection system. The difference is that the injectors in the 6.7 Cummins are rated for higher pressure and are specifically designed to handle it. They also require less voltage for operation.
Each injector in the 6.7 is specifically programmed so that they can’t switch into different cylinders. On the other hand, cylinder swap is possible on the 5.9 Cummins units. Higher-pressure injectors allow for more torque and more power.
Combustion time is limited when an engine is running at high RPM at full load. The higher pressure injectors in the 6.7 force fuel into the engine quickly to yield maximum power.
In summary, the 6.7 Cummins does offer higher performance in nearly every area as compared to the 5.9 Cummins. It also emits less harmful pollutants. However, the 5.9 Cummins is a simpler piece of technology that offers almost unbeatable reliability even in high mileage trucks.
Now that you know the main differences between the 5.9 and 6.7 Cummins engines, you can make an informed decision on whether you want to go with the newest diesel technology or stick with an older, but still very durable engine. Whichever you choose, ProSource has the parts you need to maximize performance and longevity of your Cummins truck here.