Understanding Twin-Scroll and Compound Turbocharging

Achieving maximum power from a forced induction system often requires moving beyond a single turbocharger. The Mishimoto Twin-Scroll Compound Turbo System combines two advanced technologies: twin-scroll turbine housings and a compound (series) turbo arrangement. A twin-scroll design separates exhaust pulses from the engine’s cylinders to reduce interference and improve spool. In a compound setup, a small high-pressure turbo feeds a larger low-pressure turbo, dramatically increasing boost pressure and airflow without sacrificing responsiveness. This system is ideal for high-horsepower diesel and gasoline engines that need strong low-end torque and top-end power. Mishimoto’s kit is engineered for specific vehicles, typically heavy-duty trucks and high-performance street builds, offering precision-machined components and all necessary hardware.

Before diving into installation, it’s important to understand that this upgrade significantly changes your engine’s air and fuel requirements. Proper tuning is essential to realize the full benefit and to keep the engine reliable. This guide provides a comprehensive walkthrough, but always consult the official Mishimoto instructions for your specific vehicle fitment.

Tools and Materials – What You’ll Need

Having the right tools on hand is critical to a smooth installation. The Mishimoto kit includes the turbochargers, wastegates, piping, gaskets, and hardware, but you will need standard shop tools and consumables. Below is a detailed list, including items that are often overlooked.

Essential Hand Tools

  • Socket Set: ¼-inch and ⅜-inch drives with metric and SAE sockets. Most turbo fasteners are metric, but some brackets may use standard sizes.
  • Wrench Set: Combination wrenches (open-end and box-end) in sizes 8 mm to 19 mm. A line wrench is helpful for oil lines.
  • Screwdrivers: Flathead and Phillips head in multiple lengths.
  • Pliers: Needle-nose, slip-joint, and a set of hose clamp pliers.
  • Torque Wrench: A click-type torque wrench capable of reading in inch-pounds and foot-pounds (range: 20-150 ft-lb).
  • Hex (Allen) Keys: For any set screws or banjo bolts.

Consumables and Safety Gear

  • Engine Oil: High-quality synthetic or conventional oil meeting your engine’s specifications. You will need enough for a full oil change plus a bit extra to prime the turbo.
  • Coolant: Proper coolant mixture for your vehicle. Drain and refill the cooling system during the installation.
  • Gasket Maker: High-temperature RTV silicone gasket maker for flanges that don’t have a dedicated gasket. Use one rated for exhaust heat.
  • Safety Glasses: Protect your eyes from debris, oil, and coolant.
  • Mechanics Gloves: Nitrile or latex gloves protect your hands from fluids and sharp edges.
  • Shop Rags and Drain Pans: For catching oil and coolant, plus cleaning surfaces.
  • Penetrating Oil (e.g., PB Blaster): For rusted bolts on older vehicles.

Preparation – Setting Up for Success

Proper preparation is the difference between a weekend job and a multi-day nightmare. Start by parking the vehicle on a level surface and ensuring it is securely supported on jack stands if you need to work underneath. Disconnect the battery negative terminal to prevent accidental shorts or engine starts while working.

Drain Fluids

To avoid a messy cleanup, drain the engine oil and coolant before removing any turbo components. Place a drain pan under the oil pan and remove the drain plug. Similarly, open the radiator petcock or lower radiator hose to drain coolant. Dispose of fluids according to local regulations. If the vehicle has been running, wait until the engine is cool enough to touch before draining to avoid burns.

Remove Obstructing Components

The turbo area is often crowded. Remove the following items to create clear access:

  • Intake airbox and intake tubing
  • Exhaust manifold heat shields (if equipped)
  • Any intercooler piping that crosses the engine bay
  • Radiator fan shroud (on some vehicles, it may be easier to remove the whole fan assembly)
  • Engine covers and decorative trim
  • Bolts holding the old turbo and manifolds at the exhaust flange

Keep all bolts and brackets organized in labeled bags. Taking photos before disassembly can be a lifesaver during reassembly.

