Introduction

Upgrading your N55 engine with the Aerocharger Stage 2 turbo kit is one of the most effective ways to unlock significant horsepower and torque gains. Unlike conventional turbochargers, Aerocharger’s variable-vane technology provides near-instant spool and broad power delivery, making it a popular choice among BMW enthusiasts. This guide provides a comprehensive, step‑by‑step walkthrough of the entire installation process, from preparation through post‑install testing. Whether you are an experienced DIY mechanic or a motivated enthusiast, following these instructions will help you complete the swap safely and correctly.

Understanding the Aerocharger Stage 2 N55 Kit

The Aerocharger Stage 2 kit is designed as a direct replacement for the factory twin‑scroll turbocharger on N55 engines found in 335i, 435i, and similar BMW models. Key features include a larger compressor wheel, a billet wastegate actuator, and an oil‑less bearing cartridge that simplifies installation by eliminating the need to tap into the engine’s oil system. The kit typically includes the turbocharger assembly, a cast aluminum intake pipe, a stainless steel downpipe adapter, hardware, and gaskets. Before beginning, verify that your kit is complete and that you have any required tuning software (such as Bootmod3, MHD, or a custom flash tune) on hand – the Stage 2 upgrade requires a custom calibration to run safely at higher boost levels.

Tools and Materials Needed

Gathering the correct tools before you start will save time and prevent mid‑job frustration. Many of these tools are standard for a turbo swap, but a few are specific to the N55 platform.

  • Socket and ratchet set – ¼‑ and ⅜‑drive, with deep and shallow sockets (metric: 8mm, 10mm, 13mm, 15mm, 18mm, and E‑Torx sockets E8, E10, E12).
  • Torque wrench – capable of both Nm and ft‑lbs ranges (0‑150 ft‑lbs) for fastener tightening.
  • Allen/hex keys – 4mm, 5mm, 6mm for clamps and some hardware.
  • Screwdrivers – flathead and Phillips (JIS crosshead preferred).
  • Pry bar or long flathead screwdriver for separating stubborn connections.
  • Pick set for removing o‑rings and clips.
  • Oil catch can (optional but recommended) – to prevent oil blow‑by from entering the intake system.
  • Jack and jack stands or a lift – you’ll need to work under the vehicle for the downpipe and oil drain lines.
  • Coolant drain pan and container – you will lose some coolant when disconnecting turbo lines.
  • Thread locker (medium strength – blue Loctite) for bolts subject to vibration.
  • Gaskets and seals – even if your kit includes them, consider replacing the exhaust manifold gasket, turbo gasket, and o‑rings for the oil and coolant lines.
  • Shop manual for N55 engine – handy for torque specs and component locations.
  • Safety goggles, gloves, and shop towels – engine bay work is dirty and potentially caustic.

Preparation Steps

Proper preparation reduces the risk of mistakes and ensures a clean install. Begin with the vehicle on level ground, engine completely cool, and battery disconnected.

Disconnect the Battery

Using a 10mm socket, loosen the negative terminal clamp and remove the lead cable. Wrap the terminal in a rag or tape to prevent accidental contact. This step eliminates the risk of shorting electrical connections during the removal process and resets the ECU adaptations for the upcoming tune.

Drain Coolant and Oil (If Needed)

While the turbo swap does not require a full oil drain, you will disconnect coolant hoses from the factory turbo. Place a drain pan under the driver‑side front of the engine and carefully loosen the coolant expansion tank cap to relieve pressure. Disconnect the lower coolant hose at the turbo and allow fluid to drain. Some builders also prefer to change the engine oil at this point to ensure clean lubrication for the new turbo. If you do, drain the oil through the pan plug and replace the filter.

Remove Engine Cover and Air Intake System

Unclip the engine beauty cover by pulling upward; it is held by four rubber grommets. Next, remove the factory air filter housing – unscrew the T‑clamp at the turbo inlet and the screws securing the box. Disconnect the MAF sensor connector and set the entire intake assembly aside. You will also need to remove the charge pipe that runs from the turbo outlet to the intercooler; it is held by a spring clip at the throttle body and a T‑clamp at the turbo. Slide it off carefully – it may be stuck due to heat and age.

