Understanding the Holset HX35 Turbocharger

The Holset HX35 is a wastegated turbocharger from the Holset HX series, designed for medium-duty diesel applications. It features a 60mm compressor inducer and a 76mm turbine exducer, with a turbine housing typically ranging from 12cm² to 16cm². However, not all HX35s are identical. The most common variants include the HX35W (with a slightly larger compressor wheel) and the HY35W (which has a smaller turbine housing and is often found on 2003-2004 Dodge Rams). For 600+ hp goals, the standard HX35 with a 60/60/12 or 60/60/14 configuration is a good starting point, though many enthusiasts prefer the 60/60/16 for better top-end flow.

  • Compressor Wheel: 60mm inducer, 76mm exducer – flows approximately 65 lb/min at 40 psi
  • Turbine Wheel: 76mm exducer; divided turbine housing options (12, 14, 16, or 18 cm²)
  • Maximum Continuous Boost: 35-40 psi (higher with upgraded wastegate and head studs)
  • Ideal Operating RPM: 2,500-3,500 engine RPM (can be improved with tuning)
  • Weight: ~35 lbs (depending on housing)

One key advantage of the HX35 is its affordability and availability. Many used units can be found for under $300 on forums like Cummins Forum or at local salvage yards. New units from reputable rebuilders like Diesel Power Products offer warranties and balanced assemblies. The HX35’s compressor map shows a broad efficiency island, making it suitable for both daily driving and high-load towing – provided the supporting mods are in place.

HX35 vs. HY35W vs. HE351

Many Dodge owners confuse the HX35 with the HY35W or the later HE351. The HY35W has a 9cm² turbine housing, which spools very quickly but chokes top-end flow, limiting it to around 450 hp. The HE351 (found on 2004.5-2007 5.9L common-rail engines) has a larger compressor but often suffers from variable-geometry issues. For 600+ hp, the HX35 with a 14-16 cm² housing offers the best balance of quick spool and high-flow capability.

Benefits of Upgrading to the HX35

Swapping to an HX35 on a 12-valve or 24-valve Dodge Cummins can transform the driving experience. Here are the primary benefits with realistic numbers:

  • Horsepower Gains: A properly tuned HX35 setup can support 550-650 hp on stock-style injectors (with aftermarket fuel system). Many dyno-proven builds achieve 580 hp with 7x0.010” injectors and 35 psi of boost.
  • Torque Increase: Expect 900-1,100 ft-lbs of torque at the wheels, especially when combined with a fuel plate or tuning. This makes the HX35 excellent for heavy towing – as long as EGTs stay controlled.
  • Throttle Response: Unlike larger turbos like the S300/S400 series, the HX35 spools relatively quickly due to its smaller turbine wheel and divided housing. On a 12-valve, boost can reach 15 psi by 1,800 RPM with a 14cm² housing.
  • Fuel Efficiency: When not pushing hard, the HX35’s efficient compressor means lower drive pressure and better fuel economy compared to stock turbos. Many owners report a 1-2 MPG improvement under moderate loads.
  • Cost-Effective: Total cost for a used HX35 rebuild kit, new seals, and billet wheel upgrade can be under $600 – considerably less than a new aftermarket turbo.

Selection Criteria for Your 600+ HP Build

Not all HX35s are created equal. Choosing the right core and configuration is critical. Consider the following factors:

  • Engine Version: 12-valve (P-pump or VE pump) vs. 24-valve (VP44 or common-rail). The fuel system dictates how much air you need. 12-valves typically require larger injectors and more boost to reach 600 hp.
  • Target Horsepower: For 550-600 hp, a stock HX35 with a 60mm compressor and 14-16 cm² housing works. For 600-650 hp, consider a billet compressor wheel upgrade (e.g., 62mm inducer) and a larger turbine housing (16-18 cm²).
  • Transmission and Drivetrain: At 600 hp, the stock 47RE automatic or NV4500 manual will need upgrades. Include a built transmission with billet input shaft, upgraded torque converter, and possibly a triple-disc clutch.
  • Intended Use: Daily driving and towing favor the 12-14 cm² housing for fast spool; street/strip or pulling favor the 16 cm² housing for top-end airflow without over-speeding the turbo.
  • Oiling: HX35s require a pressurized oil feed (1/8” NPT) and a -10 AN drain. Ensure your engine block can supply adequate oil – if running high idle for extended periods, consider an auxiliary oil cooler.

