Open Today Sales: 9 AM-4 PM
The 2027 GMC Sierra HD introduces a new Duramax 8.3L Turbo-Diesel V8 producing 555 hp and 1,230 lb-ft of torque. That torque figure marks a significant jump from the 2026 Sierra HD, which topped out at a lower capacity and towing ceiling. This page breaks down both engine options, which trims carry them, and what shoppers can expect from the upcoming Sierra HD at Greg Hubler Buick GMC.
The 2027 Sierra HD arrives with two engine paths. The base powertrain is the updated 6.6L V8 gas engine, paired with an Allison 10-speed automatic transmission across every trim from Pro through Denali. The headline addition is the new available Duramax 8.3L Turbo-Diesel V8, also paired with the Allison 10-speed automatic, which brings the truck’s best-in-class available diesel horsepower and torque numbers to the heavy-duty segment.
Both engines represent the most capable Sierra HD powertrains GMC has produced. The diesel’s 1,230 lb-ft of torque enables a maximum available towing capacity of 38,000 lbs, making the 2027 Sierra HD the strongest Sierra HD ever built. For buyers in Marion and the surrounding Indiana region who haul livestock trailers, fifth-wheel RVs, or heavy equipment, these numbers carry real operational weight.
The 6.6L V8 gas engine is the standard powertrain across the Sierra HD lineup. It pairs with the Allison 10-speed automatic transmission, a gearbox known in the heavy-duty segment for its durability under sustained load. The gas engine suits buyers who prioritize straightforward fueling, lower upfront cost, and strong everyday performance without diesel maintenance considerations.
The new 8.3L displacement marks a larger turbo-diesel than any previous Sierra HD offering. At 555 hp and 1,230 lb-ft of torque, this engine is engineered specifically for maximum pulling strength. Diesel torque characteristics, which produce peak output at lower RPM compared to gasoline engines, translate directly into confident towing from a dead stop, a critical factor when pulling a loaded gooseneck or a heavy flatbed up an incline. The Allison 10-speed automatic manages power delivery across all ten ratios to keep the engine in its most efficient torque band throughout the pull.
The 8.3L Duramax also works in tandem with the segment-first eBoost electronic brake boost system, designed to reduce stopping distances while trailering and provide a firm, predictable brake response on descents. The segment-first electric rack-and-pinion power steering system complements the engine upgrade by giving drivers precise control at higher gross vehicle weights.
Not every Sierra HD trim carries both engines. Here is how availability breaks down across the seven trim levels:
Pro runs the 6.6L V8 gas engine with the Allison 10-speed automatic. It comes standard with Adaptive Cruise Control with trailering capability, a Trailer Brake Controller, and a full Trailering Package. The Pro is the starting point for serious work use.
SLE adds the 13.4-inch diagonal Premium GMC Infotainment System and a 12.3-inch diagonal digital Driver Information Center, while retaining the 6.6L V8 gas powertrain.
SLT steps up interior content with available heated and ventilated front bucket seats and unlocks the ProGrade Trailering System with In-Vehicle Trailering App. The 6.6L V8 gas engine powers this trim.
AT4 adds off-road hardware, including an Off-Road Suspension Package with twin-tube shocks and skid plates, exclusive 20-inch Gloss Black wheels, and available BFGoodrich All-Terrain tires. The 6.6L V8 gas engine is paired with these trail-capable credentials.
AT4X debuts Terrain Mode on the Sierra HD for the first time. Terrain Mode adjusts shift mapping, traction control, and adds automatic braking to maximize low-speed control on uneven surfaces. Multimatic DSSV dampers and a factory 1.5-inch suspension lift round out the package. The AT4X runs the 6.6L V8 gas engine.
Denali brings luxury finishes, 12-way heated and ventilated power front seats, Denali-exclusive wood and patterned aluminum trim, and 20-inch Ultra-Bright machined aluminum wheels. The 6.6L V8 gas engine carries this trim.
