Why the RX-8 Needs a Turbocharger

The Mazda RX-8 is one of the last production cars to feature a rotary engine, specifically the 13B-MSP Renesis. While the original 250-horsepower rating (in the manual six-speed version) is respectable for a naturally aspirated 1.3-liter, the torque curve is notoriously flat below 4000 RPM. Enthusiasts often describe the stock RX-8 as a car that needs to be revved hard to deliver excitement. Adding forced induction—especially a properly matched turbocharger—transforms the driving experience by filling in the lower rev range and producing power curves that rival much larger engines.

The HKS GTII Turbo Kit has become a benchmark for RX-8 owners who want a reliable, turn-key 400-horsepower setup. HKS has decades of experience with rotary engines, and the GTII line is specifically engineered to suit the unique flow characteristics of the Renesis. This article provides a complete, technical review of the kit, covering installation, tuning, supporting modifications, and what it actually feels like to drive a turbocharged RX-8.

Understanding the HKS GTII Turbo Kit

The HKS GTII kit is more than just a turbocharger—it is a comprehensive system designed to bolt onto the Renesis with minimal modification. Every major component is included, from the exhaust manifold to the intercooler and fuel injectors. Below is a breakdown of the key parts and why they matter for a rotary engine.

Turbocharger Selection: Why the GTII?

HKS chose the GTII-2835R turbocharger for the RX-8 kit. This unit features a dual-ball bearing cartridge, a GT-style compressor wheel, and a stainless steel turbine housing. The ball bearing design reduces spool time, which is critical for rotary engines because they have low displacement and rely on high RPMs to build boost. The GTII-2835R is capable of flowing enough air to support 400–420 horsepower while maintaining excellent throttle response.

Unlike piston engines, rotary engines have no reciprocating parts and spin to 9000 RPM easily. The turbo must be matched to the engine’s VE (volumetric efficiency) curve. HKS spent considerable R&D to ensure the GTII delivers linear boost from around 3500 RPM all the way to redline, avoiding the sharp spike that can overload the Renesis’s fragile side seals.

Kit Contents in Detail

  • Cast exhaust manifold – Designed to equalize exhaust pulses from the twin-rotor housing. It replaces the restrictive factory manifold and provides a T25/T28 flanged outlet.
  • GTII-2835R Turbocharger – Equipped with a 60mm compressor inducer and a 68mm turbine exducer. Maximum boost of 14–16 psi is recommended for pump gas.
  • Large bar-and-plate intercooler – Front-mount unit with 2.5-inch inlet/outlet piping. It effectively reduces intake temperatures under sustained boost.
  • 3-inch downpipe – Connects the turbo outlet to the factory catalytic converter or a test pipe. Includes a bung for the stock oxygen sensor.
  • 680cc/min fuel injectors – Drop-in replacements that fit the factory fuel rail. Injectors are sized to supply enough fuel for 400 horsepower with a 30% safety margin.
  • HKS F-Con V Pro piggyback ECU – Pre-loaded with a baseline mapping for 400 HP. The F-Con V Pro allows full control over fuel and ignition timing, which is essential for a rotary engine.
  • All necessary gaskets, hoses, and clamps – High-quality silicone couplers and T-bolt clamps ensure no boost leaks.

Installation Overview and Requirements

Installing the HKS GTII kit is a project best tackled over a weekend with a well-equipped garage. While the kit is designed to be a bolt-on affair, certain steps require care to avoid damaging the engine or turbo. Below is an expanded installation guide highlighting critical areas.

Tools and Preparation

  • Socket and ratchet set (metric), including deep sockets for the exhaust manifold bolts.
  • Torque wrench (range 10–50 lb-ft for small fasteners, up to 80 lb-ft for turbo mount).
  • Jack and jack stands or a lift.
  • Engine hoist (optional, but helpful for supporting the engine if you need to tilt).
  • Safety glasses, gloves, and fire extinguisher.

