M135i F20/F21 Tuning Guide (N55)
The BMW M135i F20/F21 with the N55 engine is a fantastic platform for tuning, offering strong performance potential while maintaining everyday usability. The N55’s robust design, combined with a wealth of aftermarket support, allows owners to significantly enhance their car’s power and handling. This tuning guide covers the best upgrades and tips for maximizing the performance of your M135i while ensuring reliability.
Understanding the N55 Engine for Tuning
The N55 is a 3.0-liter turbocharged inline-six engine equipped with a single twin-scroll turbocharger. Known for its smooth power delivery, it responds well to modifications. However, like any turbocharged engine, proper supporting upgrades and regular maintenance are critical when increasing power to avoid premature wear or damage.
Stage 1: Basic Tuning
A Stage 1 tune typically involves software modifications without requiring hardware upgrades.
ECU Remap: The first step for any tuning project is a remap or flash tune. A high-quality tune can safely increase power from the stock 320 hp to approximately 380 hp, with improved throttle response and torque delivery. Reputable tuners like BM3 (Bootmod3), MHD, or Cobb Tuning provide reliable options.
Air Intake: Upgrading to a high-flow air intake system improves airflow to the engine. While gains are modest, this can enhance throttle response and provide a sportier induction sound. JXL Performance recommends the AIRTEC Motorsport Induction kit for BMW N55
Spark Plugs and Coils: Upgrade to colder spark plugs (such as NGK 97506) and replace ignition coils if necessary. These help handle the increased boost levels and prevent misfires. JXL Performance recommends upgrading to the MMX High Performance Ignition Coilpacks for BMW N55
Stage 2: Supporting Modifications
A Stage 2 setup involves hardware upgrades to support higher boost pressures.
Downpipe: Replacing the factory catalytic converter with a high-flow or catless downpipe reduces exhaust backpressure, increasing turbo efficiency and power. Note that catless options may not comply with emissions regulations. JXL Performances recommends the Scorpion downpipes for the N55 - Check out EWG options here - Check out PWG options here
Intercooler: An upgraded intercooler is essential to manage the increased heat generated by higher boost pressures. A larger, front-mounted intercooler ensures consistent performance during spirited driving. JXL Performance recommends the AIRTEC Motorsport Intercooler Upgrade for N55
Charge Pipe: The factory plastic charge pipe is a weak point in the N55. Upgrading to a durable aluminum charge pipe prevents cracking under higher boost pressures. JXL Performance recommends the MMX Charge Pipe Kit - Intake Side | Turbo Side
Exhaust System: A cat-back exhaust system enhances sound and may provide modest power gains by reducing backpressure. Look for systems that match your desired noise level. JXL Performance recommends the Milltek Sport F20 Cat-Back Exhaust.
Fuel System: While not always necessary at this stage, ensuring the high-pressure fuel pump (HPFP) is in good condition is critical for reliability.
Stage 3: Advanced Tuning
A Stage 3 setup pushes the N55 engine further with additional upgrades.
Turbocharger Upgrade: Replacing the factory turbo with a hybrid or larger unit significantly increases power potential. Common options include Pure Stage 2 or Vargas GC turbos, which can deliver power outputs of 450-500 hp with proper tuning.
Fuel System Upgrades: To support higher power levels, consider upgrading the fuel pump and injectors. Ethanol (E85) compatibility also requires flex-fuel sensors and tuning adjustments. A common modification is to fit the B58TU fuel pump from the A90 Toyota Supra, please note this also requires remapping to accomodate the extra fuel flow.
Methanol Injection: A methanol injection kit can improve cooling, prevent detonation, and enable higher boost levels safely.
Custom Tuning: At this level, a custom tune tailored to your specific modifications is essential to optimize performance and reliability.
Handling and Suspension Upgrades
Power is only part of the equation. Enhancing the handling and braking capabilities of your M135i is crucial for a balanced driving experience. The F20/F21 chassis from factory is more tailored towards performance than handling, so upgrades in this area are ESSENTIAL!
Suspension: Upgrading to coilovers or performance springs improves cornering and ride quality. Adjustable suspension components allow you to fine-tune the setup for track or street use. JXL Performance recommends the Bilstein B14 Street Performance Coilover Kit
Brakes: Install performance brake pads, stainless steel brake lines, and high-temperature brake fluid to improve stopping power. For aggressive driving, consider upgrading to a big brake kit.
For the Big Brake Kit, JXL Performance recommends the V-Maxx 355mm Big Brake Kit (TUV Approved)
If you don't want to upgrade to a BBK, consider the Full Car Brake Upgrade by JXL Performance
Tires and Wheels: High-performance tires enhance grip and handling. Lightweight aftermarket wheels reduce unsprung weight, improving acceleration and suspension responsiveness. There are a lot of options for these, we personally use ProTrack One wheels paired with Michelin PS5 on our M240i.
Tuning Tips for Reliability
Regular Maintenance: Increased power puts additional stress on engine components. Stay on top of oil changes, spark plug replacements, and other routine maintenance.
Monitor Engine Health: Invest in a quality OBD-II scanner or logging software to monitor key parameters like oil temperature, boost pressure, and air-fuel ratios.
Cooling: Ensure the cooling system is in excellent condition. Overheating can lead to engine damage, particularly with aggressive driving or track use.
Professional Installation and Tuning: Work with reputable shops and tuners to ensure modifications are installed and calibrated correctly.
Considerations for Modified M135i Ownership
While tuning unlocks incredible potential, it may also impact reliability, fuel economy, and emissions compliance. Insurance premiums and warranty coverage could be affected. Ensure you understand the implications before modifying your car.
Check out the M135i Buyers Guide, or Maintenance Guide here
Conclusion
The BMW M135i F20/F21 with the N55 engine is an excellent platform for tuning, offering significant performance gains with a variety of aftermarket upgrades. Whether you aim for a mild power boost or a fully built project, balancing performance with reliability is key. By choosing quality components, performing regular maintenance, and working with trusted professionals, you can transform your M135i into a high-performance machine tailored to your driving style.