The HKS GTII 70mm turbo kit stands as one of the most respected bolt-on power solutions for the performance enthusiast. Designed to transform a modest factory powertrain into a formidable street machine, this kit bridges the gap between daily drivability and serious track performance. With the ability to elevate output from approximately 160 horsepower to well over 350 horsepower, the 70mm HKS GTII turbo kit represents a significant upgrade path for specific inline-4 and inline-6 platforms. Achieving these power gains requires more than simply bolting on hardware; it demands a comprehensive understanding of engine management, fuel delivery, and installation best practices.

The Legacy and Engineering of the HKS GTII 70mm Turbo

HKS has been a dominant force in Japanese performance engineering since the 1970s. The GTII series was developed as an evolution of the earlier HKS turbos, incorporating modern compressor wheel geometry and more efficient bearing systems. The 70mm variant specifically targets enthusiasts who want substantially more power than a stock turbocharger can provide, without stepping into the realm of massive, laggy units that sacrifice street manners. The reputation of the GTII line is built on reliability and predictable power delivery. When you purchase a 70mm HKS GTII kit, you are investing in a turbocharger that has been tested extensively on some of the most popular engines in the tuning world, including the Nissan SR20DET, the Toyota 2JZ-GE, the Mitsubishi 4G63, and the Honda K20/K24 series. This engineering heritage provides a strong foundation for a build that can withstand the demands of aggressive driving, track days, and long-term daily use.

Ideal Applications and Vehicle Compatibility

The 70mm HKS GTII turbo kit is not a universal fitment. It is designed primarily for vehicles where a turbocharger upgrade is a well-documented path to high horsepower. Understanding which platforms benefit most from this specific kit is crucial before making a purchase. The kit is often selected for vehicles that came naturally aspirated from the factory, such as the Nissan 240SX (SR20DE or KA24DE) or the Toyota Supra (2JZ-GE). These cars have strong engine blocks but lack forced induction. Adding the 70mm GTII kit can bring them to power levels that rival or exceed their factory turbocharged counterparts, such as the SR20DET or 2JZ-GTE. For owners of already turbocharged cars, like a Subaru WRX (EJ20/EJ25) or a Mitsubishi Eclipse (4G63T), the HKS GTII 70mm kit offers a direct upgrade path over the smaller factory turbocharger, allowing for higher boost levels and more airflow without completely overhauling the engine bay layout.

  • Nissan SR20DE (240SX / Sentra SE-R): One of the most popular applications. The SR20DE responds very well to turbocharging, and the 70mm GTII kit can easily push it past 350 wheel horsepower with proper fuel and management.
  • Toyota 2JZ-GE (Supra N/A / GS300 / IS300): The 3.0L inline-six provides excellent torque and spools the 70mm turbine quickly. The 2JZ-GE bottom end is strong enough to handle the power, but the compression ratio requires careful tuning.
  • Mitsubishi 4G63/4G64 (Eclipse / Talon / EVO): A staple platform for the GTII series. The robust iron block and proven drivetrain make it an easy candidate for the 350+ hp range.
  • Honda K20/K24 (RSX / Civic Si / Accord): A newer but well-established application. The VTEC engagement combined with the 70mm turbo provides a unique and power highway pull.

Component Breakdown: What Comes in the Kit

A comprehensive HKS GTII 70mm turbo kit includes several key components that are engineered to work together. Purchasing a complete kit simplifies the build process significantly compared to sourcing parts individually. Each component plays a specific role in achieving the target power output and ensuring the engine operates within safe parameters.

The 70mm Turbocharger Core

The heart of the kit is the HKS GTII series turbocharger, featuring a 70mm compressor wheel. This size compressor wheel is capable of flowing roughly 45 to 50 pounds of air per minute, which is sufficient to support 400 to 450 engine horsepower. The turbine housing is typically available in A/R ratios of 0.63 or 0.86, with the smaller ratio providing quicker spool and the larger offering better top-end power. The HKS GTII uses a journal bearing center cartridge, which is durable and cost-effective when paired with a proper oil supply and cooldown procedure. Some later variations or specific applications may feature ball bearing upgrades, but the standard GTII journal bearing unit is well-proven for street builds.

Exhaust Manifold and Wastegate

The kit includes a cast iron or tubular exhaust manifold designed specifically for the turbocharger. The manifold is built to withstand high exhaust gas temperatures without cracking, which is a common issue with cheaper, thin-wall manifolds. The kit also includes an external wastegate, typically a 38mm or 40mm unit. The wastegate controls boost pressure by bypassing exhaust gas away from the turbine wheel. Having an external wastegate is essential on a 70mm turbo build because it provides more precise boost control than an internal wastegate, which is often overwhelmed at higher boost levels (15-25 psi).

Intercooler and Piping

To handle the increased air density from the turbocharger, a front-mount intercooler (FMIC) is standard in most complete kits. The intercooler reduces the temperature of the compressed air, increasing oxygen density and reducing the risk of detonation. The piping included in the kit is mandrel-bent to ensure smooth airflow with minimal pressure drop. The piping is typically aluminum and will require silicone couplers and T-bolt clamps to secure the connections. Proper routing of the piping is critical to avoid rubbing against other engine components or chassis parts.

