What has GM actually confirmed?
On September 24, General Motors released the major specifications for its all-new 8.3-liter Duramax Turbo-Diesel V8 in the refreshed 2027 GMC Sierra HD and Chevrolet Silverado HD. GM is calling the engine "Mongoose."
The 8.3L Duramax produces 555 horsepower and 1,230 lb-ft of torque. GM pairs it with the Allison 10-speed automatic and says it has upgraded the exhaust braking and cooling systems for heavy towing.
GM also says properly equipped trucks can offer up to 20,000 pounds of conventional trailering or up to 38,000 pounds with a gooseneck. Those are maximum available ratings, not promises for every Sierra HD configuration.
Why is 8.3 liters such a big deal?
The obvious part is displacement. Moving to an 8.3-liter diesel is a significant change for GM's heavy-duty truck strategy, and GM clearly isn't being subtle about how it's positioning the engine.
What matters more than displacement is how the complete truck turns that output into usable capability. For an HD buyer, the important questions are how much the actual truck can tow, what its payload is, how it behaves under load, and what it costs.
Cab, bed, drivetrain, axle, equipment and the specific configuration still determine the rating.
The gas engine got stronger too
The diesel will get most of the attention, but GM also updated the 6.6L gasoline V8. For 2027 it produces 412 horsepower and 472 lb-ft of torque, and it is also paired with the Allison 10-speed automatic.
That matters for buyers who need HD chassis capability or occasional towing but do not automatically need a diesel. The right answer depends on the job, the actual configuration, and how long you plan to keep the truck.
The 2027 changes aren't just under the hood
GM says the 2027 Sierra HD moves to rack-and-pinion electric power steering, adds electronic brake boost, larger front brakes and four-piston front calipers, and updates Tow/Haul and Off-Road modes.
Other available changes include Terrain Mode, up to 14 camera views, Transparent Trailer View, Trailer Side Blind Zone Alert, and trailer tire pressure and temperature monitoring. Equipment remains configuration-dependent.
What does the new Terrain Mode do?
The AT4X is getting another change that matters if you actually plan to take an HD truck off pavement. GM says its new Terrain Mode is designed for low-speed off-road driving and adds one-pedal control in 4HI and 4LO.
In simple terms, lifting off the accelerator automatically applies braking, giving the driver more precise control around steep grades and technical obstacles. That can be useful when you are placing a heavy truck carefully around rocks, ruts or other obstacles.
It does not make every Sierra HD an AT4X, increase towing capacity, or replace normal braking. It is a trim-specific feature for buyers who actually plan to use the off-road equipment.
Should you wait for the 2027 Sierra HD?
If you're shopping for a heavy-duty truck right now, I wouldn't automatically stop shopping because a new diesel was announced. The current Sierra HD is a known truck, and some buyers need a truck today.
If you specifically want the newest diesel, the 8.3L is now a concrete reason to wait. GM says the 2027 Sierra HD is expected to begin reaching U.S. dealers in late 2026, and pricing has not yet been announced.
What about the Sierra 1500?
This is a completely separate conversation from the redesigned 2027 Sierra 1500. The new light-duty Sierra has its own engine lineup, including new sixth-generation 5.7L and 6.6L gasoline V8s, an enhanced TurboMax and a Duramax diesel. GMC has also announced a redesigned interior and major technology changes for that truck.
The 8.3L Mongoose announcement is specifically about GM's refreshed heavy-duty trucks. So if you're comparing a Sierra 1500 with a Sierra 2500 HD or 3500 HD, don't choose based simply on which engine sounds more impressive. Start with what you actually need the truck to do.
If you are comparing the light-duty redesign with the current generation, the existing 2026-vs-2027 Sierra 1500 guide covers that separate decision.
What I'm waiting to see next
The final horsepower and torque numbers are now official. I'm still more interested in the complete truck.
I want to see the exact towing configurations, payload ratings, how the engine is packaged across Sierra HD trims, and what the ownership experience looks like once these trucks reach customers.
If you're around Clare, Mount Pleasant, CMU, Midland or elsewhere in Mid-Michigan and you're trying to decide whether a current Sierra HD makes sense or whether the 2027 is worth watching, you can compare the trucks currently available on SmartCarMatch or send me what you're planning to tow. Sometimes the engine question gets a lot easier once we start with the trailer.
— Kahleel Rider, SmartCarMatch
Sources and useful references
Final thoughts
Browse current HD inventory or use Find My Car and tell Kahleel what you are towing. The right truck starts with the job, not just the engine headline.
Use Find My Car for your HD truck search