GM Built Two Different 6.6-Liter V8s — And They’re Not Nearly as Similar as They Sound

GM is doing something you don't see every day: offering two naturally aspirated 6.6-liter V8 engines in its truck lineup. Both have the same displacement and bore-and-stroke dimensions, but they use different designs and serve different purposes. One is new for light-duty pickups, while the other has been updated for heavy-duty work trucks.

The new engine is called the L78, and it will be available in the 2027 Chevrolet Silverado 1500 and GMC Sierra 1500. It belongs to GM’s next-generation Gen 6 Small Block family. The second engine, the L8S, is based on the older Gen 5 Small Block architecture. It will become the standard gasoline V8 for the Silverado HD and Sierra HD, replacing the outgoing L8T.

The differences start with the engine blocks. The L78 uses cast aluminum with cast-iron cylinder liners, while the L8S has a cast-iron block with nodular-iron main caps. The heavier construction of the L8S fits its role in trucks built for tougher jobs. GM has also given the two engines different fuel systems, so sharing the same displacement doesn't mean they share all their hardware.

The L78 combines direct injection with port fuel injection, a setup designed to help balance power, emissions, and efficiency. The L8S sticks with direct injection alone. It's an interesting split, especially when both engines have the same basic dimensions. Rather than use one V8 across its entire truck range, GM has taken two different approaches for two different types of pickup.

The power figures also set them apart. The L78 produces 481 hp and 501 lb-ft of torque, which GM says makes it the most powerful naturally aspirated V8 in the light-duty full-size truck class. The L8S delivers 412 hp and 472 lb-ft. That's an increase of 11 hp and 8 lb-ft over the L8T it replaces.

GM hasn't detailed every reason behind the decision to keep two similar-sized V8s in production. Still, the differences in their construction and fuel systems show that the company hasn't simply reused the same engine for every application. The L78 gets the newer architecture and dual-injection system, while the L8S retains a cast-iron foundation for heavy-duty trucks.