Fuel System Upgrades (Often Required)

While not always physically included in the turbo kit box, a reliable fuel system is a non-negotiable requirement for the 350+ hp range. Most reputable vendors offer the HKS GTII kit as a package that can be bundled with fuel components. At a minimum, you will need high-impedance fuel injectors in the 550cc to 750cc range, a high-flow fuel pump (such as a Walbro 450 LPH or AEM 340 LPH), and an adjustable fuel pressure regulator. The factory fuel pump and injectors on almost any engine will be maxed out well before reaching 300 hp, and attempting to run the 70mm kit on the stock fuel system will lead to a lean condition and catastrophic engine failure.

The Power Transformation: From 160 HP to Over 350 HP

The 160 horsepower baseline represents a typical naturally aspirated engine like the SR20DE or a stock base-model Supra. The jump to 350+ horsepower represents a 119% increase, which completely transforms the vehicle's performance character. However, it is important to set realistic expectations based on the specific engine and supporting modifications.

Understanding the Power Band

On a 2.0L to 3.0L engine, the 70mm HKS GTII turbocharger will begin to spool noticeably around 3,000 to 3,500 RPM. Full boost (14-20 psi) is typically achieved by 4,000 to 4,500 RPM. This means the car will feel relatively tame below 3,500 RPM, similar to a slightly above-stock naturally aspirated engine. Once the turbo spools, the power delivery is strong and linear, pulling hard all the way to the redline (7,000-8,000 RPM depending on the engine). This type of power band is excellent for road courses, autocross, and highway passing, but requires the driver to keep the RPMs in the sweet spot for maximum acceleration.

Reliability and Power Output Ceilings

The 350+ hp figure is a safe, reliable power level for a properly built engine with 93 octane pump gas. With ethanol (E85), the same setup can often be pushed to 400-450 wheel horsepower due to the fuel's higher octane rating and cooling effect. Pushing beyond 450 hp typically requires upgrading the turbine housing, adding larger injectors (1000cc+), and potentially building the bottom end with forged rods and pistons. The HKS GTII 70mm kit provides a power ceiling that allows the builder to grow into the setup without immediately hitting a wall.

Installation Guide and Critical Considerations

Installing the HKS GTII 70mm turbo kit is an intermediate to advanced mechanical task. It requires a solid understanding of automotive systems, including cooling, lubrication, and exhaust. Allocating a full weekend for the installation is a realistic timeline, especially if it is your first time performing a turbocharger swap on the specific chassis.

Step 1: Vehicle Preparation

Disconnect the battery to prevent any electrical shorts. Drain the engine oil and coolant. Removing the radiator and front bumper support will dramatically improve access to the exhaust manifold and the area where the turbo will mount. On many platforms, it is also easier to remove the intake manifold to access the exhaust manifold bolts on the underside.

Step 2: Mounting the Manifold and Turbo

Remove the factory exhaust manifold and any heat shields. Clean the cylinder head mounting surface thoroughly. Install the new HKS manifold using a high-quality multi-layer steel (MLS) gasket. Torque the manifold bolts to the factory specifications in the correct sequence. Mount the turbocharger to the manifold using the supplied gasket. It is critical to properly clock the compressor housing so that the outlet points toward the intercooler piping, and the oil drain points downward.

Step 3: Routing Oil and Coolant Lines

Oil supply typically comes from a port on the engine block or the cylinder head. Use a high-quality braided stainless steel line. The oil return line (drain) is the most critical part of the oil system. It must run downhill from the turbo center cartridge to the oil pan without any dips or kinks that could cause oil to back up and leak past the turbo seals. If the kit uses water cooling, connect the water lines to the engine's cooling system, typically tapping into the heater hose circuit.

Step 4: Intercooler and Intake

Mount the front-mount intercooler using the provided brackets or custom fabrication. Route the intercooler piping from the turbo outlet to the intercooler, and then from the intercooler to the throttle body. Use T-bolt clamps to secure the silicone couplers. Install the air filter on the turbo inlet. Ensure the piping does not contact the radiator, fans, or any moving suspension components.

Step 5: Exhaust and Downpipe

Install the downpipe from the turbo outlet to the existing exhaust system. This may require a custom section if the kit's downpipe does not perfectly align with the factory cat-back exhaust. It is highly recommended to upgrade the entire exhaust system to a 3-inch diameter mandrel-bent system to reduce exhaust backpressure and allow the 70mm turbo to breathe freely.

Tuning: The Most Crucial Step for 350+ HP

Installing the turbo kit is only half the battle. The engine will not run correctly or safely without a proper tune. The factory ECU is programmed for the stock engine's airflow and fuel requirements. Introducing the 70mm turbocharger without reprogramming the ECU will result in a rough idle, poor throttle response, and potentially destructive detonation under boost. You have two primary paths for engine management: standalone ECU or flash tuning.

