Test driving the new Cummins X15 engines Not content with one 15-liter big bore engine offering, Cummins recently unveiled two iterations of its 2017 X15 engine lineup—the X15 Efficiency Series and the X15 Performance series. The X15 Efficiency Series will be the engine of choice for the majority of line-haul and regional fleets. It offers a range of 400 to 500 Horsepower, with 1,850 lb. ft. of peak torque available at 1,000 RPM. The Performance Series packs a range of 485 to 605 horsepower with peak torque of 2,050 lb. ft for applications like heavy-haul vehicles that need the added power to get the load rolling. At the Transportation Research Center in East Liberty, Ohio, I talked with Tim Proctor, technical leader of heavy-duty engineering for Cummins, about the differences between the two engines and what fleets can expect from the new offering.