The Recession Killed GM's 4.5L Duramax V8, But One Escaped the Factory

This story starts like so many others: A car company developed something cool and before it ever entered production, financial restraints forced an audible at the last minute. Unlike most other examples throughout history, though, GM's canceled 4.5-liter Duramax diesel V8 lived a little bit of life outside factory walls. The pre-2000s recession engine is as rare as hen's teeth. But over the past decade and a half, there have been reports of at least one prototype example making it into private hands.

Now, if only I could find it.

Officially designated as the Duramax LMK, the oil burner was announced in 2008 and destined for GM's half-ton pickups. A short time later, in March 2009, GM announced the engine's cancelation in a statement saying, "Given the current economic climate, GM has reviewed and updated its U.S. product portfolio and has decided to place on indefinite hold its previously announced plan to add a Duramax 4.5L V8 diesel engine in 2010 to its Chevrolet Silverado and GMC Sierra light-duty trucks."

This would have been a big deal at the time, considering no 1500-series truck had sported a diesel engine since 1998, when the 6.5-liter Detroit was available in the Silverado and Sierra. Over the past decade, we've seen Ford, GM, Ram, and even Nissan bring diesel half-tons to market, though GM is now the only one left doing it with the 3.0-liter Duramax inline-six.

The 4.5-liter was much different than what we have today, with larger displacement and more cylinders arranged in a V. GM designed it with reverse-flow heads, which can be seen in a handful of official photos released by the manufacturer. According to Diesel Hub, it also had a 72-degree bank angle with a block made of lightweight compacted graphite iron. Dual-overhead cams gave it four valves per cylinder. And while specs like bore and stroke were never confirmed, using the same formula as the larger 6.6-liter Duramax lets us extrapolate a bore figure of 3.56 inches (90.5 millimeters) and a stroke of 3.423 inches (87 millimeters).

Reports say this Duramax was capable of 310 horsepower and 510 lb-ft of torque, which was especially impressive for the period. The modern 3.0-liter Duramax makes 305 hp and 495 lb-ft of torque, for reference. In the newer engine's defense, it's likely much more fuel efficient than its would-be predecessor could have hoped for, and it also has far stricter emissions regulations to comply with.

In the 16 years since GM first showed the 4.5-liter V8, the lore surrounding the engine has only gotten richer. That's mainly because an alleged prototype of the power plant was listed on eBay in 2016. The for-sale ad is long gone now, though a post from enthusiast site GM Authority claims the description was incredibly vague. It offered zero details of how the seller came to own the engine, and it's unclear if someone actually bought it or not.

There was apparently another attempted eBay listing for an LMK Duramax prototype in January 2021. However, a post on the Competition Diesel forum claims it was pulled by the auction site. It's impossible to know for sure if this was the same engine listed just five years earlier, but it seems likely. This time around, more details and even photos of the diesel were provided. The engine had just an hour of run time for an emissions certification, according to the seller, but it had effectively sat on a stand for eight years at that point. It also had a stuck turbo that wouldn't spin and none of the attached accessories were included in the sale. Still, the engine's crankshaft spun freely when turning the flywheel by hand.

Now marks three years since a word has been breathed of the LMK Duramax and its whereabouts, at least from what I've found. Its story is shared on Facebook ever so often with grainy photos of the engine, and right now, there's a Spanish-language post making its rounds. I'm not sure the LMK Duramax will ever see the light of day again, but I'd love to find it for, uh, personal reasons.

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