HO Upgrade for Renix (Manifold/Injector/PCM Swap)
Convert a 87-90 Renix 4.0 to 91+ HO spec: intake, injectors, exhaust manifold, PCM, harness. Real 30+ HP gain on the same short block.
The Renix 4.0 (1987-1990) makes 173 hp / 220 lb-ft. The HO 4.0 (1991+) makes 190 hp / 225 lb-ft using the same block. The difference is entirely upper-end:
- HO head with revised port shapes
- HO intake manifold (better runners, larger plenum)
- HO exhaust manifold
- 21 lb/hr injectors vs 19
- HO PCM with revised fuel/timing maps
- HO ignition (coil-on-trigger with distributor)
The full conversion requires: HO head, intake, exhaust, fuel rail and injectors, PCM, harness adapter (the C-101 location changes between Renix and HO), and ideally an HO distributor. Donor parts from a 91-95 XJ (best year range — same OBD-I architecture) are commonly $200-500 in a junkyard pull.
The upgrade is well-documented but labor-intensive. Most Renix owners stop short of full conversion and do HO head + injectors + a TBI upgrade, which captures most of the gain.
Alternative path: full engine swap. A 91+ HO long block bolts in with the Renix bell housing and accessories, reducing the wiring complexity considerably. Pick your battle.
Why it works
- 20+ HP free
- modern engine management
- more parts availability
Trade-offs
- wiring complexity
- harness changes
- labor-heavy
Tools required
- engine R&R tools
- wiring tools
- diagnostic scanner
Parts
| Part | Vendor | Est. price |
|---|---|---|
| HO head (used 7120 or 0630) | junkyard | ~$200 |
| HO intake manifold (91-98) | junkyard | ~$100 |
| HO fuel rail and injectors | junkyard | ~$120 |
| HO PCM 91-95 | junkyard | ~$100 |
Sources
Related
Written and maintained by an AZ wheeler and driveway wrencher. How we source guides. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.