Understanding the Holset HX35W Turbocharger

The Holset HX35W is a turbocharger originally designed for heavy-duty diesel applications produced by Holset Engineering, a Cummins subsidiary. Its reputation among automotive enthusiasts comes from robust construction, massive airflow potential, and budget-friendly pricing. Key specifications include a 60mm compressor inducer, 64mm turbine exducer, and a twin-scroll divided turbine housing (typically T3 or T4 flanged). The “W” denotes a water-cooled center section, which helps manage heat soak during high-boost street use. The HX35W is often paired with a 12cm² or 14cm² exhaust housing, both of which provide excellent top-end power while maintaining surprising spool characteristics for its size. For an Evo VII, the Holset offers a substantial upgrade over the factory TD05-16G or TD05-20G turbos, capable of supporting 400–550 wheel horsepower with the right supporting mods.

Why the Evo VII Is an Ideal Platform

Mitsubishi’s Evolution VII, built from 2001 to 2003 with the CT9A chassis, features the legendary 4G63 engine. This iron-block, DOHC, 2.0-liter four-cylinder is renowned for its strength aftermarket support and ability to handle high boost levels. The factory ECU and sensors can be remapped via standalone or flashed solutions, making the HX35W integration straightforward. The Evo’s active yaw control and all-wheel-drive system can handle the extra torque, while the engine’s oiling system already provides sufficient pressure for turbocharger lubrication. However, the stock fuel system will require upgrades when pushing past 350 wheel horsepower, making planning essential before installation.

Pre-Installation Preparation

Proper preparation prevents poor performance and costly mistakes. Gather the following parts and tools before starting.

  • Holset HX35W turbocharger – new or low-mileage rebuild; inspect shaft play and turbine wheel for cracks.
  • Manifold – aftermarket tubular or cast T3/T4 manifold designed for Evo 4–9; consider a divided manifold to match the twin-scroll housing.
  • Downpipe and dump tube – required to match the HX35’s turbine outlet (typically 3-inch or custom).
  • Oil feed line – stainless steel braided -4AN or -3AN line with restrictor (the HX35W needs a 0.060" restrictor in the feed to avoid smoking).
  • Oil return line – -10AN push-lock or braided hose with correct banjo fitting to the pan.
  • Water lines – -4AN fittings and hose to connect from engine block to turbo center section and back.
  • Intercooler and piping – upgrade to a larger front-mount intercooler with 2.5” or 3” piping to match the HX35’s compressor outlet (4-inch inlet, 2.5-inch outlet common).
  • Boost controller – electronic boost controller recommended for precise boost control.
  • Intake and filter – 4-inch aluminum intake pipe with a high-flow cone filter.
  • Gaskets and seals – manifold-to-block gasket, manifold-to-turbo gasket, downpipe gasket, crush washers for oil lines.
  • Tools: metric and SAE wrenches, torque wrench (ft-lb and in-lb), E12 torx socket, tap and die set for NPT fittings, engine hoist or transmission jack for lifting turbo.

Work in a clean, well-lit area. Drain coolant and oil prior to removing the factory turbo. Label all connectors and vacuum lines during disassembly.

Step-by-Step Installation

1. Remove the Stock Turbo and Manifold

Disconnect the negative battery terminal. Lift the vehicle and remove the under-tray. Drain the engine oil and coolant. Unbolt the downpipe from the turbo, then remove the front O2 sensor. Disconnect the oil feed and return lines, water lines, and any coolant hoses attached to the turbo. Unbolt the manifold from the head (Evo VII uses 10mm nuts; use penetrating oil if seized). Remove the turbo-manifold assembly as one piece. Clean the head surface and block oil return hole with a rag to prevent debris entry.

2. Install the Aftermarket Manifold

Install new manifold studs if required. Place the new manifold gasket (use OEM or high-quality composite) and carefully mount the manifold. Torque in two stages: first 20 ft-lb, then final to 35 ft-lb following a crossing pattern. Use anti-seize on exposed threads.

3. Prepare the Holset HX35W

Inspect the turbo for shaft play (axial and radial). Install oil restrictor in the oil feed port (usually 1/8 NPT with a 0.060” orifice). Do not skip this: the HX35W’s journal bearings don’t require high oil flow from a 4G63, and too much oil pressure causes smoke and seal failure. Apply Teflon tape on NPT fittings but avoid overlap at the pipe ends. Install water fittings and plug the unused center section port if water-cooling isn’t used.

4. Mount the Turbo to the Manifold

Attach the turbine housing to the manifold using stage-8 locking fasteners or high-temp studs. Use a new manifold-to-turbo gasket. Tighten in a cross pattern to 18–22 ft-lb. Ensure the compressor cover is oriented correctly – the HX35’s compressor outlet typically points forward or slightly downward for Evo VII fitment; you may need to clock the housing by loosening the snap ring, rotating, and reseating.

5. Connect Oil and Water Lines

Oil feed: use a -4AN braided line from the factory oil pressure switch port (or a sandwich plate) to the turbo restrictor. Oil return: -10AN line from return flange on turbo to a new bung welded into the oil pan above oil level. Water supply: tee into the heater hoses or use coolant feed from the block near cylinder 3; return to the thermostat housing. Tighten all fittings with wrenches; do not over-torque aluminum threads.

