The Mazda RX-7 FD3S remains one of the most celebrated sports cars ever produced, largely thanks to its lightweight chassis and the unique 13B-REW twin-turbo rotary engine. While the factory sequential turbo system was innovative, it has limitations in reliability and peak output. For enthusiasts seeking a significant, reliable power upgrade, the single-turbo conversion is a proven path. Among the most respected turbochargers for this application is the HKS GT3037. This article provides an in-depth look at the GT3037, covering real-world performance gains, a detailed installation guide, and a transparent cost breakdown to help you plan your build.

Understanding the HKS GT3037 Turbocharger

The HKS GT3037 is a single-turbo conversion kit designed specifically for the FD RX-7’s 13B-REW rotary engine. It combines a GT30-series compressor wheel with a GT37 turbine housing. This hybrid design offers a balance of quick spool-up and high-flow capacity, making it suitable for street-driven cars that also see track time. The kit typically includes the turbocharger, a cast-iron exhaust manifold, oil and coolant lines, and necessary gaskets. Unlike generic turbo swaps, the HKS GT3037 kit is engineered to fit the FD’s engine bay with minimal modification.

Key Specifications

  • Compressor Inducer: 52.8mm (GT30)
  • Turbine Exducer: 62.5mm (GT37)
  • Housing AR: Typically comes with a .73 or .86 twin-scroll housing for the turbine side.
  • Wastegate: Usually an internal wastegate (though some builds opt for an external unit for better boost control).
  • Bearing System: Dual ball bearing for low friction and rapid spool.

Performance Gains: Real-World Numbers

The HKS GT3037 is known for transforming the driving experience of the FD RX-7. Here’s what you can realistically expect:

Dyno-Proven Results

On a properly tuned 13B-REW with supporting fuel and exhaust mods, the GT3037 at 15-18 psi of boost produces approximately 400-450 wheel horsepower and 380-420 lb-ft of torque. This represents a gain of 150-200 horsepower over a stock twin-turbo setup (which typically dynos at 220-250 whp). Some tuners have pushed the turbo to 22+ psi to achieve over 500 whp, though this requires a built engine and race gas or E85.

Drivability Characteristics

  • Spool Time: The GT3037 begins building boost around 3,200-3,500 RPM and reaches full boost by approximately 4,000 RPM. This is noticeably quicker than larger turbos like the GT3540 or GT3582.
  • Torque Curve: The twin-scroll housing helps maintain strong mid-range torque, providing punchy acceleration without excessive lag.
  • Top-End Pull: The turbo holds power well through the 7,500 RPM redline, offering a broad powerband suitable for road and track use.

Comparison to Other Common Turbo Upgrades

Enthusiasts often compare the GT3037 to the Garrett GT3076R (x2 for twin variants) or the BorgWarner S300SX. The GT3037 tends to spool slightly faster than the GT3076R due to the larger turbine housing on the 3076, while delivering similar peak power. It is a well-rounded option for those who want strong power without sacrificing daily drivability.

Installation Process: A Detailed Walkthrough

Installing the HKS GT3037 is a substantial job requiring mechanical knowledge, especially with rotary engines. Below is a step-by-step outline of the process. Always consult the specific HKS GT3037 kit instructions and a reliable service manual for torque specs and procedures.

Preparation and Safety

Tools required: Metric socket set, wrenches, torque wrench, sealant, vacuum line kit, boost controller, turbo timer (optional), jack and jack stands.

Safety: Disconnect the battery, drain the engine oil and coolant, and work in a well-ventilated area. The rotary engine’s exhaust manifold gets extremely hot—allow the engine to cool fully before starting.

Step 1: Remove the Stock Twin-Turbo System

  • Remove the intake ducting, air filter, and intercooler piping.
  • Disconnect the secondary turbo control vacuum lines and solenoids.
  • Unbolt the stock exhaust manifolds (both primary and secondary turbo housings).
  • Disconnect the oil and coolant feed and return lines from the turbos.
  • Remove the turbochargers, downpipe, and catalytic converter if still present.
  • Inspect the engine oil system for sludge; consider replacing the oil pan gasket and pickup tube O-ring while the turbos are off.

Step 2: Install the HKS GT3037 Manifold and Turbo

  • Clean the engine block mounting surfaces for the new manifold.
  • Install the HKS cast-iron manifold using the supplied gasket. Apply anti-seize to the studs and torque to spec (typically 30-35 ft-lbs in a cross pattern).
  • Mount the GT3037 itself to the manifold. The internal wastegate rod may need adjustment so the gate opens fully when actuated.
  • Attach the oil feed line from the engine block (usually from the pressure port near the oil filter) to the turbo. Use a restrictor if the turbo is ball bearing (recommended: 0.045-0.060 inch restrictor).
  • Connect the oil return line to the oil pan using a proper AN fitting. Ensure the drain is downhill and not kinked.
  • Attach the coolant lines; the kit often provides a feed from the water pump and return to the radiator. Use hose clamps and double-check for leaks.

