March 4, 2024

Which is better: 5.9L or 6.7L Cummins Diesel Engines?


For a while now, Cummins has been a standard engine in heavy-duty Dodge and Ram vehicles. In 1989, the renowned 12 valve edition of the 5.9l was introduced to Dodge pickups. Models from 1989 to 2007, when the new 6.7 Cummins was launched, were powered by 5.9 engines. The task of replacing the formidable 5.9 won’t be simple by any means.

The original Cummins engine has undergone several upgrades throughout time, going from a 12-valve to a 24-valve ISB engine. Despite all of the changes, the 5.9 was never intended to function with contemporary emmision systems.

Features 5.9 Cummins

  • Production run 1989 to 2007
  • Cylinder heads: Cast Iron
  • Compression ratio: 17:1 to 17.2:1
  • Horsepower: 160 to 325 hp
  • Torque: 460lb-ft to 610lb-ft
  • Engine Block: Cast Iron
  • Valve Train: 2v and 4v
  • Bore: 4.02 Inches
  • Stroke: 4.72 inches

Features of 6.7 Cummins

  • The 6.7 brought a larger stroke, higher compression ratio, larger bore and more horsepower and torque.
  • Production Run: 2007.5 to Current
  • Cylinder Heads: Cast Iron
  • Engine Block: Cast Iron
  • Valve Train: 4v
  • Bore: 4.21 Inches
  • Stroke: 4.88 Inches
  • Compression Ratio: 16.2:1 to 19:1
  • Horse Power: 350hp to 370hp out of the all new 2020 Cummins 6.7l I6 Engine
  • Torque: 610lb-ft to 850 LB-FT

Dodge was forced to switch over its 5.9L engine for a 6.7L because of rising emissions regulations over time. Long before the Diesel Clean Air Act was enacted, the first generation Cummins engine was created. It was therefore imperative to update to the more potent 6.7.

5.9L Cummins Problems

The early 12 Valve had a lethal dowel pin problem that would essentially take out your engine if it went bad, but the 5.9 was a pretty reliable engine with very few problems. Replace the dowel pin if you want to purchase a secondhand 12-valve Cummins; it will be worth it! Casting number #53 was a poor casting used in the 24 valve; these blocks were terrible and frequently fractured.

A Dodge pickup manufactured between 1999 and 2001 should have its casting imprinted onto the block. We would advise finding another vehicle if it is stamped “53”! Lift pump failures were common in Dodge pickups with the 5.9L Cummins diesel engine from 1998 to 2004.

If you are considering a Dodge pickup from 1998 to 2004, it’s probable that this problem has already been fixed. Additional problems include broken manifolds and wiring-related ECM problems. With a few exceptions, the 5.9L Cummins is a reliable engine.

6.7 Cummins Problems

There are a few problems with the 6.7, most of which are caused by the new emissions needed to make a diesel engine suitable for the market today. Clogged DPF filters are a regular problem with this type of engine, much as problems with powerstroke engines. Another problem, which affects all late-model vehicles regardless of manufacturer, is the EGR valve sticking and clogging.

When Cummins debuted the 6.7 engine in 2007, they made the first switch to a variable geometry turbocharger. This resulted in certain problems with the turbo systems. It’s common for these turbos to become trapped or stuck. The head gaskets on the 6.7 are another problem.

In our opinion, if we had to choose a diesel engine, we would choose an earlier model in order to escape the pollution control that comes with later model diesel trucks. These late-model trucks are not as reliable as their older counterparts now that the EPA is strictly enforcing pollution regulations.

When purchasing a late-model diesel vehicle, it is crucial to adhere to a fairly strict maintenance schedule. On these late-model pickup vehicles, the expense of skipping even one oil change may add up. Maintaining your car’s maintenance records up to date will help you save money. If you’re purchasing a vehicle for dependability, you have to consider the following five.

6.7 is the Real Deal

The 6.7 engine is quite powerful, producing a tremendous amount of torque and horsepower. Although we adore the power this engine generates, we would always choose a 5.9 over a 6.7. Dependability is improved with the 5.9, especially when compared to an older manual injection 12-valve Cummins. However, there isn’t actually a bad response. The 5.9 is a better option if you truly want to alter a diesel engine. Get a 6.7 if you want to keep it stock or only make a few modifications.

Why then do we recommend a 5.9 for modifications rather than a 6.7? It’s actually rather simple: late-model diesel vehicle modifications are becoming more and more difficult due to new rules. Depending on the year, the emissions of the early versions ranged from almost nothing. if you already have modifications on your truck, making it much simpler to locate a company to service it.

Many shops won’t even handle a newer truck that has been removed because of the new EPA regulations. As everyone knows, the less emissions the better when it comes to performance building. Choosing the Cummins 5.9 liter eliminates the need to remove the EGR and convertor, especially on the very early vehicles.

Facilitating the development of horsepower even further. Without a doubt, you can still create a somewhat healthy 6.7 with today’s emissions, but the cost will be far higher and there will still be limitations. We have to admit that the 6.7 would be the engine of choice for daily driving and hauling. A contemporary vehicle will likely be far more dependable than one from 1998 and the newer motors will use less gasoline.

Final Thoughts

That basically concludes this essay; We are sure there are many other viewpoints. This is simply our personal view; maybe, it will assist those of you who are unfamiliar with diesel engines and are having trouble deciding. If you choose to purchase one of these two vehicles, there are plenty more considerations to make. Budget first: are you able to buy a late-model truck? Next, which choices are you interested in? An outdated 5.9 stereo with Bluetooth and navigation is probably not what you should be considering if you’re searching for a new audio system.

If all you want is a truck and you don’t need all the new technology, go for an older 5.9 Dodge 2500 pickup or a Ram tradesman with a diesel engine. Whatever you are transporting, these two incredibly capable vehicles will get the job done.

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