Overview of the Holset HX35 Turbo Kit for the Jeep Cherokee XJ

The Jeep Cherokee XJ, especially its 4.0L inline-six engine, has a dedicated following of off-roaders and street enthusiasts. While the 4.0L is known for durability and torque, its factory output of around 140–160 horsepower leaves many wanting more. The Holset HX35 turbo kit has become a go-to solution for XJ owners seeking a significant power bump without breaking the bank. Originally a diesel turbocharger from Holset—a Cummins‐owned company—the HX35 is rugged, well‐proven, and surprisingly adaptable to gas engines. This article covers everything you need to know about the horsepower these kits add, the supporting mods you’ll need, and the real‑world results you can expect.

The HX35 is not a typical “small frame” turbo. It was designed for medium‑duty diesel applications like the 5.9L Cummins in Dodge Ram trucks. The key specs that make it attractive for the 4.0L XJ are its robust construction, large compressor capacity, and oil‑cooled center housing.

  • Compressor wheel: 60 mm inducer, 76 mm exducer (typically a 6+6 blade design)
  • Turbine wheel: 76 mm with a 9 cm² or 12 cm² housing depending on the variant (the 9 cm² housing spools faster)
  • Maximum boost: 30+ PSI (but the 4.0L engine’s limitations usually cap usable boost around 12–18 PSI)
  • Flow capacity: Supports up to 400–450 horsepower in diesel applications, so on a 4.0L gas engine it offers ample headroom

Because the HX35 is available cheaply (often pulled from wrecked trucks), aftermarket adapter kits and boost control parts make a custom turbo setup feasible for under $1,500—far less than a purpose‑built turbo system. However, the horsepower gains depend heavily on how you integrate it into your XJ.

Realistic Horsepower Gains: Expectations vs. Reality

The OG article gave a broad range of 250–300 HP from a stock 140 HP XJ. That’s a realistic ballpark, but the actual gain varies based on boost level, fuel system, engine health, and tuning. Let’s break it down more granularly.

Stock Engine + Basic Kit (Low Boost, 8–10 PSI)

Many XJ owners start with a mostly stock 4.0L (cast pistons, stock rods, head gasket). At 8–10 PSI and with a basic tune (often a piggyback or reflashed ECU), you can expect 200–230 horsepower at the wheels. Torque jumps to about 280–300 lb‑ft. This is safe for daily driving if you upgrade the fuel system enough to avoid lean conditions.

Stock Engine + Moderate Boost (12–15 PSI) with Supporting Mods

Once you add a larger fuel pump (Walbro 255 or similar), 36‑lb injectors, a good intercooler (air‑to‑air front mount), and a 3‑inch exhaust, you can run 12–15 PSI. That pushes output to 270–320 horsepower at the crank (about 230–270 at the wheels). Many real‑world builds fall here. The 4.0L’s stock bottom end is the limiting factor; above 15 PSI, the cast pistons are at risk of cracking or ring land failure.

Built Engine + High Boost (15–20 PSI or More)

For those who rebuild the 4.0L with forged pistons, stronger rods, and a multi‑layer steel head gasket, the HX35 can deliver 350–400+ horsepower. A few dedicated XJ forum members have hit over 500 HP with a built short block, but that typically requires a larger turbo (e.g., HX40). The HX35 is best suited for the 300–400 HP range on a built engine.

Supporting Modifications: Why They Are Non‑Negotiable

Slapping a Holset HX35 on a stock XJ and cranking up the boost will destroy the engine quickly. The 4.0L runs a compression ratio of 8.8:1, which is too high for significant boost without detonation. You must reduce compression (thicker head gasket or dished pistons) and upgrade these critical systems.

Fuel System Upgrades

  • Injectors: Stock 21‑lb/hr injectors max out at around 250 HP. Upgrade to 36‑lb/hr (Ford “blue top”) or 42‑lb/hr injectors.
  • Fuel pump: The stock in‑tank pump can’t supply enough fuel above 10 PSI. A Walbro 255 LPH pump is standard.
  • Fuel pressure regulator: Optional but helps maintain consistent pressure, especially on return‑style systems.
  • Boost reference: A rising‑rate fuel pressure regulator (RRFPR) is common for piggyback setups to increase fuel pressure under boost.

Intercooling

The HX35 compresses air, heating it significantly. Without an intercooler, intake air temperatures can exceed 250°F, causing detonation and power loss. A front‑mount intercooler (FMIC) from a universal kit (e.g., 24x12x3 inches) paired with 2.5‑inch piping is the standard upgrade. It reduces charge temps by 100–150°F and adds about 15–30 HP.

Exhaust System

The 4.0L’s restrictive exhaust manifold and downpipe must be replaced. A 3‑inch downpipe into a 3‑inch cat‑back (or straight pipe) cuts backpressure and allows the HX35 to spool faster. The stock exhaust is a major bottleneck—without upgrading it, boost response suffers and peak horsepower drops by 20–40 HP.

Engine Internals (Optional for Higher Boost)

  • Head gasket: Stock composite gasket fails above 12 PSI. Use a multi‑layer steel (MLS) gasket from Fel-Pro or Cometic.
  • Head studs: ARP head studs are essential to clamp the head down against boost pressure.
  • Pistons and rods: For over 15 PSI, swap to forged pistons (e.g., Wiseco or Ross) and upgrade the connecting rods to forged units. A budget alternative is a “stout stock” rebuild with fresh rings and bearings, but boost must be limited.