Removing the Old Turbo System

Now that you have clear access, it’s time to remove the existing turbo or turbos. This process assumes you are replacing a single turbo or a simpler compound setup. Work slowly and methodically to avoid damaging surrounding components.

Step 1 – Unbolt the Exhaust Manifold

Beginning at the cylinder head, carefully unbolt the exhaust manifold studs or bolts. If the manifold is rusted, apply penetrating oil and wait 10–15 minutes. Remove the manifold and set it aside. You may need to remove the heat shield first.

Step 2 – Disconnect Oil and Coolant Lines

Locate the oil feed line (usually from the engine block) and the oil return line (typically a larger line going to the oil pan). Also find any coolant lines that feed the turbo. Use a line wrench to prevent rounding the fittings. Plug the open ends with clean rags or plastic caps to keep dirt out.

Step 3 – Remove the Old Turbocharger

With the exhaust manifold off and lines disconnected, unbolt the turbo from its mounting bracket or the manifold itself. Lift it straight out, being careful of the downpipe connection. You might need to loosen the downpipe flange bolts first. Remove the old gaskets and thoroughly clean the mounting surfaces on the engine block and exhaust manifold flange using a razor blade or gasket scraper. Any leftover gasket material will cause leaks with the new gaskets.

Step 4 – Clean and Inspect

Use brake cleaner and a rag to wipe down the engine mounting area. Inspect the oil drain tube and the block for any debris. If the old turbo failed, consider adding an inline oil filter to protect the new turbos. Check the condition of the exhaust studs; replace any that are damaged or stripped.

Installing the Mishimoto Twin-Scroll Compound Turbo System

This is the main event. The Mishimoto kit is designed for a direct fit, but attention to detail is crucial. Follow the kit’s specific instructions for your application, as the exact orientation may vary.

Position and Secure the Primary (High-Pressure) Turbo

Start by placing the smaller high-pressure turbo onto its mounting surface. Use new gaskets supplied in the kit. Hand-tighten the bolts, then torque them to the manufacturer’s specification (typically 30–40 ft-lb for M8 bolts, but verify with Mishimoto’s spec sheet). Do not over-tighten cast iron flanges—they can crack.

Attach the Oil Feed and Return Lines

Connect the braided stainless steel oil feed line from the engine block to the turbo center cartridge. Use new sealing washers (copper or aluminum crush washers) on banjo fittings. Tighten the banjo bolt to 15–20 ft-lb. Connect the oil return line (drain) from the turbo back to the oil pan, ensuring there are no kinks. The drain line must slope downward.

Install the Exhaust Manifold and Twin-Scroll Housing

Mount the new exhaust manifold (if included in the kit) or use the supplied adapter. The twin-scroll turbine housing has two separate inlet ports—one for cylinders 1–2–3 and another for 4–5–6 (or similar split). Ensure the manifold correctly routes the exhaust pulses to each scroll. Torque the manifold bolts in a crisscross pattern to ensure even pressure.

Mount the Secondary (Low-Pressure) Turbo

If your Mishimoto kit is a true compound setup, the second (larger) turbo mounts downstream. Secure it to its bracket using the provided hardware. Connect the outlet of the high-pressure turbo to the inlet of the low-pressure turbo with the crossover pipe included in the kit. Clamp all connections tightly with T-bolt clamps to handle high boost.

Connect Sensors and Electrical Connections

Many modern turbo systems include a boost control solenoid, wastegate actuator, and possibly a MAP sensor. Route the electrical harness carefully, avoiding hot exhaust components. Connect the wastegate vacuum lines as shown in the Mishimoto diagram. Mishimoto typically uses a dual wastegate setup for precise boost control. Use silicone vacuum lines and secure them with zip ties away from moving parts.

Connecting the Intake and Exhaust Systems

With the turbo assembly in place, it’s time to plumb the air intake and exhaust paths.

Intake Piping

Attach the large-diameter intake tube from the air filter to the compressor inlet of the low-pressure turbo (or directly to the high-pressure turbo depending on your system configuration). Ensure a smooth flow path with no sharp bends. Use silicone couplers and T-bolt clamps. If your kit includes a blow-off or bypass valve, mount it according to instructions.

