The MAP (manifold absolute pressure) adjuster was the only part we put on that was not going to improve fuel economy. It provides adjustability of the fuel curve, therefore allowing you to run the engine richer or leaner. The MAP adjuster plugs in to the MAP sensor's harness and has a 12-volt wire which needs to be hooked up to a switched 12-volt power source. Once that is done, you can use a voltmeter to watch the voltage reading as you adjust the fuel curve with the turn of a screw built onto the unit. We curved our unit to give us a 5-volt reading from the MAP sensor. Cherokees are set to run lean from the factory, and we were going to increase spark efficiency with the Performance Distributors ignition kit, and therefore only saw the potential for a more complete combustion.