The Cummins ISB and QSB engine family represents the pinnacle of mid-range diesel performance, renowned for exceptional durability and advanced technology . These engines feature a rugged cast-iron block design with forged steel crankshafts, ensuring longevity even under the most demanding conditions . Available in 4.5L, 5.9L, and 6.7L displacements, they deliver power outputs ranging from 200-385 horsepower with massive torque—up to 930 lb-ft in later 6.7L variants .