For years, the Ford Motor Company has held the crown of having the burliest diesel in America. The high-output version of the 6.7-liter Ford Power Stroke V8 with 500 horsepower and 1,200 lb-ft of torque beat the rest of Detroit in power, and not exactly by a small margin. Today, General Motors has announced the final answer in diesel pickup truck engines. The new 8.3-liter Duramax V8 diesel engine isn’t just the biggest diesel fitted to a mass-production pickup truck, but it puts Ford into a solid second place. Yet, there’s one metric that, despite GM’s best efforts, it could not beat Ford on.
The rumor mill has been busy printing out speculation about the future of Duramax diesel engines for longer than two years now. Back in January 2024, the Fast Lane Truck (TFL) reported that “a source with knowledge of GM’s internal powertrain development alerted us to a new, bigger, and far more potent powertrain about to emerge as the new king of the hill: a massive, 8.3-liter beast.” There was a lot of skepticism following the publication of TFL‘s news, with some, including GM enthusiast blogs, concluding that chasing best-in-class power numbers was more of a Ford thing. TFL nicknamed the project “Megamax” in its coverage, which was fitting.
As it turns out, TFL was right from the jump. While GM did not confirm to TFL that it was designing a new, bigger Duramax diesel engine, it was secretly working to create the very engine that someone leaked to TFL. The real engine does not make 600 horsepower as originally speculated. Instead, it still puts out more than enough thoroughbreds and twist to unseat the Blue Oval from its long-held throne. In the process, it has also built an engine that, so far as I can tell, is the largest diesel engine ever fitted to a mass-market production diesel pickup truck in America.
Credit: AJ Mast for General MotorsThe Rumor Mill’s Rumored Mill
The Fast Lane Truck got Chevrolet Silverado and GMC Sierra fans excited for what sounded like the impossible. In 2024, TFL explained the rumor they heard and why it was a big deal:
As for the headline specs, we’ve been told this new 8.3L diesel engine (which we are calling “MegaMax” — there’s no indication that’s the name GM has in mind for it) puts out up to 600 horsepower and up to 1,200 lb-ft of torque.
For reference, the current 6.6-liter L5P Duramax turbodiesel available in 2024 and newer Chevy Silverado and GMC Sierra HD models manages up to 470 horsepower and 975 lb-ft of torque, according to SAE J1349 power and torque certification measurements. If the automaker does see such an engine through to serial production, GM would leapfrog ahead of Ford, whose 6.7-liter Power Stroke diesel V8 cranks out 500 horsepower and 1,200 lb-ft of torque in its high-output form. Despite packing two fewer cylinders, the 6.7-liter H.O. Cummins in Ram’s heavy-duty trucks produces 420 horsepower and 1,075 lb-ft of torque.
Ram’s heavy-duty pickup trucks have since upgraded to 430-horsepower mills, but GM’s L5P Duramax has stayed the course with its output. This put General Motors in a weird position in the diesel pickup truck market. It couldn’t touch any version of the 6.7-liter Ford Power Stroke V8, and only bested the Cummins on horsepower.
Duramax L5P. Credit: GM
Yet, understandably, some didn’t believe that GM would go as far as to make a mammoth diesel to rule them all. While the 6.6-liter Duramax V8 was technically third in its field, it wasn’t like the Silverado and the Sierra were suffering. They still benefited from a 36,000-pound towing capacity in a properly equipped truck pulling a fifth wheel or gooseneck trailer. Sure, this was 4,000 pounds short of the Ford F-450’s headline-grabbing 40,000-pound limit, but 36,000 pounds is still tons of weight for a pickup truck to haul. For reference, a Ram 3500HD pickup truck could tow up to 36,610 pounds with a regular cab, a 6.7-liter Cummins straight-six, and a long bed. The best towing configuration for a Ford F-450 is similar.
Apparently, General Motors did not agree with the notion that the L5P Duramax V8 was enough by itself, because its DMAX division has taken the biggest of swings by crafting a whole new 8.3-liter V8 diesel engine from a clean-sheet design.