The Critical Role of Tuning

No matter what hardware you bolt on, the tune makes or breaks the build. The XJ’s stock ECU (Renix 1984–1990 or Chrysler 1991–2001) was not designed for boost. It runs a speed‑density system (no mass air flow sensor) and relies on a pre‑programmed fuel map. You have three tuning options.

Option 1: Piggyback ECU (e.g., MegaSquirt, AEM FIC)

A piggyback unit intercepts sensor signals and modifies fuel and spark curves. This is the most common approach for budget XJ turbo builds. MegaSquirt (MSPNP) is a popular standalone that replaces the factory ECU altogether. It gives full control over fuel, ignition timing, and boost control. With a piggyback, expect to pay $400–$1,000 plus tuning time.

Option 2: Reflash of Stock ECU

Some tuners can reflash the Chrysler ECU for boost, but it’s not common because the tables are locked. A $50 “tune” file from an online seller is risky. Most reputable XJ turbo builders recommend a standalone system for safety and performance.

Option 3: Speeduino or DIY Systems

For the technically inclined, Speeduino (an Arduino‑based ECU) can run the 4.0L with boost. It’s a low‑cost entry but requires significant wiring and tuning knowledge.

Proper tuning includes: dialing in air‑fuel ratio (AFR targets about 11.5‑12.0:1 under boost), adjusting ignition timing (retard 1‑2 degrees per pound of boost), and monitoring knock. Dyno tuning is optimal; street tuning with a wideband O2 sensor is possible but less precise.

Dyno‑Tested Results from Enthusiast Builds

To give you a concrete idea, here are real dyno results from XJ owners who documented their builds on forums like Jeep Cherokee XJ Enthusiasts and NAXJA.

  • Build 1 (Stock motor, 10 PSI, piggyback): 217 whp and 278 lb‑ft on a Dynojet. Boost came on at 2800 RPM, full spool by 3500 RPM. The owner used a Walbro 255, 36‑lb injectors, a front mount intercooler, and a 3” exhaust.
  • Build 2 (Stock bottom end, 12 PSI, MegaSquirt): 264 whp and 298 lb‑ft. This car had an MLS head gasket and ARP studs. The tuner stopped at 12 PSI due to fuel pump limitations (original Walbro 255 was failing).
  • Build 3 (Built 4.0L, 15 PSI): Forged pistons, Eagle rods, ported head. Made 351 whp and 376 lb‑ft on a Mustang dyno. The HX35 started to run out of steam at 5,000 RPM but the curve was flat from 3,000‑5,000 RPM. The builder later switched to an HX40 for more top end.
  • Build 4 (Moderate boost, 9 PSI, minimal mods): Stock engine except for injectors and tune. 194 whp. This demonstrates that even a mild setup yields a 50–60% power increase from stock.

These numbers are wheel horsepower. At the crank, multiply by roughly 1.2–1.25 (the XJ’s automatic transmission and transfer case sapping about 20‑25% of power). So a 264 whp build equals roughly 310‑330 crank horsepower.

Common Pitfalls and Reliability Concerns

Boosting an XJ is not a set‑and‑forget project. Even with a conservative tune, you must pay attention to several weak points.

Oil Feed and Drain

The HX35 is oil‑cooled and needs a pressurized oil feed from the engine. Many XJ builders tap the oil pressure sender port or drill the block. The oil drain must be a large (‑10 AN or larger) line that dumps above the oil level in the pan. Improper oil drain causes smoking seals and turbo failure.

Cooling System

The 4.0L already has marginal cooling for towing. Adding a turbo increases heat load significantly. Upgrade to a three‑row radiator, an electric fan, and possibly an oil cooler. Overheating is the #1 killer of turbo XJ engines.

Exhaust Manifold Cracking

Cheap turbo manifolds (from eBay or bottom‑of‑the‑barrel brands) crack easily due to heat cycles. A good manifold made from thick wall tubing (or a cast version designed for the HX35) is worth the investment. Also use a flex joint in the downpipe to relieve stress.

Ignition System

Boosted engines need a strong spark. Stock coil packs and wires are marginal. Upgrade to a high‑output coil (e.g., MSD Blaster) and lower the spark plug gap to around 0.035 inches to prevent blow‑out under boost.

Transmission

The AW4 automatic is durable, but above 300 whp it will slip on hard shifts. A shift kit or full rebuild with upgraded clutches is recommended. Manual AX‑15/NV3550 transmissions handle up to 350 HP with careful driving.

Conclusion: Is the Holset HX35 Right for Your XJ?

The Holset HX35 turbo kit is an excellent choice for Jeep Cherokee XJ owners who want a substantial, real‑world horsepower increase (200–400 HP) on a budget. It is not a plug‑and‑play solution; you’ll need to invest in supporting modifications, tuning, and attention to the engine’s internal strengths. For a daily driver who wants 250–300 HP, the HX35 with good tuning and a moderate boost level (10–12 PSI) provides a reliable and exciting upgrade. For those chasing 350+ HP, a built engine and dedicated standalone ECU are mandatory.

If you’re preparing to start your own XJ turbo build, spend time researching on forums like NAXJA or Cherokee Forum. Detailed build threads there will show you exactly what parts work and what mistakes to avoid. Also consider factory turbo resources for the 4.0L—though rare, the 4.0L engine page provides baseline specs that help when planning modifications. For tuning, a resource like DIYAutoTune (home of MegaSquirt) has guides specific to Jeep XJ turbo installations.

The HX35 kit proves you don’t need a crate engine swap or a $10,000 supercharger to wake up your XJ. With careful planning, realistic expectations, and proper upgrades, you can transform your Cherokee into a powerful, responsive machine that surprises sports cars at stoplights and crawls rocks with authority.