Standalone ECUs

For a 350+ hp build, a standalone ECU like a Haltech Elite 750, AEM Infinity 506, or Link G4X is the recommended choice. Standalone ECUs offer complete control over fuel maps, ignition timing, boost control, and idle stabilization. They also allow you to add modern sensors like a wideband O2 sensor and a flex fuel sensor for E85 tuning. The initial cost is higher, but the tuning flexibility and safety features make it a worthwhile investment.

Flash Tuning and Piggyback Systems

For some platforms (like the Nissan SR20DE or Honda K-Series), flash tuning solutions like Nistune, Hondata, or ECUTek provide a more budget-friendly path. These systems modify the factory ECU's software to accept the new injectors and MAF sensor scaling. Flash tuning can be very effective up to the 350-400 hp range, but it may lack the advanced features of a standalone system, such as boost-by-gear or anti-lag.

Monitoring and Safety

Regardless of the tuning method, you must install a wideband air/fuel ratio gauge, a boost gauge, and an oil pressure gauge. These gauges provide real-time data that is essential for ensuring the engine is running safely. A single pull with a lean air/fuel ratio can destroy a built engine. Work with a reputable professional tuner who has experience with the HKS GTII kit on your specific engine platform. Remote tuning is an option, but dyno tuning is far superior for maximizing power and ensuring reliability across the entire RPM range.

Driving Experience and Real-World Benefits

The 70mm HKS GTII turbo kit completely changes the character of the vehicle. It transforms a modest daily driver into a performance machine that demands respect. The sound of the external wastegate and the suction of the 70mm compressor wheel provide an auditory experience that is deeply satisfying for any gearhead. The throttle response, while not as instant as a naturally aspirated setup, is predictable and builds torque in a linear fashion as the turbo spools.

One of the key benefits of this specific kit is its top-end power. Unlike smaller turbos that drop off in power after 6,000 RPM, the 70mm GTII continues to pull hard all the way to the fuel cut limiter. This makes it an excellent choice for high-speed driving on a racetrack or during highway pulls. The kit also improves the overall efficiency of the engine at high speeds, as the turbo helps the engine overcome aerodynamic drag with less effort.

Maintenance of the HKS GTII kit is straightforward but requires diligence. The journal bearing center section requires high-quality synthetic oil and frequent oil changes (every 3,000-5,000 miles). It is also critical to allow the turbo to cool down after a hard drive before shutting off the engine. A turbo timer can be installed to automate this process, preventing oil coking and extending the life of the turbocharger.

Budgeting for a Complete 350+ HP Build

Understanding the total cost of the project is essential. The HKS GTII 70mm kit itself is a significant investment, but it is only one part of the overall budget. Skimping on supporting components to save money is a common mistake that leads to reliability issues or poor performance.

  • HKS GTII 70mm Turbo Kit: $2,500 - $4,000 (includes turbo, manifold, wastegate, and oil lines)
  • Fuel System (Pump + Injectors + FPR): $500 - $1,200
  • Standalone ECU + Tuning: $1,500 - $3,000
  • Front Mount Intercooler + Piping (if not included): $500 - $1,000
  • Clutch Upgrade: $400 - $1,000
  • Exhaust System (Downpipe + Cat-back): $500 - $1,500
  • Gauges and Boost Controller: $400 - $800

The total cost for a reliable 350+ hp build typically falls between $6,000 and $12,000, depending on the platform and component choices. If you are paying a shop for installation labor, add another $1,500 to $3,000. Plan your budget carefully and prioritize the supporting modifications.

Frequently Asked Questions

Is the HKS GTII 70mm kit daily drivable?

Yes, with a proper tune and a quality boost controller, the kit provides excellent daily drivability. The car will feel normal at low RPM, and the power is only available when you step into the throttle. The external wastegate can be loud, which may be a consideration for some daily drivers.

Can I run the kit on a stock engine?

It depends on the engine and the power target. Many stock engines (like the SR20DE or 2JZ-GE) can safely handle 300-350 hp with a proper tune. For 400+ hp, or if the engine has high mileage, forged pistons and rods are highly recommended to ensure long-term reliability.

What is the difference between the GTII and a cheap turbo kit?

The primary differences are material quality, fitment, and support. The HKS manifold is less likely to crack. The turbocharger uses quality bearings and is balanced from the factory. The piping is designed for specific chassis, reducing the need for custom fabrication. Cheap kits often fit poorly, require modifications, and fail prematurely.

Conclusion: Is the HKS GTII 70mm Kit Right for Your Build?

The HKS GTII 70mm turbo kit is a proven, well-engineered solution for achieving a dramatic increase in horsepower. It offers a balanced combination of spool characteristics and top-end power that is difficult to match with cheap, generic components. For the enthusiast who wants a car that can serve as a reliable daily driver and a potent weapon on the track, this kit is a top-tier choice. The journey from 160 hp to 350+ hp requires dedication, mechanical skill, and a solid budget for tuning and supporting mods. If you are prepared for the commitment, the result is a vehicle that provides an exhilarating driving experience every time you get behind the wheel. For further technical details, consult the official HKS product page for your specific vehicle or reach out to a professional tuner who is familiar with the HKS GTII series.