In the process of reviving the 4.0L I-6 engine in our '97 Jeep Cherokee, we'll address some of the main elements that make it run: air, fuel, and spark. Spark, of course, was covered in the Aug. '07 4WD&SU with the installation of fresh spark plugs and wires along with a new distributor cap and rotor and coil. Fuel will be addressed in an upcoming issue. In this article, we'll replace the factory air-intake system with a new intake tube and open-element filter from K&N.
With the K&N FIPK Gen II system, the air filter is positioned behind a K&N heat shield, allowing it to draw more air than the constrictive factory airbox. Also, the system is designed to reduce intake restriction by smoothing and straightening the airflow path from the filter to the throttle body. This permits the engine to take in a greater volume of air than the OEM-style filter and results in more power and acceleration throughout the engine's rpm range.
K&N indicated that we could expect to gain almost 8 hp through installation of the Cherokee FIPK Gen II system, but this figure will vary from one vehicle to the next. The 4.0L engine in our Cherokee has very high mileage and is a bit tired so we may not have achieved this much, but a definite seat-of-our-pants improvement was detected. Improved throttle response is one noticeable gain, and fuel mileage also showed slight improvement. A smoother idle was also achieved and was especially noted in stop-and-go traffic where the stock system acted deprived of airflow.
More Air, Not More DirtWhen converting to an open-element-style air filter, one concern is that efficient particle filtration will be lost. However, the K&N filter is said to do an excellent job of keeping dirt out and letting air in. K&N testing has revealed that its air filters have an overall efficiency rating of between 97 and 99 percent. The filters feature a molded, pliable, rubber flange to absorb vibration and create a secure attachment to the air tube. Within the flange and top cap are multiple layers of white cotton gauze and wire mesh, which make up the filter. When oiled, the gauze shows a pinkish color since the oil has a red dye added to indicate how much has been applied. If the filter looks pinkish-red, it is oiled. Once this color fades, it's time for a cleaning.
The K&N FIPK Gen II air-intake system is shown installed on the 4.0L I-6 engine in our '97 Jeep Cherokee. K&N manufactures a variety of air-intake systems and open-element-style air filters to accommodate many vehicle applications. | 
The stock air-intake system was adequate, but we wanted an air filter that we could pull and clean after dusty trips. The K&N system would also increase throttle response through the rpm range. | 
We removed the stock intake tube and upper airbox assembly. The lower airbox will also be removed. |