Step-by-Step Installation

  1. Disconnect battery and drain engine coolant and engine oil. Remove the factory air intake, intercooler (if equipped with an aftermarket one), and plastic shroud.
  2. Remove the exhaust system from the manifold back. Unbolt the factory header and mid-pipe. You may need to cut or loosen the catalytic converter bolts if they are seized.
  3. Remove the factory fuel injectors and install the 680cc units. Carefully clean the injector seats before reinstallation. Reinstall the fuel rail with new O-rings.
  4. Install the exhaust manifold. Apply high-temperature anti-seize to the studs. Torque the manifold bolts in a crisscross pattern to 25 lb-ft. Mount the turbocharger to the manifold with the provided gasket, then install the oil feed and drain lines.
  5. Connect the intercooler piping. Test-fit all couplers and clamps before tightening. Ensure the blow-off valve is oriented so that it vents to atmosphere (if using a vent-to-atmosphere BOV) or recirculates as per the kit design.
  6. Install the downpipe. Use a new gasket between the turbo outlet and downpipe. Tighten to 30 lb-ft. Reconnect the catalytic converter or test pipe.
  7. Route the oil drain line to the oil pan. HKS provides a fitting that requires drilling and tapping the pan. Drain the oil first, then remove the pan. Mark the location carefully to avoid interfering with the crank or oil pump.
  8. Connect the coolant lines. The water-cooled center housing requires a supply from the engine coolant system. T into the heater hose return line using the supplied brass tee.
  9. Install the intercooler. Mount it behind the front bumper using the provided brackets. Ensure it is securely fastened and that the upper radiator hose does not rub against the piping.
  10. Wiring and ECU. Connect the F-Con V Pro according to the manual. This usually involves splicing into injector triggers and the MAF sensor signal. If you are not experienced with wiring, consider professional installation at this point.
  11. Refill fluids with fresh engine oil (5W-30 synthetic), coolant, and check the transmission fluid. Perform a preliminary boost leak test with the system sealed.

Total installation time for an experienced home mechanic is typically 15–20 hours. A professional shop may charge 8–12 hours of labor. Do not skip the boost leak test—rotors do not tolerate lean mixtures.

Performance Gains and Tuning

The HKS GTII kit is advertised to take the RX-8 from 250 to 400 horsepower at the flywheel. Real-world dyno results typically show 360–380 wheel horsepower on 93 octane pump gas at 12–14 psi. With ethanol (E85) and higher boost (16–18 psi), 420–450 wheel horsepower is possible, but fuel system modifications (larger lines, higher-flow fuel pump) become necessary.

Tuning Requirements

The Renesis rotary engine has very specific tuning needs. The long, narrow intake ports and the unique combustion chamber shape require careful ignition timing. The HKS F-Con V Pro provides a good starting map, but a professional tune on a dyno is strongly recommended. A knowledgeable tuner will:

  • Set the fuel target to 11.0–11.5:1 air-fuel ratio under boost (richer than a piston engine).
  • Retard ignition timing at peak torque to prevent detonation.
  • Activate a water injection or methanol injection system if higher boost is used.
  • Adjust the boost control solenoid to achieve smooth, progressive boost onset.

Without proper tuning, a turbocharged rotary engine can fail in minutes due to detonation. Heat is the enemy: the Renesis has side-mounted exhaust ports that already run hot. A conservative tune is far better than chasing peak numbers.

Supporting Modifications for 400+ HP

To handle the increased power and heat, the following upgrades are recommended:

  • Clutch – A single- or twin-plate clutch from ACT, Exedy, or Spec (capable of 450 lb-ft of torque).
  • Differential – Regear to 4.44 or 4.77 for better acceleration; upgrade the stock viscous limited-slip to a Torsten or clutch-type unit.
  • Brakes – Upgraded pads (Hawk HP+ or similar) and braided stainless steel lines. Consider a big brake kit if tracking the car.
  • Cooling – Larger aluminum radiator, high-flow oil cooler (the stock Renesis oil cooler is marginal), and a thermostatically controlled fan.
  • Fuel system – Walbro 400 LPH fuel pump, return-style fuel pressure regulator, and larger fuel lines (if exceeding 400 whp).
  • Engine management – Many owners prefer swapping the F-Con V Pro for a standalone ECU like the Haltech Elite 1500 or Link Fury. These allow better control over the secondary injectors and ignition.

Driving Experience and Characteristics

A turbocharged RX-8 is a completely different animal from a naturally aspirated one. The most obvious change is the massive surge of torque around 3500 RPM. Where the stock engine feels weak below 4000 RPM, the HKS GTII car pulls hard from 3000 RPM and only gets stronger until redline.

Boost Response and Powerband

Using the ball-bearing GTII-2835R, boost comes on quickly. At 12 psi, you will feel significant boost by 3200 RPM, with full boost achieved by 4000 RPM. The spool is smooth enough that the car remains predictable during corner exits. On the track, you can apex at 4000 RPM and carry boost through the corner, unlike a large turbo that would lag.

The powerband feels linear—more like a large-displacement V8 than a peaky four-cylinder. The rotary engine’s high rev ceiling (8500–9000 RPM) allows you to stay in the power without shifting unnecessarily. Many owners report that the car feels 800+ pounds lighter because of the torque.

