The partnership between Cummins and Dodge goes a long way. It officially began a little more than 30 years ago when the former started supplying diesel engines to Dodge, whose truck division was later on rebranded Ram. With the model year 1989.5, the hugely respected 5.9L Cummins started its supreme run, before putting to rest in 2007. That’s when the Cummins 6.7L came to the scene, took stricter emissions norms head-on, and as of today, continues to power a variety of Ram pickup trucks. In terms of availability, it wins the game, in comparison to its predecessor, but the question often lingers — if it’s better than the 5.9. The answer to that isn’t simple, let us tell you right away, but if you can weigh the pros and cons, you’re more likely to make the decision.

Why should you make that decision now?

There are a few possibilities why you’d be looking at comparing an engine that’s way past its prime and one that’s successfully been on the market for longer than its predecessor. The first is if you’re looking at buying a used Dodge Ram/Ram pickup truck. Or if you’re planning on buying one used for more specialist needs — and not just daily driving. For instance, if you were to make a race truck or off-road truck, you don’t get constrained by things that render modern trucks unfit for public road use after some time. Having said that, if you’re buying new, then the choices are limited to only the most-modern iterations of the 6.7 Cummins.

What are the main differences between the 5.9 Cummins and the 6.7 Cummins?

The biggest difference between the two of the most popular diesel engines on sale in America is, of course, the displacement. The newer engine is bigger, and on the whole, more powerful and produces more torque. You must keep in mind that when the 6.7 Cummins was introduced, it carried over a fair bit of parts from the 5.9. That means the new engine got a respectable foundation to begin with.

The 6.7L Cummins has a wider bore and longer stroke. It also comes equipped with a variable geometry turbocharger, whereas the 5.9L Cummins used to get a fixed-geometry turbocharger only. Another inclusion on the 6.7 is the exhaust brake, which is absent from the 5.9. But if you think the changes between the two end there, you got to think again.

You see the 6.7 came in a time when the world had begun getting strict on emissions. And it’s safe to say that the 6.7L Cummins was one of the victims of that. The engine came strapped with all kinds of emission control devices — EGR, DPF, SCR, to name a few. Although this meant the truck wasn’t spewing pollutants wherever it went, it did hamper the fuel economy on the 6.7.

How does the 5.9L Cummins prove to be a better engine?

To start with, there are the innumerable great reviews for the 5.9L Cummins that mention its longevity. Although its inclusion in the Dodge Ram range has been shorter than the 6.7’s, owners swear by the engine’s problem-free nature. And considering they are talking about an engine that last went into a truck in 2007, that’s saying something, isn’t it!

Since it wasn’t burdened with emission-control devices, the engine itself has fewer possible issues to deal with. And that makes living with the 5.9L Cummins slightly easier. The turbocharger also, for instance, is a fixed-geometry turbine. That means you’re less likely to run into trouble with it.

5.9 Cummins (Schematic of 5.9 Cummins Engine)

What works in the 6.7L Cummins’ favor?

A lot. Since it’s the newer of the two, the 6.7 Cummins makes more power and torque (stock form). And once modified, it matches the 5.9’s reputation and sometimes overtakes that without causing any problem at all.

Over the generations, the engine’s reliability and power have been worked on, and as a result, the 6.7 is much more drivable. The credit to that goes to the variable-geometry turbine turbocharger present on the vehicle. That enables lower turbo lag and improved engine response — something apparent when you jump in from a 5.9-engined truck. There are also countless modifications that can be made with the 6.7 cummins for extra performance including tuning, turbos and delete kits. 

The new-to-6.7L Cummins feature of exhaust brake is also tremendously helpful. It comes thanks to the 6.7’s VGT. With this turbo-powered brake, the engine helps slow the truck down and thus reduces the chances of overburdening the truck’s braking system. It can be useful when carrying large loads or towing. Such a system will help keep a check on the overall speed but without having to burn the brakes.

6.7 Cummins (Schematic of 6.7 Cummins Engine)

What works in the 6.7L Cummins’ favor?

A lot. Since it’s the newer of the two, the 6.7 Cummins makes more power and torque (stock form). And once modified, it matches the 5.9’s reputation and sometimes overtakes that without causing any problem at all.

Over the generations, the engine’s reliability and power have been worked on, and as a result, the 6.7 is much more drivable. The credit to that goes to the variable-geometry turbine turbocharger present on the vehicle. That enables lower turbo lag and improved engine response — something apparent when you jump in from a 5.9-engined truck. There are also countless modifications that can be made with the 6.7 cummins for extra performance including tuning, turbos and delete kits. 

The new-to-6.7L Cummins feature of exhaust brake is also tremendously helpful. It comes thanks to the 6.7’s VGT. With this turbo-powered brake, the engine helps slow the truck down and thus reduces the chances of overburdening the truck’s braking system. It can be useful when carrying large loads or towing. Such a system will help keep a check on the overall speed but without having to burn the brakes.

So which one should you go for?

It’s easy to sway towards the 5.9 because of the track record of the engine. But in reality, the 6.7L Cummins is every bit as good, save for a few possible niggles. It makes more power, has immense potential when it comes to modifications, and since it’s newer, the truck that you’re going to buy will have much better creature comforts, among other things.

The weak points on the 6.7L Cummins include the chances of blown-up head gaskets and complications due to the presence of emission-control equipment. You may think that getting rid of the latter can solve things a great deal. And that’s partially true, but that will make your truck unfit for driving on public roads legally. The variable-geometry turbo isn’t free from problems.

Alternatively, some suggest a 6.4 Cummins (never launched officially, but it’s essentially a 5.9 with a larger bore) can be a good choice. It offers more power than a stock 5.9 but without the possible issues of a 6.7 Cummins.

An ideal scenario will be to look for a well-kept 6.7 Cummins, factor in the expenses that might be inevitable, and make the purchase. Do your market research before making the purchase. But once you do, we’re certain you’ll have a time worth remembering irrespective of the one you choose.

The 6.7 Cummins can be a great platform for modifications. And since it’s a new engine, the third-party manufacturers keep on introducing new products. You can make it more powerful, drivable, or change the way it delivers power by the push of a button. If you’re looking at using the 6.7 Cummins as a competition truck, you can even delete it. But do keep in mind that deleting comes with legal implications. 

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