Back at the turn of the century, Ford and GM trucks were separated by philosophy. GM's approach was to build massive, road-going juggernauts that could drive over small planets without slowing down. Ford specialized more in small, user-friendly haulers that were effectively Model T cars with boxes on the back. The Ranger was the latest in a long line of such consumer haulers -- unlike the Johnny-come-lately S-10 from GM.
As of 2014, nobody makes a true cold-air intake for this generation of 4.0-liter Ranger. A few companies make higher-flowing intakes -- technically for the Ranger-based Explorer -- with cone-style air filters, but those aren't true CAIs in the sense that they draw air from the outside. For that, you're going to need to do a bit of fabrication.
On an Explorer 4.0-liter, a good intake is worth 5 to 7 horsepower, provided that you replace the mass-airflow sensor along with it. The MAF is easily the biggest cork in the intake system, so consider an aftermarket MAF a requirement if you're going through the trouble of building a CAI. As for the CAI itself: You'll pick up about 1 percent more power for every 11-degree drop in intake air temperature, which can make a noticeable difference, depending on how hot the air under your hood is. If you take the time to build a heat shield or box to draw air in from a scoop or the fender well, expect a 3 to 5 percent increase in power -- that's another 5 to 8 horsepower, for a total of 10 to 15 horsepower for a true CAI and new MAF sensor combo.
