Replaces the restrictive factory airbox with a conical filter on a short metal tube. Most accessible 4.0L bolt-on, but real gains on a stock engine are modest (often 3-5 whp).
The factory airbox on the 4.0L is actually fairly well designed for the application, drawing relatively cool air from the inner fender. A short ram intake substitutes a conical filter mounted downstream of the throttle body or MAF/IAT. The K&N 57-series, Spectre 9908, and a host of DIY 3" mandrel-bend builds all share the same architecture. Expect a louder induction note and a small bump in top-end response.
On a stock HO 4.0L (190hp/225 lb-ft) the dyno gain is small — typically 3-7 whp at the top of the rev range with no meaningful change in low-end torque. The engine is volumetrically limited by the head and cam, not the airbox. The mod is more meaningful on a stroker (4.6/4.7) where airflow demand actually exceeds the stock box.
The primary tradeoff is hot air ingestion. With the filter sitting over the valve cover, IAT readings climb significantly at idle and in stop-and-go, which the PCM will compensate for by pulling timing. Many builders shield the filter or duct cold air to it (see fender intake entry). For Renix-era trucks (1987-1990) the MAF location can complicate the swap.
Verdict: cheap, manageable, sounds good. Don't expect a transformation on a stock engine.
| Part | Vendor | Est. price |
|---|---|---|
| K&N 57-1513 FIPK | K&N | ~$320 |
| Spectre 9908 intake kit | Spectre | ~$130 |
| AFE Stage 2 Pro Dry S | AFE | ~$290 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.