6. Install Downpipe and Dump Tube

If using a twin-scroll housing, ensure the downpipe is divided for the first 12–18 inches to maintain separation. Use a 3-inch downpipe with a flex section to reduce vibration. Attach the downpipe to the turbine housing, torque to 30 ft-lb, and secure with a support bracket. Reinstall the O2 sensor extended harness or use a new wideband bung in the downpipe.

7. Install Intercooler Piping

Run 2.5” or 3” aluminum piping from the compressor outlet to the intercooler, then to the throttle body. Use silicone couplers and T-bolt clamps. Position the intercooler so that it receives ram air. Install a blow-off valve (atmospheric or recirculated) on the hot side or cold side pipe.

8. Reassemble and Reconnect Systems

Reattach the intake pipe and filter. Connect the wastegate actuator line (if using external wastegate) or use a boost controller connected to the compressor cover. Fill the engine with fresh oil and coolant. Reconnect the battery. Double-check all connections for tightness, especially oil and water lines.

Common Installation Pitfalls to Avoid

  • Oil over-restriction or under-restriction: Using too large a feed orifice (over 0.080”) causes oil smoke and seal failure. Too small (<0.040”) leads to bearing starvation. A 0.060” restrictor is safe for most HX35W builds.
  • Compressor clocking interference: The HX35W’s compressor housing is large; clearances to the alternator, power steering lines, and engine mount are tight. Test fit the turbo before finalizing all lines.
  • Wastegate setup: The HX35W with internal wastegate has a small flap that struggles to control boost beyond 25 psi. An external wastegate (44–46mm) fitted to a twin-scroll manifold is recommended for reliability.
  • Crankcase ventilation: High boost can pressurize the crankcase; a proper catch can system is necessary to prevent oil leaks and seal damage.
  • Incorrect turbine housing A/R: A 16cm² housing spools slowly but makes big top end; 12cm² offers quicker spool but may choke at high RPM. Choose based on your power goals (12cm² for street, 14–16cm² for drag).

Post-Installation Break-In and Checks

Before the first start, prime the turbo by cranking the engine with the fuel pump fuse removed until oil pressure registers on the gauge (about 10–15 seconds). Then reinsert the fuse and start the engine. Let it idle at 900–1000 RPM for 10–15 minutes while checking for leaks. Oil weep from the compressor seal or turbine seal indicates a problem—shut down immediately. Once idle is stable, bring RPM to 2500 hold for a few minutes, then let idle again. Monitor oil pressure (should be above 10 psi at hot idle). Check water temperature and oil temperature.

After the break-in, do a thorough visual inspection of all clamps, lines, and bolts. Re-torque the manifold bolts after one heat cycle. Go for a moderate test drive, keeping boost below 15 psi. Data log AFR (target 11.5–12.0 at wide-open throttle) and knock. If all looks good, proceed to full boost runs after a professional tune.

Tuning for the HX35W

The HX35W flows significantly more air than the factory turbo. Without proper tuning, you risk detonation, overheating, or an engine-destroying lean mixture. Use a standalone engine management system such as Link G4+, AEM Infinity, or Haltech Elite 2500. If you prefer to retain the factory ECU, consider a flash-tune from a reputable Evo specialist with support for larger injectors.

  • Fuel system upgrades: 1000cc injectors and a Walbro 450LPH fuel pump are mandatory for E85 or even 93-octane above 400 hp. Surge tank setup may be needed for track use.
  • Boost control: Use a MAC solenoid valve to control external wastegate. Target linear boost curves; start with 22 psi and increase to 28–32 psi on ethanol.
  • Ignition timing: Retard timing in high-load areas to prevent knock. On pump gas, keep peak torque at 14–16 degrees BTDC; on E85, 18–20 degrees is safe.
  • Air-fuel ratios: 11.0–11.5:1 for pump gas; 11.8–12.2:1 for ethanol. Use a wideband O2 sensor (e.g., AEM or Innovate) for tuning and datalogging.
  • MAFS vs. MAP: The HX35W often causes flow limit in the factory MAF sensor. Convert to speed-density (MAP-based) tuning for increased boost range.

For reference tuning guides, check authoritative resources like Let’s Tune – Evo Tuning Guide or ECMTuning Evo Wiki.

Performance Expectations and Reliability

With proper fueling, tuning, and supporting mods (cams, valve springs, intercooler, exhaust), a Holset HX35W on an Evo VII can produce 450–500 wheel horsepower on pump gas and 550+ on race fuel or E85. Spool is moderate: expect 15 psi by 3800–4200 RPM (depending on housing size and cam selection). The HX35W is capable of 40+ psi, but factory rods begin to fatigue above 30 psi. Consider forged rods and pistons if running sustained high boost. The turbo itself is durable—many enthusiasts report 50,000–100,000 miles of street use with proper maintenance.

For more detailed data on the HX35W flow maps and dimensions, visit Holset’s official product page. For community build logs, EvolutionM.net has extensive threads documenting real-world results.

Conclusion

Installing a Holset HX35W turbocharger on your Mitsubishi Evo VII is a rewarding project that delivers significant power gains at a fraction of the cost of high-end ball-bearing units. Success hinges on meticulous preparation, correct oil feed restriction, and a professional tune. By following the steps and tips in this guide, you can achieve a smooth upgrade that provides reliable performance for street or track use. Always verify parts compatibility, invest in the proper engine management, and don’t cut corners on the fuel system. With the right approach, your Evo VII will become a formidable force—spooling hard and pulling strong to redline.