Exhaust Downpipe

The downpipe connects the turbine outlet of the low-pressure turbo to the rest of the exhaust. Use the supplied gasket and bolts to attach it to the turbo. Torque to spec—M10 bolts typically 35–45 ft-lb. Make sure the downpipe doesn’t contact the frame or engine, as vibration can cause noise or cracking. Use the included hanger to secure it to the vehicle’s exhaust system.

Intercooler Piping

If you are using an intercooler (highly recommended for compound turbo setups), route the charge air piping from the compressor outlet to the intercooler, then from the intercooler to the throttle body. Use a boost leak tester to verify all connections are airtight before starting the engine.

Final Steps – Refill, Prime, and Test

Before cranking the engine, you must properly fill and prime the system.

Refill Fluids

Pour new engine oil into the crankcase. Use a high-quality oil that meets your engine’s specs; many compound turbo builds prefer 15W-40 diesel oil for its heat tolerance. Refill the cooling system with the proper coolant mixture. Bleed any air pockets by running the engine with the radiator cap off until the thermostat opens.

Prime the Turbo

To prevent dry start damage, disconnect the fuel pump or ignition system (pull the fuel pump relay or fuse) and crank the engine for 10–15 seconds until the oil pressure gauge shows reading. This forces oil into the turbo bearings. Alternatively, you can pre-fill the oil feed line with a small hand pump before starting.

Reconnect the Battery and Start

Reconnect the negative battery terminal. Start the engine and let it idle. Immediately check for oil, coolant, and exhaust leaks. Listen for unusual noises from the turbos. Let the engine reach operating temperature. Then, carefully tighten any clamps that may have loosened from thermal expansion.

Test Drive and Break-In

Take the vehicle for a gentle test drive. Avoid hard boost for the first 50–100 miles to allow the turbo bearings to seat. After the break-in period, gradually increase boost pressure as you verify the wastegate operation and boost gauge readings. Do not exceed the boost level recommended by Mishimoto for your kit (usually 35–50 psi for compound setups, depending on application).

Tuning Considerations – Unlocking Full Potential

A compound twin-scroll turbo system dramatically increases airflow, which requires recalibration of the engine’s fuel and timing maps. You must visit a professional tuner with experience in compound turbo systems. A custom tune will ensure proper air/fuel ratios, avoid detonation, and maximize power. Many tuners use stand-alone engine management or piggyback controllers. Expect to add larger fuel injectors and a high-flow fuel pump if you are pushing beyond 600–800 horsepower. For further reading on tuning, check out resources like HP Academy’s guide on compound turbocharging and Mishimoto’s engineering blog for setup tips.

Maintenance and Common Issues

After installation, regular upkeep is vital. Change oil more frequently (every 3,000–5,000 miles) due to the increased thermal load. Inspect oil lines for cracks and check all clamps monthly. Common issues include boost leaks from intercooler couplers, wastegate creep, and oil leaks from banjo fittings. If you notice a loss of boost, perform a boost leak test immediately. The twin-scroll design also requires that the exhaust manifold gaskets remain sealed—retorque them after the first heat cycle.

Always use OEM-quality gaskets and hardware to prevent catastrophic failure. Mishimoto provides high-quality hardware in the kit, but if you substitute parts, ensure they are rated for high temperatures and pressures.

Conclusion

Installing the Mishimoto Twin-Scroll Compound Turbo System is a rewarding project that can transform your vehicle’s performance. By following this detailed step-by-step guide—preparing properly, removing the old system carefully, installing the new turbos with precision, and tuning appropriately—you will achieve reliable, massive power gains. Remember that compound turbocharging demands respect; monitor boost levels, oil pressure, and exhaust gas temperatures. With proper installation and maintenance, this system will deliver exhilarating acceleration and towing capability for years to come. For additional support, the Mishimoto technical support team can answer questions specific to your vehicle kit. Happy boosting!