GM’s HD Trucks Could Get New Gas and Diesel Engines Before Their 2029 Redesign
General Motors is reportedly planning a significant powertrain refresh for the 2027 Chevrolet Silverado HD and GMC Sierra HD — two full years before the trucks are due for a complete generational overhaul. According to sources cited by GM Authority and corroborated by a dealer ordering guide, both the gas and diesel engines will be replaced for the 2027 model year, giving buyers access to updated powertrains without waiting until 2029.
The news lands at an awkward moment in GM’s product calendar. The light-duty Silverado 1500 and Sierra 1500 are arriving later this year as fully redesigned 2027 models, while their heavy-duty siblings continue on the current platform for another two years. These engine swaps are GM’s way of keeping the HD lineup competitive in the interim — and given how far the current diesel lags behind Ford and Ram on the torque charts, the timing is hard to argue with.
New RPO Codes Signal Real Engine Changes, Not Just a Rebadge
On the gas side, the current 6.6-liter L8T naturally aspirated V8 — rated at 401 horsepower and 464 lb-ft of torque for 2026 — is being replaced by a unit carrying the L8S designation. The displacement stays at 6.6 liters, but the new code signals a meaningful engineering update rather than a simple tune. Output figures haven’t been confirmed, but the competitive target is clear: Ford’s 7.3-liter “Godzilla” V8 in the Super Duty produces 430 hp and 485 lb-ft, so expect the L8S to land somewhere around 450 hp and 500 lb-ft. The iron-block construction that makes the HD gasser so well-suited to heavy towing is expected to carry over.
The diesel story is more consequential. The L5P Duramax — which debuted in the 2017 Silverado HD and Sierra HD and received a fuel-system upgrade in 2024 to reach 470 hp and 975 lb-ft — is being replaced by the L6P. The ordering guide surfaced by Jones Chevrolet in Wisconsin lists the new code but omits displacement, which has fueled speculation that GM may be upsizing the engine. Unofficial estimates from autoevolution point to a possible 8.3-liter “Megamax” configuration with outputs as high as 600 hp and 1,200 lb-ft, though GM Authority has not confirmed displacement details. What is clear is that the L6P needs to clear 1,000 lb-ft to be taken seriously — Ram’s 6.7-liter Cummins produces 1,075 lb-ft, and Ford’s 6.7-liter Power Stroke is available in tunes producing either 1,050 or 1,200 lb-ft.
GM Has Done This Before, and Buyers Benefited Then Too
Mid-cycle engine swaps are not unprecedented for GM’s HD trucks. The L5P Duramax itself arrived in 2017, three years before the trucks’ planned 2020 redesign — buyers who didn’t want to wait for new sheetmetal still got GM’s then-latest diesel technology. The same logic applies here: the 2029 redesign will bring a new platform based on GM’s updated T1-2 architecture and a next-generation centralized computing platform, but there’s no reason to hold back updated powertrains in the meantime.
Production of the 2027 Silverado HD is scheduled to begin in November, with Crew Cab models starting November 16, Double Cab on November 23, and Regular Cab on November 30. Order banks open September 17. That timeline is firm enough that buyers planning a near-term purchase have a concrete decision to make: buy a 2026 now, or hold a few months for the new engines.
For buyers who need a truck today, the 2026 Silverado HD and Sierra HD remain capable, proven workhorses — but the powertrain gap with Ford and Ram on the diesel side is real. The 975 lb-ft L5P trails both the Cummins and Power Stroke by 75 to 225 lb-ft depending on the Ford tune, which matters when you’re pulling at the upper limits of the spec sheet. If your purchase timeline is flexible, waiting for the 2027 L6P makes practical sense.
For 2026 buyers who proceed now, resale value is worth considering. Mid-cycle powertrain updates tend to soften demand for the outgoing engine variant, particularly in the HD diesel segment where buyers are spec-sheet conscious. That’s not a reason to avoid a 2026, but it’s a factor in the total-cost calculation. One open question the sources flag: the EPA’s tighter 2027 emissions standards for heavy-duty vehicles, which may be a key driver behind both new engine programs, and whether the L6P will revise its diesel exhaust fluid strategy in response to recent EPA guidance on DEF-related power derate issues.
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