Remove Heat Shields and Brackets

The N55 engine bay contains a large turbo heat shield bolted to the exhaust manifold. Using an E10 socket, remove the five or six bolts securing the shield and lift it out. There may also be a small bracket supporting the oxygen sensor wiring – unclip it and move the harness aside. If your car is equipped with a factory water‑cooled wastegate actuator, note its routing; the Aerocharger kit uses a different actuator that requires its own mounting bracket.

Removing the Stock Turbocharger

With the engine bay cleared, you can now access the factory turbocharger. Take your time – the fasteners are often tight and rusted.

Disconnect Exhaust Components

From underneath the vehicle, support the exhaust system with a jack or stand. Unbolt the downpipe from the turbo outlet – three 13mm bolts connect the downpipe to the exhaust housing. You may need to spray penetrating oil on these if they are seized. Separate the downpipe from the rest of the system (V‑band clamp at the catalytic converter) and remove it entirely. This step provides clearance to drop the turbo downward.

Remove Oil and Coolant Lines

Locate the oil feed line entering the top of the turbo (a banjo bolt with a copper washer) and the oil return line at the bottom (a rubber hose clamped to a steel pipe). Use a 17mm wrench to loosen the banjo bolt – have a rag ready to catch residual oil. Disconnect the coolant inlet and outlet hoses at the turbo using a pick to release the spring clips. Cap all open ports to prevent debris from entering.

Unbolt the Turbo from the Exhaust Manifold

With all lines free, remove the four 15mm nuts that secure the turbo to the exhaust manifold. There is also a support bracket connecting the turbo to the engine block – one bolt on the turbo side, one on the block side (E12 socket). Carefully wiggle the turbo free from the manifold studs and lower it out from the top or bottom, depending on your preference. For most N55 chassis, pulling it upward is easier if you tilt the unit slightly. Watch for the manifold gasket – it may stick to the manifold and need to be scraped off.

Installing the Aerocharger Stage 2 Turbo

Before installing, compare the new turbo’s ports with the factory lines. The Aerocharger’s oil‑less bearing design means you will not route an oil feed line; instead, the turbo is self‑lubricated. However, you still need to connect coolant lines (if equipped) and the intake/exhaust systems.

Pre‑Installation Checks

Inspect all supplied gaskets and fasteners. Apply a light coat of anti‑seize to the threads of manifold studs to prevent future seizing. Verify that the wastegate actuator arm moves freely and that the wastegate flap seats tightly against the housing – if not, adjust the actuator rod length per Aerocharger’s instructions.

Mount the Turbo to the Exhaust Manifold

Place a new manifold gasket onto the studs. Lift the Aerocharger turbo into position – it may require slight persuasion. Slide the turbo onto the studs and hand‑tighten the four 15mm nuts. Do not torque them yet. Reattach the turbo support bracket to the block and the turbo housing (use blue Loctite on the bracket bolts). Now torque the manifold nuts to 28 Nm (21 ft‑lbs) in a criss‑cross sequence. Recheck the bracket bolts at 22 Nm (16 ft‑lbs).

Connect Coolant Lines

If your Aerocharger kit includes coolant circulation (some models do for heat management), reconnect the coolant hoses using new o‑rings. Ensure the hose clamps are secure and positioned so they do not rub against other components. For kits without coolant connections, simply cap the engine coolant ports with appropriate plugs.

Attach Intake and Exhaust Pipes

Use the supplied cast aluminum intake pipe to connect the air filter to the turbo inlet. Tighten the T‑clamp uniformly. For the exhaust side, install the downpipe adapter (provided) onto the turbo exhaust housing using the included v‑band clamp. Torque the clamp to 8 Nm (6 ft‑lbs). Then reconnect the factory or aftermarket downpipe to the adapter, again using new gaskets.

Reconnect Sensors and Electrical Components

Plug in the oxygen sensor(s) into their respective ports – the Aerocharger kit usually includes a bung for the wideband sensor. For vehicles requiring a narrowband sensor, verify that the location is correct. Reattach any grounding straps or additional wiring harnesses that were moved aside.