The ‘Mongoose’ 8.3-Liter Duramax Diesel V8
AJ Mast for General Motors
General Motors says that the new engine, codenamed “Mongoose,” was designed from scratch in-house specifically for the heavy-duty pickup truck market. The 8.3-liter engine sports a compacted graphite iron cylinder block, a forged crankshaft, forged connecting rods, and steel pistons. GM says the engine’s new architecture features an integrated intake, an “advanced combustion system,” and crankshaft isolation for reduced noise and vibration. It also features a compact dual-stage air-to-water charge cooler, a high-efficiency variable-geometry turbocharger, and upgraded engine cooling.
Backing up all of these goodies is an Allison 10L1000 10-speed transmission, an exhaust braking system, and a diesel aftertreatment system that’s supposedly fit for a truck that lives a hard life of towing and hauling. It all adds up to a behemoth 8.3-liter diesel V8 that pumps out 555 horsepower and 1,230 lb-ft of torque. For those of you counting, this readily beats Ford’s best and leaves Ram in the dust.
AJ Mast for General Motors
It also has the distinction of being the largest diesel ever offered in a mass-market pickup truck in America. I’ve done some digging, and the closest that I could find was Ford’s own legendary 7.3-liter Power Stroke V8 of the 1990s and the 2000s. To be clear, the semi-tractor market has had much larger mass-produced V8 diesels, like the 12.1-liter Detroit Diesel 8V92 and Scania’s whole range of V8 diesels, which reach as high as 770 horsepower and 16.4 liters of displacement. The 18-liter Caterpillar 3408 is another seriously chunky V8 diesel.
Historically, however, the diesel pickup truck market in America has had smaller diesels than their big rig and construction machinery counterparts. So, a mega version of the Duramax is pretty wild. As for that name, well, that one is just GM picking its target. The mongoose is a predator to the scorpion, and “Scorpion” was used as Ford’s codename for its 6.7-liter Power Stroke engine.
GM
General Motors says the new engine is a rather huge deal. The automaker invested $920 million into its Duramax diesel program to enable it to create this behemoth, which will be built in the GM DMAX complex in Brookville, Ohio. In describing the new engine, GM says that “The all-new available diesel is GM’s first clean-sheet HD V8 designed, developed and manufactured in-house for heavy-duty pickup applications.”
As I have written in the past, the Duramax diesel dates back to a partnership between General Motors and Isuzu that minted its first production engines in 2001. What’s interesting is that some sources say that the L5P Duramax engine, which went into production during the 2017 model year, is a clean-sheet design. Other sources say that it’s merely a “near clean-sheet” design. GM’s claim would seem to support the idea that previous Duramax diesels at least had some of the original GM-Isuzu DNA still left in them.
AJ Mast for General Motors
The new engine isn’t all that’s coming for 2027. GM says it has announced the heavy-duty pickup truck segment’s first electric power steering system for its new Sierra and Silverado pickups. According to GM, this new rack and pinion system provides 30 percent more steering power than previous hydraulic systems, while also offering fine-tuning through drive modes, improved on-center feel, and less kickback on rough surfaces. The electric power steering system is reportedly also built to take some hits and survive the worst that America’s saltiest roads could throw at it.
Another new feature is an electronic brake-boost system, which GM says is another first in the heavy-duty pickup truck market. The new trucks will come with four-piston calipers up front, which will chomp on 14.6-inch rotors. The big brakes are said to have a better thermal capacity, while the electric brake booster reduces pedal travel, makes pedal feel more linear, and allows for better integration into active safety systems like adaptive cruise control and automatic emergency braking. While perhaps not of the most importance in a heavy-duty pickup truck, the electric booster also shaves some weight compared to a hydraulic version.
GM
Finally, off-road versions of GM’s heavy-duty pickups, namely the Chevrolet Silverado ZR2 and the GMC Sierra AT4X, will be getting a new Terrain driving mode that will allow for one-pedal off-road driving. While this feature is new to GM, its competition, like Ford, has had a sort of off-road cruise control in heavy-duty pickup trucks for years.
For those who don’t like diesel, General Motors says it’s also updated the 6.6-liter gasoline V8 for 2027. GM says that the updated engine “gains horsepower and torque, with improved low-end pull and updated emissions hardware supported by a new Turbo-Jet intake manifold, revised lubrication system, engine-mounted oil cooler and digital oil-level display.”
