Understanding Twin Scroll Turbo Systems

Twin scroll turbochargers split the exhaust flow from an engine into two separate, distinct scrolls inside the turbine housing. Each scroll is fed by a specific set of cylinders, typically arranged to take advantage of the engine’s firing order and exhaust pulse separation. For example, on a four-cylinder engine with a 1-3-4-2 firing order, cylinders 1 and 4 might feed one scroll, while cylinders 2 and 3 feed the other. This pulse separation prevents reversion and keeps the exhaust energy directed at the turbine wheel in a more continuous, high-velocity stream.

The primary advantage over a single-scroll turbocharger is a significant reduction in turbo lag. Because each scroll sees a more constant flow of exhaust gas, the turbine wheel spools faster, especially at low RPM. Twin scroll systems also improve volumetric efficiency and can increase top-end power by reducing backpressure in the exhaust manifold. This makes them a favorite in high-performance builds, from street cars to dedicated track machines.

However, the true potential of a twin scroll system is unlocked when the turbo internals are upgraded to handle higher boost levels, greater airflow, and more extreme operating conditions. The stock components are often a compromise between cost, durability, and performance. Upgrading them transforms a capable factory turbo into a precision high-performance unit.

Why Upgrade Turbo Internals?

Factory turbochargers are designed for reliability under normal driving conditions. When you push them beyond their intended boost pressure or airflow limits, internal components become the bottleneck. Upgrading the internals delivers several measurable benefits:

  • Increased boost capacity: Stronger compressor and turbine wheels can withstand higher RPM and pressure ratios without fatigue or failure.
  • Faster spool and response: Lighter, more aerodynamic wheels and low-friction bearings reduce rotational inertia, helping the turbo reach operating speed sooner.
  • Higher peak power: Improved airflow through the compressor and exhaust side allows the engine to produce more horsepower and torque.
  • Enhanced reliability under stress: Ceramic bearings, upgraded seals, and stronger wastegate actuators prevent premature wear and failure during extended high-load operation.
  • Better exhaust flow management: High-flow turbine wheels and ported housings reduce backpressure, allowing the engine to breathe more freely at high RPM.

These upgrades are especially valuable in twin scroll systems, where exhaust pulse timing is critical. A well-matched set of internals can improve spool by hundreds of RPM and add 50–100+ wheel horsepower on a typical performance build.

Key Internal Components to Upgrade

When planning a twin scroll turbo upgrade, focus on the components that directly affect airflow, boost control, and durability. Here’s a detailed breakdown of each area.

Compressor Wheel

The compressor wheel is the first stage of air induction. Upgrading to a larger diameter or a more efficient blade design (such as billet aluminum 7075 or extended-tip technology) increases the mass airflow (CFM) the turbo can deliver. A wheel with a higher trim ratio can also improve efficiency at high boost levels. Beware of oversizing: an excessively large compressor wheel may cause surge at low boost or slow spool. Work with a turbo specialist to choose a wheel that matches your engine’s displacement and target power.

Turbine Wheel

The turbine wheel extracts energy from the exhaust to drive the compressor. For twin scroll applications, the wheel’s blade shape and material are crucial. A high-flow Inconel 713C or Mar-M-247 turbine wheel can handle higher exhaust gas temperatures (EGTs) and resist creep. Additionally, a wheel with a smaller exducer diameter or a higher blade count can reduce backpressure while maintaining good response. The A/R (area/radius) ratio of the turbine housing also interacts with the wheel – a lower A/R housing spools faster but can choke top-end power. Upgrading the turbine wheel often requires a matching housing modification or replacement.

Bearings and Journal

Stock journal bearings work well at moderate speeds but become a friction and heat source at high RPM. Upgrading to a ceramic hybrid bearing (also called a ball bearing cartridge) reduces friction by up to 50% compared to journal bearings. This results in significantly faster spool and improved transient response. Many performance turbo kits use a Garrett GTX or GBC ball bearing cartridge for twin scroll setups. When upgrading, ensure the cartridge is properly sized for your turbo’s center housing and that oil feed and return lines are compatible.

Wastegate and Actuator

Twin scroll turbochargers often use a dual-port wastegate or two separate wastegates to relieve excess exhaust pressure from each scroll independently. Upgrading to a high-performance external wastegate (e.g., Turbosmart or Tial) with a larger valve seat or a stronger spring allows for precise boost control at high pressures. The actuator must have sufficient stroke and a robust diaphragm to resist heat and fatigue. Proper wastegate setup prevents boost creep and overspeed, which can destroy a turbine wheel.

Seals and Gaskets

Leaks around the turbine flange, compressor outlet, or oil return line can cause boost loss and oil starvation. Upgrade all gaskets to multi-layer steel (MLS) or copper rings where applicable. Use high-temperature RTV silicone on oil pans and block off plates. For the center housing, consider upgrading to a viton oil seal for better heat resistance.

Compressor Housing and Inlet

Porting the compressor housing to remove casting flash and match the wheel’s inducer diameter reduces turbulence and increases flow. Some aftermarket housings feature a larger or more aerodynamic bell-mouth inlet for smoother air entry. A ported shroud (anti-surge) can also help prevent compressor surge at part-throttle high boost conditions.

