The Real Cost of a 400 HP Eclipse GSX Build

The Mitsubishi Eclipse GSX, especially the 2G (1995-1999) model, remains a legend in the tuner world. Its 4G63 engine is famously robust, capable of handling 400 horsepower with the right upgrades. But "capable" doesn't mean "cheap." Many build threads start with a dream and end with a half-disassembled car in the garage because the budget was underestimated. This guide provides a realistic, line-by-line breakdown of what it takes to build a reliable 400 HP GSX, including parts, labor, and the outcomes you can genuinely expect.

Understanding the 400 HP Target

Achieving 400 wheel horsepower (whp) or crankshaft horsepower? Most tuners target wheel horsepower because that's what the dyno measures. For a GSX, 400 whp is a sweet spot—fast enough to embarrass many modern sports cars, yet not so extreme that the car becomes unusable on the street. To get there, you need to address the engine's air, fuel, and spark delivery, strengthen the drivetrain, and ensure the cooling system can handle the extra heat. The stock 4G63 bottom end is surprisingly strong, but it does have limits.

Comprehensive Parts Breakdown

This is the core of your budget. Prices below are estimates based on current market rates (2025) and include both new and quality used parts where applicable. Always prioritize reputable brands such as MAPerformance, ExtremePSI, or STM for critical components.

Engine Internals

The stock 4G63 rods are forged from the factory and can handle around 450 hp, but the cast pistons are the weak link. For a reliable 400 whp, you can either go with a set of factory 1G or Evo pistons (which are stronger) or aftermarket forged pistons and rods if you plan to push higher later. Budget $1,500–$2,500 for a basic forged piston/rod set, new rings, bearings, and a full gasket kit. If you keep the stock rods (they are fine at this power level), you can save some money.

  • Forged pistons (e.g., Wiseco, CP): $600–$900
  • Forged rods (e.g., Manley, Eagle): $400–$700
  • Bearings, rings, gaskets: $300–$500
  • Machine work (bore, hone, deck: $400–$600

Turbocharger Upgrade

A 400 whp goal typically requires a turbo in the 50–60 lb/min range. The Garrett GT3076R, BorgWarner S256, or a Precision 5858 are popular choices. A quality turbo kit including a manifold, wastegate, and blow-off valve adds up quickly. Do not skimp here; a cheap turbo can fail catastrophically.

  • Turbo (e.g., Garrett GT3076R): $1,200–$1,800
  • Cast iron manifold (e.g., FP, Punishment Racing): $300–$600
  • External wastegate (e.g., Tial 44mm): $300–$450
  • Blow-off valve (e.g., Tial Q): $200–$300
  • Oil feed/drain lines: $100–$150

Total for turbo system: $2,100–$3,300

Fuel System

A stock fuel pump and injectors won't cut it. You'll need at least 1000cc injectors (or 750cc if you run E85 or race gas) and a high-flow fuel pump. Consider a rewired Walbro 450 or an in-tank brushless pump. A return-style fuel system with an aftermarket FPR is highly recommended for consistency.

  • Fuel pump (Walbro 450): $120–$200
  • Injectors (FIC, Injector Dynamics 1000cc): $400–$600
  • Aeromotive FPR and fittings: $200–$350
  • -6AN lines, filter, rail adapter: $250–$400

Total fuel system: $970–$1,550

Exhaust System

A free-flowing exhaust is critical for spool and top-end power. A 3-inch downpipe and cat-back are standard. The stock catalytic converter must be removed for tuning (or use a high-flow unit if you need to pass emissions).

  • 3-inch downpipe: $150–$350
  • 3-inch cat-back system (e.g., Thermal, Apexi): $400–$800
  • O2 housing (porting or aftermarket): $100–$200

Total exhaust: $650–$1,350

Engine Management and Tuning

The stock ECU can be flashed with a chip like a ECMLink V3 or replaced with a standalone like AEM EMS2. ECMLink is the most popular for DSM cars and offers excellent control at a reasonable price. You'll also need a wideband O2 sensor and boost control.

  • ECMLink V3 license: $545
  • ECU (core 1995 EPROM): $100–$200
  • Wideband (AEM X-Series): $200–$260
  • Manual boost controller (Hallman Pro): $60–$80
  • Tuning session on dyno (2-3 hours): $400–$800

Total management/tuning: $1,305–$1,885

Intercooler and Intake

A front-mount intercooler (FMIC) is mandatory for 400 hp. The stock sidemount will heat soak quickly. A short-route piping kit and a quality core (ETS, PR, or VRSF) will lower intake temperatures and reduce knock.

