Performance: The GMC Sierra Denali and AT4 both offer a range of powerful engine options. The Denali comes standard with a 6.2-liter V8 engine that produces 420 horsepower and 460 lb-ft of torque. The AT4, on the other hand, comes standard with a 5.3-liter V8 engine that produces 355 horsepower and 383 lb-ft of torque.