Performance Gains and Expected Outcomes

A well-executed upgrade to twin scroll turbo internals can transform a car’s powerband. On a typical 2.0L four-cylinder, swapping a stock journal-bearing turbo for a ball-bearing unit with a 10% larger compressor and turbine wheel can shift spool torque from 4000 RPM to 3500 RPM, while adding 80–100 wheel horsepower up top. On larger engines (e.g., 3.0L inline-six), the gains are even more pronounced. The reduction in backpressure often allows the engine to hold boost longer, improving high-RPM power recovery.

Dyno tests of twin scroll turbo upgrades by reputable shops show peak power increases of 15–30% with proper tuning. The response feels snappier, and the lag “hole” that many single-scroll turbos exhibit is virtually eliminated. These results come from optimizing the entire air path, not just replacing one part.

Step-by-Step Upgrade Process from Nashville Performance

Nashville Performance recommends the following sequence for upgrading twin scroll turbo internals. The process assumes the turbo is removed from the vehicle and thoroughly cleaned.

  1. Disassembly: Remove the compressor cover, turbine housing, and center housing. Mark the orientation of the compressor and turbine wheels to maintain balance reference. Clean all parts in a solvent bath to remove carbon deposits and oil residue.
  2. Inspect and measure: Check the shaft for straightness, wheel blades for damage, and bearing journals for wear. Measure clearances against factory specifications. Replace any parts that exceed tolerances.
  3. Install new bearings: If upgrading to a ball bearing cartridge, install it according to the manufacturer’s instructions, including proper shimming and axial preload. For journal bearings, install new bushings and thrust washers with the correct end float.
  4. Attach new wheels: Press-fit (or key) the new compressor and turbine wheels onto the shaft. Use a precision torque wrench for any locknuts. Never overtighten – this can distort the shaft.
  5. Balance assembly: A high-speed dynamic balancer is essential to balance the rotating assembly. Unbalanced wheels will cause vibration, seal failure, and bearing destruction. This step must be performed by a professional turbo shop.
  6. Reassemble with upgraded seals and gaskets: Install the center housing, then attach the turbine housing and compressor cover. Use new gaskets at every junction. Torque all bolts to spec in a crisscross pattern.
  7. Install upgraded wastegate: Mount the wastegate to the turbine housing or external manifold. Adjust the preload per the boost target. Use a pressure source to verify the actuator opens and closes smoothly.
  8. Final leak test: Pressurize the oil feed and water lines (if applicable) to check for leaks. Rotate the shaft by hand – it should spin freely without binding.

Common Mistakes to Avoid

  • Skipping balance: An unbalanced wheel will destroy the turbo within minutes. Always use a professional balancing service.
  • Wrong wheel material: Cast wheels are cheap but cannot handle high EGTs. Use billet aluminum or Inconel as appropriate.
  • Ignoring compressor surge: A twin scroll turbo can surge at part throttle if the compressor wheel is too large or the boost control is poor. Use anti-surge housing or a blow‑off valve.
  • Improper oil feed: Ball bearing cartridges require a restricted oil feed (usually a 0.035” orifice). Too much pressure can blow out seals.
  • Neglecting tuning: Upgraded internals change the airflow map. The engine must be retuned for ignition timing, fuel delivery, and boost control to avoid detonation.
  • Overlooking exhaust backpressure: A high-flow turbine wheel does not compensate for a restrictive exhaust manifold or downpipe. Ensure the entire exhaust path is free‑flowing.

Tools and Equipment You’ll Need

For a proper upgrade, you will need:

  • Precision micrometer and dial indicator
  • Snap‑ring pliers for C‑clips
  • Torque wrench (in‑lb for small fasteners)
  • Press (for wheel installation)
  • High‑speed dynamic balancer
  • Ultrasonic cleaner or solvent tank
  • Air compressor for leak testing
  • Turbo rebuild kit (gaskets, seals, o‑rings)
  • External wastegate (if not already installed)
  • Heat shielding for lines and actuators

If you lack access to a balancer or a press, it’s best to send the core to a reputable shop. Nashville Performance offers assembly and balancing services for twin scroll turbo builds.

Post-Installation and Tuning Considerations

After installing the upgraded turbo, follow these guidelines:

  • Prime the oil system: Crank the engine with the fuel pump relay disconnected until oil pressure registers on the gauge. This prevents dry start.
  • Start and check for leaks: Run the engine at idle, inspecting all connections. Raise the RPM to test boost control.
  • Break‑in period: Drive lightly for the first 100 miles to seat bearings and seals. Avoid sustained boost until clearances settle.
  • Data log: Use a wideband O2 sensor and boost gauge to verify air‑fuel ratios and boost targets. Adjust the wastegate spring or electronic boost controller as needed.
  • Schedule a professional dyno tune: A performance upgrade of this magnitude requires custom calibration. In addition to power gains, a good tune prevents detonation and ensures reliability.

For further technical details, consult Garrett Motion’s turbo technology resource and Turbosmart’s wastegate guide.

Conclusion

Upgrading the internal components of a twin scroll turbocharger is one of the most effective ways to increase engine performance and responsiveness. By focusing on the compressor and turbine wheels, bearings, wastegate, and seals, you can tailor the turbo’s characteristics to your specific power goals. The key to a successful build lies in careful selection of matching components, professional assembly and balancing, and proper engine tuning. Follow Nashville Performance’s guide for a reliable, high‑performance twin scroll upgrade that will transform your vehicle’s power delivery.