Reassembly and Final Checks

With the turbo installed, reverse the removal steps for the remaining components. This is a good time to install the optional oil catch can – mount it in the PCV line to prevent oil mist from coating the turbo vanes.

Reinstall the Engine Cover and Air Intake

Place the engine beauty cover back onto the grommets and press down firmly. Install the charge pipe from the turbo outlet to the intercooler, ensuring the o‑ring is seated and the spring clip engages with an audible click. Reattach the air filter housing and MAF sensor.

Refill Coolant and Prime the System

Fill the cooling system with the correct coolant mixture (BMW phosphate‑free or equivalent). Run the engine with the heater on high to purge air. After a few minutes, top up the coolant reservoir. Because the Aerocharger uses oil‑less bearings, there is no need to prime an oil line – but you should crank the engine with the fuel pump relay removed for 10 seconds to build oil pressure before the first start.

Reconnect the Battery and Check for Leaks

Reattach the battery negative terminal. Start the engine – allow it to idle for 15‑30 seconds, then inspect all turbo connections, coolant hoses, and intake piping for leaks. Listen for unusual whistles or exhaust leaks.

Testing and Tuning

The Aerocharger Stage 2 kit requires a custom tune to run safely at elevated boost. Do not drive the car hard until the ECU has been reflashed. Use a reliable tuning platform such as Bootmod3 or MHD to flash a Stage 2 map tailored to your fuel (91/93 octane or E85 blend).

Initial Drive and Data Logging

After the flash, take the car on a short drive. Keep boost below 15 psi for the first 50 miles to allow the turbo to break in. Monitor boost pressure, air‑fuel ratio, and intake air temperatures via your logging tool. Ideally, verify that the wastegate duty cycle is appropriate and that boost targets are being met without overshoot. If you notice knock activity or fuel trims that exceed 15%, pull over and adjust the tune or consult your tuner.

Boost Leak Test

A small boost leak can drastically reduce performance. Pressurize the intake system to 20 psi using a boost leak tester (available at any parts store). Listen for hissing sounds; common leak points include the charge pipe connections, throttle body gasket, and diverter valve mounting. Fix any leaks before continuing.

Maintenance and Long‑Term Care

With the added power, your N55’s maintenance schedule becomes more critical. Change engine oil and filter every 5,000 miles using a high‑quality synthetic that meets BMW LL‑01 specifications. Inspect the turbo coolant hoses and clamps annually. Since the Aerocharger does not rely on engine oil for lubrication, you can extend the life of the turbo by avoiding frequent heat‑soak shutdowns – always let the engine idle for 30‑60 seconds after a hard drive. If you installed a catch can, empty it during every oil change.

Potential Challenges and Troubleshooting

Even experienced builders encounter issues. Here are common pitfalls with this swap:

  • Wastegate adjustment: If boost is too low or too high, the actuator rod may need a half‑turn adjustment. Use a vacuum pump to verify that the wastegate opens at the specified pressure (typically 5‑8 psi).
  • Oil leaks from coolant line plugs: Use proper copper or aluminum sealing washers on any capped ports.
  • Inlet pipe rubbing: The cast intake pipe may contact the engine strut tower – you can wrap it with high‑heat silicone tape to prevent abrasion.
  • Tune compatibility: Not all off‑the‑shelf maps account for the unique spool characteristics of an Aerocharger. Custom calibration from a reputable tuner is strongly recommended. Aerocharger’s official website lists recommended tuning partners.

Conclusion

The Aerocharger Stage 2 N55 kit is a high‑quality upgrade that transforms the driving experience of your BMW. While the installation is achievable for a well‑prepared DIYer, it demands patience, attention to detail, and a willingness to learn the nuances of turbocharging. By following this comprehensive guide, you can avoid the most common mistakes and enjoy a smooth installation. Always consult your vehicle’s factory service manual and the kit’s instructions, and do not hesitate to contact BMW enthusiast forums for community support. When in doubt, enlisting a professional shop with N55 experience is money well spent. With the new turbo installed and a proper tune, you will unlock the full potential of your N55 engine – reliable, strong, and exhilarating.