We Discovered GM's Missing Duramax V8 Prototype That Had the Potential to Transform Modern Trucks.

      Henrik Bengtsson

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      More than ten years ago, General Motors created an engine that had the potential to transform the market for half-ton trucks and diesel vehicles. Unfortunately, that didn’t happen. The 4.5-liter Duramax V8, intended to power the half-ton Chevy Silverado, GMC Sierra, and Hummer H2, was scrapped due to GM’s bankruptcy in the late 2000s and never entered production. Whether this decision was an inevitable cut or a strategy to obtain GM’s criticized government bailout remains unclear, but it ultimately led to the engine being withheld from consumers, for the most part.

      However, some prototypes did escape the factory. After I wrote about it last summer, I received an intriguing email from a man in Sweden who claimed to possess one. After a deep dive into the engine's lost history and a conversation with diesel tuning expert Gale Banks, who saw the engine in person before it was canceled in 2006, I can confirm its authenticity. An unused 4.5L Duramax V8 prototype indeed found its way into private ownership. Here’s the story of how that occurred.

      The light-duty diesel, engine code LMK, was nearly ready for release before the Great Recession struck in 2008. Its existence has lingered in obscurity since then, surfacing occasionally without explanation. The last time I nearly uncovered one, the owner initially agreed to speak but then suddenly went silent. Shortly after, Henrik Bengtsson contacted me with an exciting message: “I got one.”

      We exchanged emails, texts, and had phone conversations about the diesel V8 that he had practically stumbled upon years earlier. His story of acquiring the engine from across the Atlantic was captivating; it was well worth the $66 international call fee. Additionally, he shared a folder of original photos that highlighted the engine's innovative features, which remain impressive today.

      I then reached out to Gale Banks, the turbocharging expert and diesel-tuning authority. He was my first choice for more information about the 4.5-liter Duramax. As expected, he had all the insights I needed and more. His knowledge of the project is extensive, and the stories he recounted are invaluable.

      General Motors

      While not every detail here is new, many are, and my intention is to create a centralized source for all information regarding the LMK. Instead of just delivering a dry data dump, I want to take you on the same journey I undertook to reach this point.

      Let’s begin with some history.

      The 4.5L Duramax Origin Story

      From the beginning, the LMK was intended to power vehicles on GM's half-ton truck platform during the last decade. Gary Arvan served as the chief engineer for GM's Duramax diesel engines when the 4.5-liter project began to take shape in 2005, and Charlie Freese was the executive director of GM’s diesel powertrain engineering alongside him. They were charged with creating a small-block alternative that produced more torque and offered better fuel efficiency than its gasoline counterpart while complying with increasingly stringent emissions regulations.

      “In today’s auto industry, you can’t afford to develop an average anything,” Freese told Automotive News in 2007. “You can’t set your goals to be like everyone else.”

      This statement highlights much about the LMK Duramax. It featured several attributes that were not only intriguing but also genuinely groundbreaking for its time. Among its notable hardware is the hot-vee turbo layout, which reversed the exhaust gas flow compared to traditional diesel engines that expelled exhaust outward. The variable-vane turbo was positioned between the cylinder banks, allowing for quicker acceleration of incoming air and delivering boost to the engine more rapidly due to the shorter distance. This design is utilized in high-performance vehicles like the Mercedes-AMG GT and GM's short-lived CT6-V with the renowned twin-turbo, 4.2-liter Blackwing V8.

      General Motors

      The automotive giant introduced the powertrain to the public in 2007 and even placed it in a Suburban HD Z71 concept for the following year's SEMA show before cancelling the project. This disappointed many who were eager to witness an engine that Car and Driver had deemed "revolutionary" at the time. It was also a significant setback for the GM team that dedicated years to the project. Over time, the LMK faded from public awareness.

      Ahead of Its Time—Before Its Time Ran Out

      The 4.5-liter was truly impressive. Gale Banks mentioned this when discussing his handwritten notes from previewing the prototype LMK in 2006. “It’s a victim of timing and circumstance rather than a failure of engineering,” he emphasized.

      Gale Banks was brought in so his company, Banks Power, could “represent the Duramax to smaller OEMs, such as those in the marine sector.” His extensive experience with marine power systems dates back to before he built