Step 3: Intercooling, Intake, and Exhaust

  • Intercooler: A front-mount intercooler (FMIC) is essential. The HKS kit often includes a core and piping that mates to the turbo’s compressor outlet. Expect to cut the bumper support slightly.
  • Intake: Use a high-flow intake filter with a MAF adapter (or switch to a blow-through MAF setup). If using a standalone ECU, a speed-density setup eliminates the MAF entirely.
  • Exhaust: The turbine outlet requires a custom downpipe or a ready-made HKS downpipe. A 3-inch exhaust from the turbo back is recommended for minimal backpressure.
  • Wastegate Venting: The internal wastegate should be routed back into the downpipe (“plumbed”) for quiet operation, or vented to atmosphere (screamer pipe) for more noise.

Step 4: Vacuum Lines, Boost Control, and Sensors

Relocate the boost control solenoid (if using electronic boost control) near the turbo. Run a vacuum line from a manifold pressure source to the wastegate actuator. Install a boost gauge and, ideally, an AFR gauge for tuning feedback.

Step 5: ECU Tuning

This is mandatory. The stock RX-7 ECU cannot correctly fuel or time a single-turbo setup. Common choices are the Adaptronic, Power FC, Haltech, or AEM standalone.

  • Base map: Start with a safe tune (10:1 AFR under boost, conservative timing).
  • Dyno tuning: A professional rotary tuner will dial in ignition timing and fuel delivery across the entire load range. Expect 4-8 hours of dyno time.
  • If using E85, ensure fuel injectors (e.g., 1000cc or 2000cc) and a high-flow fuel pump are installed before tuning.

Supporting Modifications: What Else You Need

To handle the HKS GT3037’s airflow, several supporting mods are essential for reliability and performance.

ComponentRecommendationApproximate Cost
Fuel Injectors1000cc (min) for pump gas; 1600-2000cc for E85.$400–$800
Fuel PumpWalbro 255 lph or AEM 340 lph in-tank$100–$200
Intercooler3.5-4 inch core, bar-and-plate design$300–$700
Exhaust3-inch full turbo-back$400–$1,000
Boost ControllerElectronic (AEM, HKS EVC)$250–$500
Oil Cooler (optional)RX-7s need robust cooling; consider a thermostatic plate with 19-row cooler$300–$600

Cost Breakdown: A Realistic Budget

Below is an itemized cost estimate for a complete HKS GT3037 installation. Prices reflect typical new parts and professional labor (if not DIY).

  • HKS GT3037 Turbo Kit: $1,600 – $2,200 (includes turbo, manifold, lines, gaskets)
  • Intercooler Kit: $300 – $800
  • Exhaust System (downpipe + catback): $500 – $1,200
  • Fuel System (injectors + pump): $500 – $1,000
  • Standalone ECU + Tuning: $1,200 – $2,500
  • Boost Controller: $200 – $500
  • Installation Labor (10-15 hours @ $100-150/hr): $1,000 – $2,250
  • Miscellaneous (gaskets, fittings, coolant, oil, vacuum line, zip ties): $200 – $400
  • Dyno Tuning (3-6 hours): $400 – $900

Total Estimated Range: $4,900 – $10,750

Doing the installation yourself reduces labor costs significantly (install and basic tuning). However, professional tuning is strongly recommended for safety and performance.

Maintenance Considerations After Installation

Once the HKS GT3037 is installed, maintenance routines change.

  • Oil Change Interval: Reduce to 3,000 miles or less. Turbocharged rotary engines shear oil quickly. Use a quality synthetic 10W-40 or 15W-50.
  • Cooling System: Monitor coolant temps; consider an aftermarket radiator (Koyo, Fluidyne) if towing or driving in hot climates.
  • Boost Leak Check: Annually or after any service. A small leak can cause lean conditions.
  • Wastegate Function: Verify the wastegate opens freely. Carbon buildup can cause overboost.
  • EGT and AFR Gauges: Permanently install these to keep an eye on engine health.

Pros and Cons of the HKS GT3037

Pros

  • Plug-and-play design for FD RX-7 (minimal fab work).
  • Excellent spool-up characteristics for a 400+ hp setup.
  • Dual ball bearing reliability.
  • HKS brand support and aftermarket parts availability.

Cons

  • Price premium over generic Garrett turbo kits.
  • Limited power ceiling (typically maxes around 500 whp).
  • Requires external wastegate for precise boost control (many owners replace the internal unit).
  • Not a “small” swap—still requires significant downtime and supporting mods.

Alternatives to the HKS GT3037

If the GT3037 doesn’t meet your budget or power goals, consider these alternatives:

  • Garrett GT3076R: Similar power potential, slightly slower spool, often cheaper. Requires custom manifold.
  • BorgWarner EFR 7670: Faster spool and integrated blow-off valve. More expensive.
  • Precision 5858: Good middle-ground, popular in budget builds.
  • Stock Twin-Scroll Upgrade: Retaining the factory twins with high-flow intakes and a boost controller is cheaper but yields lower gains.

For more information, consult the HKS official product page for specifications, or join community forums like RX-7 Club for real owner experiences and build threads. A comprehensive guide from How To Tune (example link) may also be helpful.

Conclusion

The HKS GT3037 turbo conversion remains one of the most well-engineered, reliable upgrades available for the FD RX-7. When paired with proper tuning and supporting parts, it transforms the car from a quirky 250-hp coupe into a responsive, 400+ hp machine capable of embarrassing far more modern sports cars. While the total investment can approach $10,000, the balance of power, spool, and driveability justifies the cost for those committed to owning one of the greatest rotary platforms ever built.