Classic Advisory: 12 Valve

The 12-valve is a moniker synonymous with the second coming of the diesel engine—the evolution of big diesel power in a small package with what has a reputation for limitless reliability. The Cummins 5.9L 12V engine and its baby brother, the 3.9L 4BT, are part of a series of fully mechanical on-highway diesel engines manufactured by Cummins as its B series.

The B series was originally designed jointly by Cummins and Case Corporation (later to be acquired by John Deere) for medium-duty commercial truck applications, but eventually found its way under the hood of the Dodge Ram pickup in 1989.

The 12-valve was the introductory engine to the B-series family, and used the full suite of mechanical Bosch fueling, injector, rotary pump and injection pump. It started its life as an off-highway agricultural engine for use in tractors, but Chrysler recognized the opportunity it had before them and bid for the Cummins power plant. The 12-valve replaced the seldom ordered Mitsubishi naturally aspirated diesel engine and was an immediate success.

At the time, the other diesel engine options from the Big Three were the 250lb-ft naturally aspirated 6.2L Chevy 350-based Detroit diesel engine in General Motors trucks and the 350lb-ft naturally aspirated 7.3L IDI International Harvester engine. The 5.9L Cummins 12-Valve featured direct injection, instead of indirect injection, engine bores machined directly into the block (instead of wet liners) and a shallow, one-piece head. Every engine has come equipped with a turbocharger, and gear drive camshaft for extra reliability. Cummins also specified a cast iron deep-skirt block and extra-strong forged connecting rods and crank shaft. Thanks to these improvements, the engine generated 160 hp and 400 lb-ft of torque, which was substantial and outclassed even the largest gasoline V8 available by Chrysler, with massive fuel economy savings. The inline-six coupled with the engine’s long-stroke and under square design 4.02-inch bore and 4.72-inch stroke) meant that its competition crushing torque numbers, were available right off the get-go. Seventy percent of engine torque is available at 700 rpm, with a full, broad, torque curve lasting through the entire range of engine speed.

During this period of technological advancement, the 5.9L inline-six Cummins was a physically smaller engine, with less parts and mechanical complexity compared to its V-8 competition. Its design was based on a long-running legacy of stout commercial engines, and thanks to its direct injection, was the only diesel engine on the market available that did not rely on glow plugs for cold weather starting. The legacy of the 5.9L today still does not use glow plugs, but instead uses a grid heater for extreme cold weather starting.

The 5.9L Cummins 12-valve was available in Dodge Ram trucks from 1989 to 1998 where it was replaced by the updated 24-valve 5.9L ISB with computer control. 

The 1,100lb dry weight 12-valve has an enormous cult following due to its simplicity, reliability and huge supply of replacement parts due to its wide use in global consumer, commercial, mining and agricultural fields. The engine falls under the “one-wire-diesel” category, which means that the alternator is a one-wire (positive) self-exciting setup, and that’s all the engine needs to make electricity for its own starter motor. Technically, the engine can operate without electricity all together, once it’s started.

The 5.9L 12-valve Cummins is a legend, and thanks to it being over built, it probably won’t ever die. It is a million-mile engine, albeit surrounded by a 50,000-mile truck.