GM's 8.3-Liter Mongoose Duramax Makes 555 HP, and the Rest of the HD Truck Changed Around It

GM teased it as the Mongoose, a dig at Ford's Scorpion. On Sept. 24 it put numbers on it: the new Duramax 8.3L Turbo-Diesel V8 for the 2027 Chevrolet Silverado HD and GMC Sierra HD makes 555 hp and 1,230 lb-ft. GM calls it best in class for diesel horsepower and torque, and it is the first heavy-duty V8 the company has designed, developed and manufactured entirely in-house for its pickups.

GM's reasoning is simple market math. The company says more than 60 percent of heavy-duty customers choose a diesel, and GM's brands already account for the majority of diesel share in the segment. A new engine is how it intends to keep it.

The engine is the headline, but GM rebuilt a lot of the truck around it. Steering, brakes and drive modes all changed, and some of those changes matter as much to the lifted and off-road crowd as the extra 85 hp does.

What GM has said is inside

GM's release describes a compacted graphite iron cylinder block with a fully forged crankshaft, connecting rods and steel pistons. The air side uses a high-efficiency variable-geometry turbocharger and a compact dual-stage air-to-water charge cooling system, with what GM calls an upgraded engine-cooling strategy for sustained towing. GM also lists high-flow fuel injection, an integrated intake design, a new combustion system and crankshaft isolation hardware to cut noise and vibration.

Compacted graphite iron is the material of choice for modern high-pressure diesels because it is stronger and stiffer than gray iron, which lets engineers keep walls thinner for the same strength. Forged steel pistons, rather than aluminum, point at high cylinder pressures.

What GM has not published is just as interesting. Off Road Xtreme's running list of unknowns includes bore and stroke, compression ratio, injection pressure and details of the emissions hardware. It also notes that GM Authority reported a new Duramax with the RPO code L6P in 2027 order guides, replacing the L5P, and that the two are increasingly likely to be the same engine, though GM has not confirmed it.

The numbers against the old engine and the rivals

Off Road Xtreme puts the outgoing 6.6-liter L5P at 470 hp at 2,800 rpm and 975 lb-ft at 1,600 rpm, so the 8.3 adds 85 hp and 255 lb-ft. Diesel Army's comparison lists Ford's 6.7-liter Power Stroke High Output at 500 hp and 1,200 lb-ft and Ram's 6.7-liter High Output Cummins at 430 hp and 1,075 lb-ft.

Towing goes up too. GM rates the trucks at up to 20,000 pounds conventional and 38,000 pounds gooseneck, with a GCWR of up to 46,500 pounds, and says every diesel dually can tow 30,000 pounds or more. Diesel Army notes Ford still lists 40,000 pounds of gooseneck towing for a properly configured F-450.

"This new diesel engine is the culmination of decades of GM diesel development and integration," said Norman Peralta, GM's executive chief engineer for global engine and battery systems.

Electric steering and brake-by-wire for an HD

The chassis news is a bigger break from the past. GM's HD trucks move from hydraulic assist to rack-and-pinion electric power steering, which GM calls a segment first. GM says it has about 30 percent more output than the hydraulic gear it replaces, provides full assist regardless of engine speed, and removes the pump, hoses and fluid entirely.

Brakes get a segment-first electronic brake-boost system, larger 14.6-inch front rotors and new four-piston fixed front calipers. GM says separating steering and braking from a shared hydraulic source reduces complexity and weight.

For anyone who lifts these trucks and runs much bigger tires, those changes will matter. Heavier wheels and taller tires load the steering and brakes harder, and GM's own pitch for the new steering mentions heavy front-axle loads, rough terrain and off-road use.

Terrain Mode and a stronger gas engine

The ZR2 and AT4X get a new Terrain Mode, a first for GM's HD trucks, that enables one-pedal off-road driving in 4HI and 4LO. Lifting off the accelerator applies the brakes automatically for control on steep grades and obstacles. Off-Road mode also loosens ABS, traction and stability control to allow more wheel slip on loose surfaces.

Tow/Haul mode now tailors engine mapping, shifting, the exhaust brake, grade braking, steering assist and brake-boost tuning together, GM says, which is the payoff of putting steering and braking under electronic control.

The standard 6.6-liter gas V8 is not left behind. GM rates it at 412 hp and 472 lb-ft, with a new Turbo-Jet intake manifold, a revised lubrication system, an engine-mounted oil cooler and a digital oil-level display. Both engines use the Allison 10-speed.

"HD customers don't hold back, and neither did we," said Mark Dickens, GM's executive chief engineer for full-size trucks and SUVs.

Built in Ohio, on sale late this year

The 8.3 is built at GM's expanded DMAX plant in Brookville, Ohio, a 1.4-million-square-foot facility that machines and assembles the engine under one roof. GM previously invested $920 million to expand it for this program. The 2027 Silverado HD and Sierra HD are expected at US dealers beginning late 2026, with pricing to come.

The aftermarket will have plenty to learn. A new block material, a new turbo system and a new aftertreatment layout mean the tuning, lift and power-adder playbook for GM's HD trucks starts over. The 555 hp and 1,230 lb-ft are where that story begins.