Full Specs On New GM Duramax Diesel-What A Beast!

General Motors is escalating the heavy-duty pickup horsepower war with an all-new 8.3-liter Duramax turbo-diesel V8 producing 555 horsepower and 1,230 pound-feet of torque. We had our first in-person look at the new engine this past week at the Texas Auto Show at the State Fair of Texas. The engine will debut in the 2027 GMC Sierra HD and the 2027 Chevrolet Silverado HD, which are expected to begin arriving at U.S. dealerships late this year.

Nicknamed “Mongoose” during development, the new engine is aimed directly at Ford’s 6.7-liter Power Stroke diesel, internally known as “Scorpion.” Apparently, even diesel-engine competition now comes with its own nature documentary.

The new Duramax replaces GM’s familiar 6.6-liter L5P diesel, which produces 470 horsepower and 975 pound-feet of torque. That means the 8.3-liter engine delivers 85 additional horsepower and 255 more pound-feet of torque. Horsepower peaks at 2,500 rpm, while the full 1,230 pound-feet arrives at just 1,400 rpm—exactly where heavy-truck owners need it when launching a large trailer or climbing a steep grade.

GM says the new Duramax offers best-in-class diesel horsepower and torque, based on the latest available competitive information. For comparison, Ford’s high-output 6.7-liter Power Stroke is rated at 500 horsepower and 1,200 pound-feet of torque, while Ram’s high-output 6.7-liter Cummins inline-six produces 430 horsepower and 1,075 pound-feet.

More importantly, this is not simply an enlarged version of the existing Duramax. GM says it is the company’s first clean-sheet heavy-duty V8 diesel designed, developed and manufactured in-house specifically for heavy-duty pickups. Its foundation includes a compacted graphite iron cylinder block, a fully forged crankshaft and connecting rods, and steel pistons.

The engine uses a high-flow fuel-injection system and a high-efficiency variable-geometry turbocharger. A compact dual-stage air-to-water charge-cooling system and an upgraded engine-cooling strategy are designed to maintain performance when the truck is pulling hard for extended periods. An integrated intake, advanced combustion system and crankshaft-isolation hardware are intended to reduce diesel noise and vibration.

2027 GMC Sierra HD AT4X. CarPro.

The 8.3-liter Duramax will be paired with an Allison-branded 10-speed 10L1000 automatic transmission. GM says the engine and transmission were engineered as a complete package, including revised shift programming, stronger exhaust-braking performance and a new emissions-aftertreatment layout.

The additional output helps increase the maximum available gooseneck trailering rating from 36,000 pounds to 38,000 pounds on properly equipped models. Maximum conventional towing is rated at 20,000 pounds, while maximum gross combined weight rating reaches 46,500 pounds. Chevrolet lists a maximum available payload of 6,170 pounds for the new Silverado HD, although exact ratings will depend on cab, bed, drivetrain and equipment configuration.

The engine is only part of GM’s extensive 2027 heavy-duty update. The Silverado HD and Sierra HD will receive a segment-first rack-and-pinion electric power-steering system, replacing the current hydraulic-assisted arrangement. GM says the electric system supplies approximately 30% more steering power while reducing kickback, noise and maintenance requirements.

The trucks will also receive electronic brake boost, larger 14.6-inch front brake rotors and new four-piston fixed front calipers. Updated Tow/Haul and Off-Road modes will coordinate the engine, transmission, steering, braking and chassis settings. ZR2 and AT4X models add a Terrain mode providing one-pedal control during low-speed off-road driving.

2027 GMC Sierra HD AT4X. CarPro.

GM will manufacture the new Duramax at its expanded 1.4-million-square-foot DMAX facility in Brookville, Ohio. The automaker previously committed $920 million to expand that operation for the new engine program.

Pricing, fuel-economy estimates and complete model-by-model towing ratings have not been announced. GM says those details will be released closer to the trucks’ arrival. What is already clear is that the heavy-duty diesel contest has entered an entirely new weight class—and 1,230 pound-feet of torque should make short work of almost anything owners can legally hitch behind it.

SPECIFICATIONS AT A GLANCE

Engine: 8.3-liter Duramax turbo-diesel V8
Horsepower: 555 at 2,500 rpm
Torque: 1,230 pound-feet at 1,400 rpm
Transmission: Allison-branded 10-speed 10L1000 automatic
Maximum conventional towing: 20,000 pounds
Maximum gooseneck towing: 38,000 pounds
Maximum gross combined weight rating: 46,500 pounds
Maximum Silverado HD payload: 6,170 pounds
Engine assembly: Brookville, Ohio
Availability: Late 2026 in 2027 Chevrolet Silverado HD and GMC Sierra HD