Fabrication And Tuning Of A Twin-turbocharger System For The Nissan 240sx

The Nissan 240SX, with its lightweight chassis and rear-wheel drive, has become a legendary platform for modification. While engine swaps are common, turbocharging the factory SR20DET or KA24DE presents a compelling alternative. Taking it a step further, a twin-turbocharger system promises even greater power potential, albeit with increased complexity. This guide delves into the intricate process of fabricating and tuning such a system.

Understanding Twin-Turbocharger Configurations

Before diving into the fabrication, it's crucial to understand the different twin-turbo configurations. The two most common are:

Parallel Twin-Turbo Setup: Each turbocharger is fed by exhaust from one half of the engine's cylinders. This configuration is simpler to implement and generally offers a broader powerband.
Sequential Twin-Turbo Setup: One turbocharger operates at low RPMs, providing quick boost response. A second, larger turbocharger kicks in at higher RPMs for increased top-end power. This setup is more complex but offers the potential for both excellent low-end responsiveness and high peak power.

For the 240SX, a parallel twin-turbo setup is often favored due to its relative simplicity and ease of integration, especially for amateur builders. This guide will primarily focus on this configuration.

Planning and Component Selection

Careful planning is paramount. Key considerations include:

Turbocharger Selection

Choosing the right turbochargers is crucial. Factors to consider include:

Engine Displacement: Smaller displacement engines (like the SR20DET) require smaller turbochargers to avoid excessive lag. Larger engines (like the KA24DE) can accommodate slightly larger turbos.
Desired Power Output: Match the turbocharger's flow rate to your target horsepower. Remember, two smaller turbos often provide better spool-up than one larger turbo for the same overall flow capacity.
A/R Ratio: A lower A/R (Area/Radius) ratio turbine housing will spool up faster but may restrict top-end flow. A higher A/R ratio will offer better top-end power but may suffer from more lag.
Budget: Turbocharger costs vary significantly. Consider reputable brands like Garrett, BorgWarner, and Precision Turbo.

For an SR20DET targeting around 400-450 horsepower, a pair of Garrett GT2554R or similar turbos would be suitable. For a KA24DE targeting a similar power level, slightly larger turbos like Garrett GT2860RS (Disco Potato) could be considered.

Intercooler Selection

An efficient intercooler is essential for cooling the compressed air from the turbochargers, increasing air density and reducing the risk of detonation.

Core Size: Choose an intercooler with sufficient core volume to handle the airflow from both turbochargers.
Core Type: Bar-and-plate intercoolers are generally more efficient than tube-and-fin designs.
Placement: Front-mount intercoolers (FMICs) are most common, but side-mount intercoolers (SMICs) may be necessary depending on space constraints.

Fuel System Upgrades

Increasing power requires a corresponding increase in fuel delivery. Essential upgrades include:

Fuel Injectors: Larger injectors are necessary to supply the increased fuel demand. Consider injectors with a flow rate of at least 550cc/min for the power levels discussed.
Fuel Pump: An upgraded fuel pump is essential to maintain adequate fuel pressure. A Walbro 255 LPH pump is a common and reliable choice.
Fuel Pressure Regulator: An adjustable fuel pressure regulator allows for fine-tuning of fuel delivery.

Engine Management System

The stock ECU is inadequate for managing a twin-turbo system. An aftermarket engine management system (EMS) is crucial for precise control of fuel, ignition timing, and boost. Popular choices include:

AEM EMS Series 2
Haltech Elite Series
Link ECU G4+

The EMS should be capable of handling boost control, air/fuel ratio monitoring, and datalogging.

Fabrication Process

Fabricating a twin-turbo system requires skilled welding and fabrication techniques.

Exhaust Manifolds

Custom exhaust manifolds are necessary to route exhaust gases to the turbochargers. These manifolds should be designed to minimize backpressure and maximize flow. Common materials include:

Mild Steel: More affordable but prone to rust.
Stainless Steel: More expensive but more durable and resistant to corrosion.

The manifolds should be carefully designed to ensure equal length runners, promoting even exhaust flow to each turbocharger. This helps ensure balanced boost and power delivery.

Turbocharger Mounting

The turbochargers must be securely mounted. Custom brackets may be required to position them properly. Consider factors such as:

Clearance: Ensure adequate clearance from the engine block, chassis, and other components.
Oil and Water Lines: Plan the routing of oil and water lines for turbocharger lubrication and cooling.
Heat Shielding: Protect surrounding components from excessive heat generated by the turbochargers.

Intercooler Piping

Custom intercooler piping is needed to connect the turbochargers to the intercooler and the intercooler to the throttle body. Aluminum piping is commonly used due to its lightweight and heat-dissipating properties.

Minimize Bends: Use smooth, gradual bends to minimize pressure drop.
Secure Connections: Use high-quality silicone couplers and T-bolt clamps to prevent leaks.
Blow-Off Valve (BOV): Install a BOV between the turbochargers and the throttle body to vent excess pressure during throttle lift, preventing compressor surge.

Oil and Water Lines

Proper oil and water lines are essential for turbocharger longevity. Use high-temperature, braided stainless steel lines with AN fittings.

Oil Supply: Tap into the engine's oil supply for turbocharger lubrication.
Oil Drain: Return oil to the oil pan via a gravity drain. Ensure the drain line is large enough to prevent oil from backing up in the turbocharger.
Water Cooling: Connect the turbocharger's water jackets to the engine's cooling system.

Tuning and Calibration

Tuning a twin-turbo system is a complex process that requires specialized knowledge and equipment. It is highly recommended to consult with a professional tuner experienced in tuning boosted engines.

Initial Setup

Before tuning, ensure that all components are properly installed and functioning correctly. Check for leaks and verify that the engine is in good mechanical condition.

Base Map

Start with a base map provided by the EMS manufacturer or a tuner experienced with similar setups. The base map will provide a starting point for fuel, ignition timing, and boost control.

Dyno Tuning

Dyno tuning is the most effective way to optimize the engine's performance. The tuner will use a dynamometer to measure horsepower, torque, and air/fuel ratio under various load conditions.

Boost Control

A boost controller is used to regulate the amount of boost produced by the turbochargers. Electronic boost controllers offer more precise control than manual boost controllers. The boost controller should be properly configured to prevent overboost and engine damage.

Air/Fuel Ratio

Maintaining a proper air/fuel ratio is crucial for engine health. Aim for a slightly rich air/fuel ratio under boost to prevent detonation. A wideband oxygen sensor is essential for monitoring air/fuel ratio in real-time.

Ignition Timing

Optimizing ignition timing can improve power and efficiency. However, advancing timing too far can lead to detonation. Retard timing as boost increases to prevent knock.

Datalogging

Datalogging is essential for monitoring engine performance and identifying potential problems. Monitor parameters such as air/fuel ratio, boost pressure, ignition timing, coolant temperature, and oil pressure.

Conclusion

Fabricating and tuning a twin-turbocharger system for a Nissan 240SX is a challenging but rewarding project. Careful planning, proper component selection, skilled fabrication, and professional tuning are essential for success. While the complexity is significant, the potential for increased power and performance makes it an attractive option for serious enthusiasts. Always prioritize safety and consult with experienced professionals throughout the process. This is not a beginner project, and extensive research and preparation are crucial.