Denali Ultimate is the only trim confirmed to include the new Duramax 8.3L Turbo-Diesel V8 with the Allison 10-speed automatic as a featured powertrain. It pairs maximum diesel output with the most premium interior in the Sierra HD lineup, including full-grain leather seating, authentic Paldao wood trim, and 16-way power front seats with a massage feature.
The practical split between these engines is straightforward. The 6.6L V8 gas engine serves drivers who need a reliable, capable heavy-duty truck for consistent work without specializing in maximum-load towing. Six-passenger seating fits crews in every configuration.
The 8.3L Duramax diesel answers a different question: what is the most the Sierra HD can pull? At 38,000 lbs of available max towing, the answer covers virtually every trailer application a commercial operator or serious private hauler will encounter. For someone moving horses in a large slant-load trailer, hauling a triple-car transport, or pulling a fully loaded fifth-wheel through Indiana’s mix of flat highways and rolling terrain near Marion, the diesel’s torque floor at low RPM reduces strain and improves control.
The ProGrade Trailering System, available on SLT and above, pairs with either engine and adds an In-Vehicle Trailering App to help drivers monitor trailer conditions, manage tongue weight data, and access up to 14 camera views. That system is most valuable when the engine is pulling at or near its rated capacity.
GMC positions the 2027 Sierra HD’s diesel output as best-in-class available diesel horsepower and torque in the heavy-duty truck segment. The 555 hp and 1,230 lb-ft figures from the 8.3L Duramax exceed what rivals offer in their comparable diesel configurations, and the 38,000-lb towing ceiling reflects that advantage directly.
The segment-first eBoost electronic brake boost system and segment-first electric rack-and-pinion power steering are also powertrain-adjacent innovations. Stronger engines generate higher braking demands under load, and eBoost was engineered specifically to match the capability the new diesel makes possible. Competitors in the heavy-duty segment do not currently offer either of these systems.
The gas engine side of the equation also receives updates for 2027, with GMC describing the 6.6L V8 as enhanced for this model year. While the primary focus of the 2027 powertrain story is the new diesel, the gas engine’s continued pairing with the Allison 10-speed automatic keeps it competitive for buyers who do not need diesel fuel infrastructure.
The 2027 GMC Sierra HD has not arrived at Greg Hubler Buick GMC yet and is expected to become available in late 2026. Shoppers interested in the new 8.3L Duramax diesel or updated 6.6L V8 gas engine can contact the dealership for release updates, ordering information, and upcoming availability. The team can also help compare the two powertrains based on towing needs, daily driving, and long-term ownership plans. In the meantime, buyers who need a heavy-duty truck sooner can browse current Sierra HD inventory.
The 2027 Sierra HD offers two engines: a standard 6.6L V8 gas engine and a new available Duramax 8.3L Turbo-Diesel V8. Both pair with an Allison 10-speed automatic transmission.
The new 8.3L Duramax Turbo-Diesel V8 produces 555 hp and 1,230 lb-ft of torque, which GMC states is best-in-class available diesel output in the heavy-duty segment.
The 2027 Sierra HD tows up to 38,000 lbs when equipped with the new Duramax 8.3L Turbo-Diesel V8.
The Denali Ultimate trim is confirmed to include the Duramax 8.3L Turbo-Diesel V8 with the Allison 10-speed automatic transmission as its featured powertrain.
The biggest addition is the new Duramax 8.3L Turbo-Diesel V8. The 2026 Sierra HD topped out at 36,000 lbs of available towing. The 2027 diesel pushes that to 38,000 lbs while adding 555 hp and 1,230 lb-ft of torque. The 6.6L V8 gas engine is also enhanced for 2027.
Yes. The eBoost electronic brake boost system is a segment-first feature designed to match the braking demands of the more powerful diesel engine, especially during downhill trailering. The electric rack-and-pinion power steering is also a segment-first system on a heavy-duty truck.
The 2027 GMC Sierra HD has not arrived yet and is expected to become available in late 2026. Greg Hubler Buick GMC in Marion can provide release updates and upcoming availability information.