Cummins 5.9L vs 6.7L: Which One is Better?

If you’re into diesel trucks, then you probably know all about Cummins. The 5.9L and 6.7L Cummins engines that Dodge has used over the last three decades have become legendary.

They’re both really solid engines with minimal downsides, but the question remains, which one is better? Today we’re going to dive in and find out.

The Basics

As you might already know, the 6.7L Cummins ultimately replaced the 5.9L Cummins in 2007. This was after multiple revisions to the original 12-valve, ultimately leading to the 5.9L ISB 24-valve engine.

The 5.9L wasn’t designed to work with modern emissions systems and although Cummins did an awesome job with the updates on the 5.9L, making it more powerful with fewer emissions with every update, it still wasn’t enough to keep up with stricter emissions.

With the jump from 5.9L to the 6.7L Cummins came a completely new architect, more displacement, a more advanced turbocharger, an exhaust gas recirculation system, a diesel particular filter, and many more significant changes.

The 6.7L was born from the need for decreased emissions output, but it also brought a significant power increase as well.

To make things easier to understand, we’ll mostly be talking about the 03 to 07 common 24-valve 5.9L Cummins and not the older 12-valve engines. Both the 5.9L and the 6.7L Cummins use a cast iron head with a cast-iron block and they both use 4-valve heads, but that’s pretty much where the basic similarities end.

The 6.7L has a larger stroke and bore, a higher compression ratio, and as I said before, outputs more power.

Emissions Systems

The 5.9L was not designed for modern emissions systems. After all, the original 12-valve was released in 1989 and that was long before the public wanted clean diesel engines.

Up to the end of the 5.9L’s life, it had no emissions components other than a catalytic converter. On the other hand, the 6.7L Cummins has an EGR and DPF to reduce emissions output.

Unfortunately, the EGR system is known for sticking and suffering from clogged EGR coolers.

With the emissions systems reducing overall reliability, a lot of 6.7L Cummins owners end up deleting some of the emissions systems. Just be careful deleting emissions components, as its not legal as per the clean air act, and you can get hit with a pretty hefty fine.

It’s something I recommend doing unless you’re not worried about getting caught.

Injection Systems

As far as fueling, both the 5.9L and the 6.7L use a similar common rail injection system. Again, we’re talking about the 03-07 Cummins here, not the earlier 12 valve engines which used a mechanical injection system.

Both of these common-rail injection systems revolve around a Bosch CP3 injection pump. There are rumors that the 6.7L’s CP3 pump is a bit better, but according to Bosch’s data, they flow the same amount of fuel.

It should be noted that the fuel rail, fuel rail feed lines, injector lines, and crossover tubes on the 6.7L Cummins measure 50% larger than what’s found on the 5.9L. Because the 6.7L’s injection system operates at a slightly higher pressure than the 5.9L’s, a pressure relief valve with an additional 3,000 psi rating is used.

The injectors on each engine are similar in physical size, however, the 6.7L’s injectors are rated for higher pressures and require less voltage to operate.

Unfortunately, the injectors on the 6.7L’s injectors are programmed to the ECU and can’t be swapped into different cylinders as they could be on the 5.9L. Other than that, the fueling systems are pretty similar.

Turbochargers

When the 6.7L Cummins was introduced, the new variable geometry turbocharger was one of the most notable features, as it’s something that its competitors had already been using that the 5.9L Cummins lacked entirely.

The 5.9L always used a fixed geometry turbo, which was fine, when the 5.9L was initially released, but the benefits of a variable geometry turbo are well worth it, and Cummins had to keep up with what the competition was offering so it ended it up on the 6.7L.

The variable turbo gives the 6.7L lots of low-end torque, lots of high-end power, and excellent throttle response. This system works by giving the exhaust housing electronically controlled vanes which change the effective internal size of the exhaust housing.

Changing the size of the housing results in a change in exhaust gas velocity in the exhaust housing. To put it simply, at low RPM the housing decreases its effective size and at high RPM it expands and effectively becomes larger.

Unfortunately, variable geometry turbos are more likely to have issues compared to fixed geometry turbos. While stuck vanes common on the variable turbos on Power Stroke and Duramax engines, the Holset HE351VE found on 6.7L Cummins is notorious for actuator failure.

On top of that, if you add more fueling to crank up the power, the extreme shaft speed and excessive pressure on the exhaust side a variable turbo can become a ticking time bomb.

With the variable turbo on the 6.7L comes the ability to exhaust brake, which allows you to slow down the truck with just the engine, which isn’t something a diesel can normally do because they don’t have throttle bodies, which means there isn’t a pumping loss to retard engine speed.

Exhaust braking is an awesome feature to have when towing and it massively improves the capability of the engine

Common Problems

As far as common issues, the 6.7L has to deal with the emissions components as I said earlier. The 6.7L also has to deal with more boost pressure at low RPMs thanks to the variable turbo.

With the increase, in-cylinder pressure comes significantly more head gasket failures. That’s not to say the 6.7L blows head gaskets like crazy because it doesn’t, but it’s an issue that is much more common than on the 5.9L.

Performance Potential

When it comes to after performance, both engines have a ton of potential. Both of them are easily capable of making 1000lb-ft of torque or much more. The newest Rans with the 6.7L Cummins makes 1,000 lb-ft of torque straight from the factory.

For those who want big power out of their engine, the Siamese bore design on the 6.7L is super strong. The Siamese bore design, just entails a lack of channels for coolant to circulate directly between cylinders, which improves cylinder stability, and it’s an overall stronger design.

At the peak of diesel racing, the 6.7L is the preferred foundation for many racers. Many of the top racers in a truck pulling and drag racing, add sleeves to the cylinders, a deck plate up top and increase the stroke of the crank.

6.7L blocks treated to this type of machining typically wind up with 390 cubic inches of total displacement and produce anywhere between 1,800 and 3,000 hp and do so in ultra-reliable fashion.

Which One is Better?

Back to the original question of which one is better, it kind of depends on your use case. The 5.9L is a little bit more reliable and lacks the annoying emissions components which plague all modern diesel engines, but the lower displacement and fixed geometry turbo limit performance unless you add a different turbo.

The 6.7L has more displacement and a larger variable turbo, allowing it to make more low-end torque and more top-end power, however, it’s more to head gasket failure and the emissions components occasionally cause issues.

If you think reliability is the most important feature of an engine, the 5.9L is the better engine, however, if you’re interested in outright performance, the 6.7L is the better engine.

Of course, this article mostly omits the older 12v engines with fully mechanical injection systems which are known for being crazy reliable.