For more than two decades, GM has been bringing a 6.6-liter diesel to heavy-duty fights. It kept up by turning the boost up, not by getting bigger. That changes for 2027. The new Duramax 8.3L Turbo-Diesel V8 is headed for the Chevrolet Silverado HD and GMC Sierra HD, and it’s the most powerful diesel you can buy in a heavy-duty pickup. GM calls it “Mongoose.” The name is aimed squarely at Ford, whose Power Stroke is known as the Scorpion.
The numbers are real. The blank spot on GM’s brag sheet is the interesting part.
The numbers, and the fine print
GM rates the engine at 555 hp and 1,230 lb-ft, and calls both figures best in class. Its footnote says the ratings were measured under SAE J1349 and compared against “latest available competitive information.”
Against its own predecessor, that’s an 85-hp and 255-lb-ft jump. Against Ford, it’s a knockout on horsepower, 55 more. On torque it’s a points decision: 30 lb-ft, about 2.5 percent. Bragging rights are bragging rights, but no driver will feel that torque gap on a grade.
Displacement did the heavy lifting
Divide output by displacement and you can see each company’s philosophy.
In plain terms, Ford squeezes its engine hardest. GM’s new V8 makes the least torque per liter of any diesel in the segment, and less peak power per liter than the engine it replaces.
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Call it a weakness if you like. For a truck dragging a gooseneck up the Rockies, it’s the point. A big engine working at a moderate state of tune generally sees lower cylinder pressures and less heat for a given amount of work. That is the recipe you want for a truck dragging a gooseneck up the Rockies all day. It also leaves obvious room for GM to raise the numbers later, whenever Ford answers.
The hardware reads like it was chosen for exactly that kind of work:
- A compacted graphite iron (CGI) block.
- Fully forged crankshaft and connecting rods.
- Steel pistons.
- A variable-geometry turbocharger.
- Dual-stage air-to-water charge cooling.
CGI isn’t new to the class; Ford was touting a CGI block for its 6.7 in a 2015 product sheet. Steel pistons are the more telling detail. Steel tolerates heat and cylinder pressure better than aluminum, which is why it shows up in engines built to live at high load.
“In-house” is doing some quiet work
GM calls this its first clean-sheet heavy-duty V8 “designed, developed and manufactured in-house.” That phrasing is deliberate.
The Duramax name was born at DMAX, which started as a 60:40 joint venture between GM and Isuzu. DMAX built its first production engine on July 17, 2000.
Ford has been needling GM about this for years. Back in 2015, a Ford marketing manager said Ford was “the only manufacturer that develops and builds our own powertrains in this class” in a Ford press release. The Mongoose is GM’s rebuttal, cast in iron.
The engine will be built at GM’s expanded DMAX plant in Brookville, Ohio. That’s a 1.4-million-square-foot facility where GM put machining and assembly under one roof, backed by a $920 million investment. GM also says more than 60 percent of heavy-duty buyers choose diesel and that its brands already hold the majority of diesel share in the segment. That explains why it spent the money.
The 6.6 Duramax is done
Chevrolet’s 2027 Silverado HD page lists two engines: the 8.3L diesel and an enhanced 6.6L gas V8. The 6.6L diesel isn’t there. If you want a Duramax in a 2027 GM heavy-duty pickup, the only one on offer is the big one.
The gas engine gets a meaningful update too: 412 hp and 472 lb-ft. GM credits:
- A new intake manifold.
- A revised lubrication system.
- An engine-mounted oil cooler.
- A digital oil-level display.
The crown GM didn’t claim
Read GM’s release closely. It claims best-in-class horsepower and torque. It does not claim best-in-class towing.
Max gooseneck towing is 38,000 pounds, with 20,000 conventional and a gross combined weight rating up to 46,500 pounds. That’s up from the current truck’s 36,000-pound ceiling. GM says every diesel dually can tow at least 30,000 pounds.
Tow ratings depend on frames, axles, brakes, cooling, and the SAE test procedures that apply to all of it, not just the engine. So the most powerful engine in the class doesn’t automatically deliver the biggest tow number. The 2027 page also lists a maximum available payload of 6,170 pounds. GM hasn’t published the new engine’s weight, and that’s worth watching given how much iron an 8.3-liter V8 carries over the front axle.
The parts owners will actually live with
The engine gets the headlines. The chassis changes will matter just as much to owners.
Steering. GM’s heavy-duty trucks switch from hydraulic assist to a rack-and-pinion electric power steering system, which GM calls a segment first. GM says it delivers roughly 30 percent more output than the old hydraulic gear. Owners lose the pump, hoses, and fluid, and with them a classic source of leaks and whine. The tradeoff is that an electric rack is an expensive sealed assembly, not a pump you swap in a driveway.
Brakes. A new electronic brake-boost system replaces the old setup, in which steering and braking assist shared the same hydraulic source. It comes with:
- Larger 14.6-inch front rotors.
- Four-piston fixed front calipers.
- Diagnostics GM says can detect brake fade or leaks sooner.
Off-road. ZR2 and AT4X models get a new Terrain Mode with one-pedal driving in 4HI and 4LO, a first for GM’s heavy-duty trucks.
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Transmission. The Allison 10L1000 10-speed carries over and now has to handle 255 lb-ft more than it did behind the 6.6 diesel. GM rates the combination, so it presumably did its homework. Still, a proven gearbox under a much bigger engine is the pairing that high-mileage fleet owners will keep the closest eye on.
Practical takeaways for buyers
- Timing: GM says the trucks arrive at U.S. dealers beginning in late 2026. Pricing hasn’t been announced. Early-build, first-year engines deserve the usual caution. There’s no field history yet, and the aftermarket parts supply will take time to catch up.
- Tuning: The new engine has a new aftertreatment layout. Don’t count on deleting it. Section 203(a) of the Clean Air Act is the statute federal regulators have used against sellers of diesel defeat devices. Tampering with a brand-new truck under warranty is also a quick way to lose coverage.
- Insurance: Expect repair costs to climb. Electric steering racks, electronic brake boosters, and a trailering system with up to 14 camera views all make fender-benders more expensive and add calibration work after the repair. Get a quote before you sign, not after.
- Used market: With the 6.6 Duramax discontinued, clean late-model L5P trucks may hold their value well among buyers who want a known quantity over a first-year engine.
The Mongoose is the biggest swing GM has taken in the heavy-duty diesel segment since the Duramax arrived. GM won the horsepower title outright and the torque title by a hair. What it hasn’t proven yet is the thing heavy-duty buyers care about most: how this engine holds up at 300,000 miles. So who’s the better bet there: GM’s first clean-sheet V8 in years, or Ford’s Scorpion with its field history? If you tow for a living, where does your money go?
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