Understanding the HKS TurboKit

The Mitsubishi Eclipse GSX (1990–1999) remains a cult favorite in the DSM (Diamond Star Motors) community, prized for its all-wheel-drive chassis and bulletproof 4G63 engine. While the stock TD05-14B turbocharger offers a decent kick, serious enthusiasts know that a factory-fresh turbo simply won't deliver the 400 hp level of power without a comprehensive upgrade. Enter the HKS TurboKit. HKS has long been a trusted name in Japanese performance, and their bolt-on turbo systems are engineered to complement the 4G63’s strengths without the guesswork of piecing together a custom setup.

The specific HKS kit for the Eclipse GSX typically includes a HKS GT3040 turbocharger (or similar model), a high-capacity intercooler core, a remote-mount wastegate, a blow-off valve, and all necessary cast piping and hardware. Unlike a budget eBay “kit” that may use thin piping and poor flanges, HKS components are precision-manufactured to withstand high boost levels—often up to 25–28 psi with proper tuning. The kit’s design is semi-direct bolt-on; while some modification of the stock intercooler piping or relocation of the battery tray may be required, the overall fitment is far cleaner than a universal turbo swap.

The HKS TurboKit is particularly valued for its balanced spool characteristics. The GT3040 variant can reach full boost around 3500–3800 rpm on a 2.0L 4G63, giving you a usable powerband for daily driving or road course use, while still supporting airflow for well over 400 wheel horsepower. This makes it a far more practical choice than a massive T4 frame turbo that would suffer lag until 5000+ rpm.

Installation Process

Installing an HKS TurboKit on your Eclipse GSX demands mechanical experience, patience, and the right tools. Plan for a weekend’s worth of labor if you are experienced; beginners should budget three to four days. Below is an expanded step-by-step guide that covers what the original article left out: torque specs, gasket selection, and pro tips.

1. Preparation and Tools

You’ll need a complete socket set (10mm to 19mm), wrenches, a torque wrench, a set of pry bars, and a jack or lift. Crucially, pick up a new oil drain gasket, OEM exhaust manifold gaskets, copper spray for reinforced gaskets, and a set of ARP exhaust manifold studs (the stock studs often snap). Also have on hand high-temp RTV silicone, anti-seize compound, and a vacuum/boost gauge to verify function.

2. Removing the Stock Turbo System

Start by disconnecting the battery and draining the coolant. Remove the air intake duct, stock intercooler piping, and the entire stock intercooler. Unbolt the O2 housing from the turbo, then the turbo from the exhaust manifold. The 4G63’s stock manifold is notorious for warping; check the surface with a straightedge. If it is not flat, you will lose boost. Replace with a quality aftermarket manifold if necessary.

Critical step: Apply penetrating oil to the oil feed line and return line bolts a day before disassembly to avoid rounding them. When removing the stock turbo’s oil return line, carefully clean the baffling inside the oil pan to prevent debris from entering the oil system.

3. Installing the HKS Turbocharger

Mount the new HKS turbo to the manifold using a steel-reinforced gasket and ARP studs torqued to 35 ft-lb. Connect the oil feed from the head (use a -4AN line if the kit provides it) and the oil return to the pan using the supplied gasket. The HKS water lines should be routed to the stock coolant ports; bleed the system slowly after refilling to avoid air pockets.

4. Mounting the Intercooler and Piping

The HKS intercooler is designed to fit behind the stock front bumper support, but you may need to trim the bumper reinforcement slightly. Use the provided silicone couplers and T-bolt clamps. Every joint should be tightened to 4–5 Nm—overtightening T-bolts can cut silicone. Perform a boost leak test at 20 psi using a DIY PVC tester before moving on.

5. Installing the Wastegate and Blow-off Valve

Mount the HKS external wastegate on the turbine housing’s designated flange. Run the boost reference line to the wastegate actuator using 4mm silicone hose. The blow-off valve (HKS SSQV is common) goes on the cold-side intercooler pipe. Adjust its spring tension to prevent compressor surge at low throttle.

6. Final Checks and Initial Start-Up

Inspect all hoses for chafing, verify the turbo rotates freely by hand before start-up, and prime the turbo oil system by disconnecting the ignition and cranking for 10 seconds. After starting, check for exhaust leaks and boost leaks immediately. Run the engine at idle until coolant temps stabilize, then drive gently for the first 50 miles to seat seals before any boost.

