1987-1990 Renix and 1991-2001 HO share the block but differ in head, intake, injectors, ECU, and accessory drive. HO makes 190 hp vs Renix 177 hp.
The Renix-era 4.0L (1987-1990) was a joint development between AMC and Renix, the French electronics firm. It uses peanut-port heads (smaller intake runners), a tubular cast intake manifold, batch-fire fuel injection with larger 21 lb/hr injectors, a separate ignition coil and HEI module, and multiple V-belts for accessory drive. Rated at 177 hp and 220 lb-ft. The 1991+ High Output (HO) 4.0L received a redesigned head (larger intake ports), a long-tube intake manifold, sequential multi-port fuel injection with 19 lb/hr injectors, a coil mounted on the distributor, and a serpentine belt. Rated at 190 hp and 225 lb-ft. The blocks interchange but the heads, intakes, and engine harnesses do not without extensive modification. Renix-era throttle bodies are about 60 mm; HO is 62 mm. Both engines share the same bore, stroke, deck, and rotating assembly. For builds, the HO head flows better and is the preferred upgrade for Renix engines (with careful attention to injector and ECU matching). Identifying year: Renix has a fuel-rail-mounted pressure regulator and oval valve cover; HO has a returnless or rail-mounted regulator and a more rectangular valve cover with bolted-on coil pack later.
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.