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5.9L VS. 6.7L: Which Cummins is really better?

When it first arrived on the scene for ’07.5 model year Dodge Rams, the 6.7L Cummins had a lot to live up to. After all, the 5.9L it replaced was highly revered for its million-mile durability, vast performance potential, and an endless supply of aftermarket parts and support. Luckily, more than 40 percent of the 6.7L’s internals were carry-over parts from the 5.9L, and the injection system remained virtually the same.

The 5.9L Cummins was produced from 1998.5-2007 and featured a fixed-geometry turbo. It put out 325hp and 610 lb-ft of torque by the end of its run, along with 359 ci of displacement coming from a bore of 4.02 in and stroke of 4.72 in. The 6.7L Cummins started its run for 2007.5 model year Ram trucks with 350hp and 650 lb-ft of torque. It used a variable geometry turbo and had a bore of 4.21 in and stroke of 4.88 in, with 408.2 ci of displacement.

However, as far as emissions control devices are concerned, the 6.7L would be saddled with the full gamut — something 5.9L owners knew absolutely nothing about. The brand new, 6.7L Cummins came with exhaust gas recirculation (EGR) and a diesel particulate filter (DPF). On 2013 model year Rams, selective catalytic reduction (SCR) would be added to the mix, which as part of the exhaust after treatment system would require the use of diesel exhaust fluid (DEF) to reduce NOx emissions.

And now the trade-offs begin! Due to the aforementioned emissions control components, fuel economy is noticeably lower on 6.7L-equipped trucks as well as its overall reliability. However, the 6.7L’s use of a variable geometry turbocharger makes it much more responsive than the 5.9L at low rpm, not to mention that it incorporates a very effective exhaust brake function. The 6.7L’s larger displacement also helps produce considerably more torque, which is ideal for truck owners that spend a lot of their time hooked to a trailer.

While plenty of folks still believe the venerable 5.9L is the end-all, be-all Cummins, many have embraced the fact that the 6.7L is basically a bigger, better version of the 5.9L. But, is the 6.7L actually the better engine? To try to settle this 50/50 split, we’ve spelled out each engine’s strong suits and key weaknesses below.