This is by far the easiest modification to do on your Jeep TJ or LJ. While you only gain 8-12 HP and 1-3 mpg in fuel efficiency, it's a nice thing to have under the hood. If for no other reason, you will never have to buy another air filter again.

I went to my local Autozone, which by my house is now open 24 hours, and chose to get the K&N model number RU3610. It passes 700 cfm vice the stock 450 cfm or so. In conjunction with the vortex inline fan, throttle body spacer, metal air tube, and a cat-back exhaust system you could probably gain 40+ HP and 10mpg. That will cost upwards of $400. The filter itself is $40, which makes 8-12 HP seem like quite a value.

Installation is easy. Loosen the screw holding the tube to the throttle body. Remove the breather hose from the tube. Remove the screws holding the stock airbox to the body, and pull the whole assembly out of the engine compartment. Once removed, Remove the flexible tube from the straight air tube. Measure 7 1/2 inches from the breather port on the air tube, and cut off the rest with a sawzall or hacksaw. File the rough edges as smooth as you can, making sure to clean out the inside of the tube before installation. Discard the airbox, and flexible tube portions of the assembly. Attach the K&N filter to the tube and re-attach it to the throttle body. Reconnect the breather hose, and start her up.

After a few days, you won't even notice the whistle while you shift, which is kind of sad since it's so cool! The only thing you have to do now is put your tiny K&N sticker on your Jeep TJ somewhere.