  • FMIC core (25x12x3 minimum): $250–$500
  • Short-route piping kit: $250–$400
  • Dejon or custom intake pipe + filter: $100–$200
  • Throttle body spacer or gasket: $30–$60

Total intercooler/intake: $630–$1,160

Drivetrain and Clutch

The stock AWD drivetrain is tough, but the clutch is the first thing to slip. A sprung 6-puck or twin-disc clutch is recommended. Also consider upgrading the transfer case yoke and doing the "t-case brace" for reliability.

  • Clutch (e.g., ACT, South Bend, Comp Twin): $500–$1,200
  • Flywheel (lightweight steel or billet): $200–$400
  • Shifter bushings and cables: $50–$150

Total drivetrain: $750–$1,750

Supporting Mods and Maintenance

Don't forget the basics: new timing belt, water pump, oil pump, and a full fluid flush. A 400 hp engine will also need a better radiator (Mishimoto or Koyo), and perhaps a oil cooler.

  • Timing belt kit (Gates Racing): $150–$200
  • Water pump: $60–$100
  • Oil pump (OEM only): $120–$180
  • Radiator (all-aluminum): $250–$400
  • Thermostat + hoses: $50–$100

Total supporting: $630–$980

Parts Total Summary

Adding up all the categories, you should expect to spend between $7,500 and $13,500 on parts alone. This range accounts for differences in brand, new vs. used (e.g., a used turbo or salvage engine core), and whether you buy a full kit or piece together each component. If you're patient, you can save 20-30% by scoring deals on forums like DSMTuners or Facebook groups.

Labor Costs Breakdown

Labor is where budgets often double. A professional shop will charge between $100 and $150 per hour. A full engine rebuild with turbo and fuel system installation can easily take 40–60 hours. However, you can split the work into stages to spread the cost.

DIY vs. Professional

If you have mechanical experience and a workspace, doing the work yourself can save $2,000–$4,000. But keep in mind you'll need specialized tools: a torque wrench, engine hoist, transmission jack, piston ring compressor, and a wideband or data logger for tuning. The 4G63 is relatively simple to work on, but there are many small steps that can trip up beginners (like the balance shaft delete or proper timing belt tension).

Typical Labour Hours for a 400 HP Build

  • Engine removal and disassembly: 8–12 hours
  • Machine work and reassembly (short block): 6–10 hours (if done by you; a shop may charge flat-rate)
  • Head disassembly, valve job, reassembly: 6–10 hours (if upgrading springs/retainers)
  • Turbo/exhaust/fuel system installation: 10–15 hours
  • Wiring, sensors, ECU installation: 4–6 hours
  • Dyno tuning: 3–5 hours (often included in tuning cost)
  • Test, debug, and final adjustments: 5–8 hours

If you hire a shop for all labor, expect a bill between $3,500 and $6,000. Some shops offer a package deal for a full build—ask around.

Expected Outcomes: Power, Reliability, and Daily Livability

After spending $11,000–$20,000 (parts + labor), what do you get?

Power and Acceleration

A properly tuned 400 whp GSX running on 93 octane (or E85) will trap 118–125 mph in the quarter mile and run low 11s or high 10s with a good driver and sticky tires. 0-60 times drop to around 3.5–4.0 seconds. The torque will be strong from 3,500–7,000 rpm, making the car feel ridiculously fast on the street.

Reliability

With forged internals, proper tuning, and maintenance, a 400 hp GSX can be a reliable daily driver. However, the weak links become the transmission (the stock 5-speed can handle it if you don't abuse it), the transfer case, and the rear differential. Many owners swap to a stronger transmission from EVO 8 or 9, or install a dogbox if they track hard. Expect to replace wheel bearings and axles more frequently if you launch aggressively.

Daily Livability

An aftermarket clutch will be heavier than stock. A large turbo may lag a bit unless you choose a quick-spooling unit like a GTX3076R. The cabin will be louder, the ride stiffer (if you upgrade suspension), and you'll likely check fluids every week. But for many enthusiasts, that's part of the fun.

Budget Planning and Pitfalls to Avoid

  • Underestimating the cost of machine work: Get quotes before you order pistons.
  • Buying used parts without inspection: A used turbo may have shaft play; a used clutch may slip after installation.
  • Ignoring the cooling system: A stock radiator and fans won't keep a 400 hp engine cool in summer traffic.
  • Not planning for tuning: You cannot tune a car with a stock ECU and a boost controller—you need a real tuning solution.
  • Forgetting the little things: Gaskets, fittings, fluids, spark plugs, and wiring supplies add up quickly.

Final Thoughts

Building a 400 hp Mitsubishi Eclipse GSX is a rewarding project, but it requires serious financial commitment. Budget a minimum of $11,000–$15,000 for a reliable build, or $8,000–$10,000 if you do all labor and hunt for deals. The result is a car that can thrill you for years, with performance that rivals modern sports cars at a fraction of the price. Plan carefully, buy quality parts, and enjoy the journey.