* 6.0L Vortec V8: This was the standard engine option and produced 290 horsepower and 360 lb-ft of torque.
* 8.1L Vortec V8: This was an optional engine that offered more power, producing 340 horsepower and 455 lb-ft of torque.
So, the base engine size depends on which engine you're referring to. The 6.0L Vortec V8 was the standard engine and therefore the base engine option for the 2001 GMC Sierra 2500HD.