As of 2015, maximum torque values for Cummins ISX truck engines range from 1450 pound-feet at 1200 revolutions per minute for the ISX 400, to 2050 pound-feet at 1200 RPM for the ISX 600. ISX engines labeled "ST" feature "SmartTorque" technology, allowing for a maximum of 200 extra pound-feet of torque at the highest gears. Torque benchmarks for specific ISX engines are available for viewing at the Cummins Engines website.
"Pound-foot" is the standard unit for measuring rotational force. One pound-foot corresponds to the twisting energy exerted when 1 pound of force pushes an object forward along a 1-foot radius from a second object. Torque in an engine is generally based on RPM, which is the crankshaft's amount of revolutions per minute. Different engines begin to dip off in their torque production at different points as RPM increases to substantial levels, creating a "torque curve" that peaks at maximum torque efficiency.
The combination of torque and RPM produces horsepower, which represents the work a vehicle does to go forward. Horsepower is calculated by multiplying the torque in pound-feet by the RPM and dividing the resulting number by 5,252. Heavy-duty diesel engines such as Cummins ISX engines typically have low RPM but high torque in order to pull heavy loads across the highway, sacrificing speed for hauling power.