Tip: When buying a used HX35, inspect the compressor wheel for nicks, check for shaft play, and look for oil coking inside the turbine housing. A Holset factory rebuild kit includes thrust washers, seals, and bearings for a reliable refresh.

Required Supporting Modifications for 600+ HP

Reaching 600+ horsepower with an HX35 requires more than just the turbo. Here’s a detailed list of supporting modifications:

Fuel System Upgrades

  • Fuel Injectors: For 600 hp on a 12-valve, use 7x0.010” or 5x0.012” injectors (approx. 200-250 hp over stock). On a 24-valve common-rail, 120-150% over injectors with a high-pressure lift pump (e.g., AirDog or FASS) are needed.
  • Fuel Pump: A minimum of a 165 GPH fuel pump is recommended for mechanical injection systems. For VP44 trucks, a draw-straw kit and a FASS diesel lift pump prevent cavitation.
  • ECU Tuning: On common-rail engines, custom tuning (EFI Live or Smarty) is mandatory. For VP44 trucks, a timing spacer or 175-200 hp injectors with a Smarty POD change fueling.

Air Intake and Intercooler

  • Cold Air Intake: A high-flow filter (S&B, AFE) with a 4-inch intake tube reduces restriction.
  • Intercooler: The stock intercooler on 24-valve trucks is adequate for 500 hp but becomes a bottleneck at 600 hp. Upgrade to a large Mishimoto or Banks intercooler with 3-inch charge pipes.
  • Charge Air Cooler (CAC) Boots: Replace factory rubber boots with silicone hoses and T-bolt clamps to avoid blow-offs under high boost.

Exhaust System

  • Downpipe: A 4-inch downpipe is essential to minimize backpressure. The HX35’s turbine outlet is 3 inches, so a 3-to-4 inch adapter is needed.
  • Muffler and Tailpipe: Go with a full 4-inch straight exhaust for minimal restriction. Some states require a muffler – choose a high-flow diesel muffler (e.g., Donaldson or MBRP).
  • Exhaust Manifold: Stock manifolds flow well up to 600 hp, but aftermarket options (e.g., Steed Speed) improve spool and reduce EGTs.

Engine Internals

  • Head Studs: ARP 2000 or 425 studs are mandatory for 600+ hp to prevent cylinder head lift.
  • Valve Springs: Upgraded springs (e.g., Hamilton) prevent valve float at high RPM.
  • Pistons and Rods: Stock pistons in the 5.9L can survive 600 hp with proper tuning, but forged pistons and rods are recommended for reliability.

Cooling and Drivetrain

  • Transmission: A built automatic with a triple-disc torque converter and heavy-duty clutches is needed. For manuals, a South Bend dual-disc clutch handles the torque.
  • EGT Monitoring: Install an EGT gauge with a probe in the manifold before the turbo. Keep sustained EGTs below 1,300°F and peak under 1,500°F during pulls.
  • Boost Controller: A manual or electronic boost controller helps control boost spikes and set desired levels.

