"I believe that the engineers working at automotive manufacturing firms spent time in school, graduate studies and intensive (not to mention, expensive) research. So, THEY would engineer the engines that they design and make to perform at at its best. Fuel efficiency and power (torque) are taken in as primary factors in the design of the engine. THEREFORE, no fuel-saving device actually work on any of the modern engines that we have because they are now performing at optimal levels."