Supporting Modifications Required for 400 hp

The HKS TurboKit alone will not get you to 400 hp; you must address the car’s fuel system, engine management, and drivetrain. Failure to do so results in knock, melted pistons, or gearbox failure.

  • Fuel System: Replace the fuel pump with a Walbro 255 lph unit and install 750–1000 cc injectors. Use a fuel pressure regulator set to 43 psi base pressure. The stock fuel lines are adequate for 400 hp but consider upgrading to -6AN lines for safety.
  • Engine Management: The factory ECU cannot handle the airflow, so you need a standalone or flashable management. ECMLink V3 is the gold standard for DSM—it lets you tune fuel, timing, boost control, and includes knock detection. Alternatively, AEM EMS Series 2 is popular. Plan for a professional dyno tune costing $400–$700.
  • Exhaust System: The stock 2.25-inch exhaust is a bottleneck. Install a full 3-inch turbo-back exhaust with a high-flow catalytic converter (or test pipe) and a quality muffler. The HKS Hi-Power cat-back is a great match.
  • Intake and Intercooler Pipes: Use a 3-inch intake pipe with a cone filter. The HKS intercooler piping is included, but upgrade the intercooler to a thicker core if you plan to push beyond 400 hp
  • Clutch and Flywheel: The stock clutch slips around 300 hp. Budget $600–$1,000 for a ACT 2600 or South Bend Stage 3 clutch kit with a lightweight flywheel.
  • Cooling System: A Mishi 3-row aluminum radiator and lower-temp thermostat are recommended to manage increased heat.

Tuning for 400 HP

With the HKS kit and supporting mods installed, your GSX should be dyno-tuned. Target AFRs of 11.2–11.5:1 under full boost (22–24 psi) and timing around 18–20° peak. The HKS GT3040 flows enough air for 400–450 bhp, but boost creep can occur if the wastegate port is inadequate. Many owners opt for a GrimmSpeed boost controller to dial in consistent boost.

Use ECMLink V3 or AEM EMS to tune in speed density, removing the MAF, which is essential for high-boost setups. A wideband O2 sensor (AEM or Innovate) must be installed. On a dyno, expect 340–370 wheel hp at 22 psi on 93 octane, and 390–420 wheel hp on E85 with larger injectors. E85 is the cheat code for 400 hp, as it cools the intake charge drastically.

Cost Breakdown – Total Estimate

Below is an itemized budget for achieving a reliable 400 hp with the HKS kit. Prices are current as of 2025 and may vary.

  • HKS TurboKit (GT3040 + intercooler + piping): $3,200 – $4,200
  • Fuel system (pump, injectors, FPR, lines): $600 – $1,000
  • Clutch and flywheel: $700 – $1,200
  • Exhaust (turbo-back, 3-inch): $500 – $900
  • Engine management (ECMLink + cable): $650 – $770
  • Professional dyno tuning: $400 – $700
  • Additional parts (gaskets, hoses, ARP studs, fluids): $200 – $400
  • Installation labor (if not DIY): $800 – $1,600

Total range: $7,050 – $10,770. This is the cost for a properly built 400 hp setup. Cutting corners on the fuel system or the clutch will lead to failure; invest once.

Performance Expectations & Reliability

After tuning, your Eclipse GSX will run low-12 to high-11 seconds in the quarter-mile with good traction. On the street, the HKS GT3040 spools faster than the stock turbo, giving excellent mid-range pull from 3500 rpm to redline. The power delivery is linear enough for daily commuting, yet potent enough for track days.

Reliability requires regular oil changes (5W-40 full synthetic every 3,000 miles) and a 2-step colder spark plug (NGK BPR7ES gapped to 0.022”). Watch for crankwalk — while rare on 6-bolt engines, a high-capacity oil pan and proper beaking procedures minimize risks. Also, a boost gauge and wideband are mandatory; don't drive without them.

For more community knowledge, visit DSM Tuners and check their “420A to 4G63 swap” and HKS kit threads. A real-world build log from this thread shows a 1997 GSX hitting 398 whp on 91 octane and 23 psi.

Conclusion

Achieving 400 hp with the HKS TurboKit on your Mitsubishi Eclipse GSX is not just possible—it is a well-documented path that yields exhilarating performance without the headaches of a full custom turbo setup. By following the detailed installation process, investing in the supporting modifications, and committing to professional tuning, your GSX will become a reliable 400 hp all-wheel-drive beast. The cost may be significant, but the result is a car that can hang with modern sports cars while still turning heads with its classic 90s DNA.