Installation Process – Step by Step

Proper installation is critical for reliability. Follow these steps:

  1. Prepare the Engine: Disconnect the battery, drain coolant, and remove the intake, exhaust manifold, and oil lines. Clean the mounting surface on the manifold.
  2. Remove the Old Turbo: Unbolt the oil feed line, drain line, and support bracket. Carefully lift the turbo off the manifold.
  3. Inspect and Modify the HX35: Clock the compressor housing if needed for your intake piping. Ensure the wastegate actuator is functional – many HX35s come with a weak actuator; upgrade to a Tial 38mm external wastegate for precise boost control at 40+ psi.
  4. Install New Gaskets: Use a new manifold gasket (Fel-Pro or OEM) and a new turbo gasket. Apply anti-seize on the bolts.
  5. Mount the Turbo: Lower the HX35 onto the manifold studs. Tighten the manifold nuts to 30 ft-lbs. Install the oil feed line with a restrictor if using high oil pressure (recommended 1/8" restrictor for street use).
  6. Connect Oil Drain: The drain should be a -10 AN line with a gravity drain to the pan. Ensure it runs downward without loops – kinks will cause seal failure.
  7. Install Intake and Exhaust: Connect the 4-inch intake pipe with a silicone coupler. Install the downpipe and charge piping.
  8. Final Checks: Prime the turbo by pouring a small amount of oil into the feed port. Crank the engine without starting to build oil pressure before the first fire-up. Check for leaks.

Tuning Your HX35 Setup for 600+ HP

Tuning is where the magic happens. A well-tuned HX35 build will be responsive, cool, and powerful. Key parameters:

  • Boost Targets: Aim for 35-40 psi for 600 hp. Use a boost controller to set an 18 psi wastegate spring and then add 5-10 psi through a controller.
  • Fuel Timing and Duration: On a P-pump, advance timing to 22-25 degrees and adjust fuel plate to deliver more fuel at lower RPM. On common-rail, request a tuner to extend injection pulse width while monitoring EGTs.
  • EGT Management: Keep EGTs below 1,350°F for sustained boost. If they exceed 1,450°F, reduce fueling or increase boost. A water-methanol injection kit can help cool intake temps and gain 20-30 hp safely.
  • Drive Pressure: The HX35 has a turbine housing that can create high drive pressure at high boost. If drive pressure exceeds boost by more than 2:1, consider a larger turbine housing or a twin-turbo setup.
  • Data Logging: Use a device like the FlashScan V2 (for common-rail) to log boost, EGT, RPM, and fuel rail pressure. Adjust fuel maps in 1° increments.

Common Issues and Troubleshooting

Even with a good setup, problems can arise. Here’s how to diagnose and fix them:

  • No Boost / Low Boost: Check for boost leaks in charge pipes, couplers, and intercooler. Test the wastegate actuator with compressed air – it should begin opening at 10-12 psi. Replace if stuck.
  • High EGTs: Usually caused by excessive fuel relative to boost. Increase boost pressure, verify injector size, and check for downpipe restriction (e.g., crushed pipe).
  • Oil in Intake or Exhaust: Seal failure occurs from high crankcase pressure, improper drain line slope, or excessive shaft play. Replace seals and ensure the drain is -10 AN or larger with a maximum vertical drop of 12 inches.
  • Turbo Surge: Fluttering or chirping noises under heavy load. This happens when the compressor is forced into the surge line. Solutions: use a larger compressor cover, recirculate to intake, or run a dual setup.
  • Wastegate Creep: Boost continues to rise uncontrollably often due to a small housing. Upgrade to a 16-18 cm² housing or add a boost controller with a dump pipe.

Conclusion

The Holset HX35 turbocharger remains a proven choice for Dodge Cummins owners chasing 600+ horsepower without breaking the bank. Its availability, simplicity, and strong performance make it ideal for both weekend racers and daily-driven monsters. Success lies in selecting the right HX35 variant for your power goals, installing it with careful attention to oiling and piping, and tuning fuel delivery and boost pressure to keep EGTs in check. While the HX35 has limits – above 650 hp you’ll need twin turbos or a larger single – it can deliver reliable, hard-hitting power for years with proper maintenance. Assemble your supporting mods, take your time with the install, and dial in the tune. Your Cummins will thank you with every full-throttle pull.