GM neglected to say just how much horsepower the updated 6.6-liter gasoline V8 will make. It also doesn’t say if the digital oil-level display means that the engine won’t have a dipstick. I have reached out to General Motors for clarification on these figures.
Ford Still Beats GM In One Metric
GM
The big number is towing. General Motors says that properly equipped 2027 Chevrolet Silverado HD and 2027 GMC Sierra HD trucks will have 20,000 pounds of conventional trailer towing capacity, and up to 38,000 pounds of gooseneck towing. The 20,000 figure is unchanged from the current-generation GM HD pickups, while the latter number is a 2,000-pound upgrade. Notably, this means that, despite having the biggest engine in the business, GM’s trucks technically still tow less weight than the 40,000 pounds that can be hauled by a properly equipped Ford Super Duty.
Still, the gap is now only 2,000 pounds, and I’m sure there will be plenty of truck buyers who won’t care just so they can say they bought the biggest, baddest, and most powerful diesel on the market. Also, the Ford’s win is only a technicality, as it’s on the F-450 chassis. GM doesn’t sell trucks with pickup beds past its 3500 class. However, the towing upgrade does mean that the Chevrolet Silverado 3500HD and its GMC twin can now haul the same weight as an equivalent Ford F-350.
But why? In a world where diesel is seen as old news, and high diesel prices break records, why would GM introduce this colossus?
AJ Mast for General Motors
As Automotive News explained recently, it’s because General Motors sees a future in diesel and is doubling down on the technology in both light-duty and heavy-duty pickups:
“It’s not one engine for everybody,” GM President Mark Reuss said during a media event at the company’s engine plant here. “It’s the freedom to choose the engine that fits your work, lifestyle and priorities.”
For Silverado and Sierra buyers, GM sees its diesel powertrain as having some advantages over a hybrid option, including driving range. The redesigned pickups’ Duramax engine allows more than 900 miles of highway driving on a 34-gallon tank. “If you look at the performance and efficiency but also raw performance and range, hybrids really don’t do it,” Reuss said. “It’s a bit different adding a hybrid system to an existing powertrain and trying to get the efficiency out of it like that.”
“Different customers need different combinations of capability, range, affordability, towing confidence and convenience,” Reuss said. “Our aim is to meet them where they are.”
As a note, GM was still being secretive about this new engine when that Automotive News article was published. The range figure stated above is for the Duramax 3.0-liter engine.
GM
Additionally, as General Motors says, over 60 percent of its heavy-duty truck buyers opt for diesel, and it also sells the most diesels in the heavy-duty pickup truck segment. To GM, making a herculean V8 diesel just cements its place in the diesel world even further.
So, while Ford, Toyota, and Ram commit to hybrids, General Motors is going the opposite direction. It remains the only automaker in America with half-ton diesel pickup trucks on the market. Now, this monolith of an engine is going to be a monument to oil-burning power.
As for the awkward situation of how this engine is going to launch into a market with historic diesel prices, it’s pretty clear that General Motors has been working on the Mongoose for at least a couple of years. Today’s diesel prices are an unfortunate coincidence.
GM
If diesel prices aren’t stopping you from getting a new diesel truck, and you want the one with the largest lump of diesel under its power barn, GM says that its 2027 heavy-duty pickups will ship to dealers late this year. Should you be one of those who go all-in on diesel, you’ll be getting an engine with a displacement so huge that each cylinder has more than the volume of one of the engines out of my Smart Fortwos.
That’s just silly, and honestly, I’m here for it. Only General Motors could have concocted an idea this crazy. In a world of hybrids and electric cars, the General is making a huge bet on diesel. It’s even wilder to think about when you realize that GM could have made an extended-range electric truck, or maybe even a diesel-electric hybrid. But nope, it instead decided to make a hilariously huge lump of diesel power.
Will GM’s bet pay off? I have no idea. I know I shouldn’t like this idea, but I’d be lying if I said I didn’t want to be the first journo in line to test it.
Top graphic images: GM