Sound and Character

The HKS kit retains the unmistakable rotary howl. The turbo muffles some of the high-frequency intake noise, but the exhaust note (especially with the provided downpipe) is deep and aggressive. The blow-off valve gives a recognizable hiss between shifts. At idle, the engine sounds more subdued, but under full throttle, the screaming, turbine-assisted wail is addictive.

Street vs. Track

For daily driving, the kit is surprisingly manageable. The F-Con V Pro provides acceptable drivability at light throttle, and the intercooler prevents heat soak in stop-and-go traffic. However, fuel economy will drop to about 12–16 MPG depending on how often you boost. On the track, the combination of excellent power and the RX-8’s already sharp handling makes for a competitive package. Upgraded cooling is essential for heavy lapping to avoid heat-related detonation.

Reliability and Maintenance Considerations

The Renesis engine is notoriously sensitive to heat and detonation. Adding a turbocharger increases the thermal load significantly. Reliability depends entirely on proper tuning, supporting modifications, and diligent maintenance. Here are the key factors:

  • Oil cooling – The stock oil cooler is barely adequate for NA street use. For turbo, upgrade to a Setrab or Derale oil cooler with at least 19 rows. Synthetic oil (5W-40) should be changed every 3000 miles.
  • Premix – All rotary engines benefit from adding 2-stroke premix oil to the fuel. For turbo cars, mix 1 ounce per gallon to protect the apex seals from low-load tuning and ethanol fuels.
  • EGT monitoring – Install an exhaust gas temperature gauge and keep peak EGT under 900°C (1650°F). If EGT rises above 950°C, engine failure is imminent.
  • Boost control – Never run more than 14 psi on 93 octane without water/methanol injection. A boost cut switch is recommended as a safety layer.
  • Frequent inspections – Check for oil leaks (common at the turbo drain), coolant leaks, and boost leaks. The rubber couplers may degrade over time.

If these guidelines are followed, many turbocharged Renesis engines last 40,000–60,000 miles before requiring a rebuild. That is shorter than a typical piston engine, but rebuild costs for a rotary (around $3,000–5,000 for a street port) are comparable to a piston engine overhaul.

Cost and Value Analysis

The HKS GTII Turbo Kit itself retails for approximately $5,000–$5,500 (depending on vendor). Installation by a professional shop costs $1,200–$2,000. Additional supporting modifications (clutch, differential, cooling, fuel system) can add $3,000–$5,000. A professional tune adds $500–$1,000. Total investment for a reliable 400 HP RX-8 is in the range of $10,000–$14,000, not including any unforeseen repairs or upgrades to the engine, transmission, or suspension.

Compared to other turbo kits (e.g., the Pettit Racing kit or the now-discontinued Greddy kit), the HKS GTII is mid-range in price but offers the best combination of component quality and support. The included F-Con V Pro ECU is a good starting point, but many owners end up upgrading to a standalone. If you are budget-conscious, a DIY turbo setup using a BorgWarner S200SX could be cheaper, but you lose the proven calibration and fitment of the HKS kit.

For owners who intend to keep their RX-8 long-term, the GTII kit is a solid investment. It significantly increases the fun factor and resale value (a properly turbocharged RX-8 can sell for $5,000–$10,000 more than a stock one).

Considerations Before Upgrading

Before ordering the HKS GTII kit, take an honest assessment of your car’s condition. A compression test is mandatory—the Renesis should have at least 7.0 bar on all three faces of each rotor. Low compression units will not tolerate boost and will fail quickly. Also check the apex seals, side seals, and oil control rings. Rebuild the engine if compression is marginal.

Legal considerations: Many states have strict emissions laws. The HKS downpipe removes the primary catalytic converter; in California, this will cause a smog test failure. Some owners add a high-flow catalytic converter later. Check your local regulations.

Warranty and insurance: A turbocharged RX-8 is a modified vehicle. Insurance companies may increase your premium or require a specialist provider. Be upfront about the modification to avoid claim denial.

Finally, decide if you can tolerate the compromises: reduced fuel economy, increased noise, more frequent maintenance, and the constant worry about heat-related issues. The HKS GTII kit is not for everyone, but for those who love the rotary engine and want to unlock its true potential, it is one of the best aftermarket upgrades available.

Conclusion

The HKS GTII Turbo Kit is a thoroughly engineered solution to the RX-8’s lack of torque. With 400 horsepower on tap, the car becomes a genuine sports car capable of competing with modern turbocharged machines. The key to success is not just bolting on the kit, but investing in proper tuning, supporting modifications, and ongoing maintenance. If you are willing to do that, the HKS GTII transforms the RX-8 into a thrilling, addictive, and surprisingly reliable performer. For more information, visit HKS’s official website, and read owner experiences on RX8Club. A professional tuner such as Rotary Performance can provide detailed guidance for your specific build.