* 3.4L V6: This was the standard engine, producing 180 horsepower and 210 lb-ft of torque.
* 3.8L V6: This was a more powerful option, producing 200 horsepower and 220 lb-ft of torque.
So, depending on the specific trim level and options chosen, the base engine size for a 2004 Chevrolet Impala could be either 3.4L or 3.8L.