GM’s 2027 heavy-duty pickups are expected to receive new gasoline and Duramax diesel engines.
The L8S retains 6.6L, while specifications for the L6P Duramax remain unconfirmed.
’27 Silverado HD production is scheduled to begin in November, with orders opening September 17.
General Motors is preparing a significant powertrain update for its heavy-duty pickups. The existing-generation 2027 Chevrolet Silverado HD and GMC Sierra HD are expected to receive new gasoline and diesel engines.
The changes are notable because the trucks are not new-generation models, unlike the 1500 series, which is all-new for the same 2027 model year. Rather than waiting for a complete redesign, GM is expected to introduce the revised engines during the current product cycle.
On the gasoline side, the new engine carries the L8S production code and is expected to keep a 6.6L displacement. It is described as an evolution of the naturally aspirated 6.6L L8T V8 currently offered in both HD pickups. Final horsepower, torque and other technical specifications have not been confirmed.
The existing L8T produces 401 hp at 5,200 rpm and 464 lb-ft of torque at 4,000 rpm. That engine currently works with an Allison-branded 10-speed automatic transmission.
GM is also preparing a new Duramax diesel identified by the L6P code. It will replace the L5P 6.6L turbodiesel V8, although the new engine’s displacement is unknown. Reports suggest a larger displacement, but no confirmation is available.
The outgoing L5P provides 470 hp at 2,800 rpm and 975 lb-ft of torque at 1,600 rpm. It is also paired with a 10-speed automatic. Specifications for the L6P, including transmission details and its diesel exhaust fluid emissions strategy, have not been disclosed, though we expect it will cross the 1,000 lb-ft mark if only to compete with Ram for bragging rights.
Evidence of development work emerged earlier in 2026 when a camouflaged Sierra HD prototype was observed testing with bodywork largely consistent with the current production model. The vehicle was suspected of evaluating a heavy-duty gasoline powertrain, which has been linked to the L8S program.
The engine revisions will help bridge the gap to GM’s next heavy-duty truck generation. The next Silverado HD and Sierra HD are reportedly planned for the 2029 model year, with production targeted for the fourth quarter of 2028. These vehicles should use a heavy-duty adaptation of GM’s updated T1-2 architecture and its newer centralized computing platform.
Before then, 2027 Silverado HD production is scheduled to start in November 2026. Crew Cab assembly is planned for November 16, followed by Double Cab production November 23 and Regular Cab production November 30. Order banks are scheduled to open September 17.
Source: GMAuthority
