The Mercedes-AMG A45 is already a formidable compact performance machine, but the thirst for more power never truly ends. For serious enthusiasts, swapping the stock turbocharger for a larger, more capable unit is one of the most effective ways to unlock the engine's true potential. The Precision Turbo 5857 A/R .63 has become a popular choice for A45 owners seeking a significant yet manageable upgrade. In this guide, we’ll walk through the reasons for upgrading, a detailed installation process, and what kind of power gains you can realistically expect when using this turbocharger on the M139 engine.

Why Upgrade to the Precision Turbo 5857 A/R .63?

The factory twin-scroll turbocharger on the M139 is engineered for a broad powerband, but it runs out of steam at higher RPMs and higher boost levels. The Precision Turbo 5857 A/R .63 offers a larger compressor wheel and turbine housing optimized for increased flow capacity. Here’s what makes it a compelling upgrade:

  • Higher Flow Capacity: The 5857’s compressor is designed to move significantly more air than the stock unit. This directly translates to the ability to make more horsepower, provided the fuel system and engine internals can handle it.
  • Improved Thermal Efficiency: Precision Turbo’s billet wheel technology reduces turbulence and heat soak, allowing for more consistent power delivery during extended pulls or track sessions.
  • Better Mid-Range and Top-End Power: While the stock turbo is punchy low down, the 5857 shifts the powerband upward. With proper tuning, you’ll retain strong low-end response while gaining substantial top-end horsepower that keeps pulling past 7,000 RPM.
  • Custom Tuning Flexibility: The Precision Turbo 5857 pairs well with aftermarket engine management options such as a custom ECU flash or a piggyback controller. It provides a solid hardware foundation that can be refined through boost targeting, ignition timing, and fuel mapping.

How It Compares to the Stock Turbo

The stock M139 turbo uses a smaller turbine housing and a cast compressor wheel. At around 20-22 psi, it achieves roughly 380-400 whp on pump fuel with a tune. The 5857, on the other hand, can comfortably handle 25-30 psi with the right supporting modifications, pushing power well past 450 whp. The spool characteristics also change: the stock turbo reaches full boost around 2,800 RPM; the 5857 will typically hit full boost around 3,200-3,500 RPM, sacrificing a tiny bit of low-end lag for a massive mid- and top-end gain.

Installation Process

Installing the Precision Turbo 5857 A/R .63 on an A45 is an intermediate-to-advanced job. If you’re comfortable turning wrenches on forced-induction engines, you can likely handle this over a weekend. However, we always recommend working with a shop experienced in M139 builds. Below is a comprehensive installation guide.

Required Tools and Parts

  • Socket set (metric, preferably ¼” and ⅜” drives with extensions)
  • Torque wrench (capable of reading in Nm / lb-ft)
  • Wrenches – combination wrenches and ratcheting wrenches
  • Flare nut wrenches for oil lines (M10 and M12 sizes common)
  • Intercooler pipe removal tools (pick tools, flat head screwdrivers)
  • New gaskets: turbo-to-manifold gasket, downpipe gasket, oil drain flange gasket, oil feed copper washers
  • Oil feed and drain lines – often the Precision Turbo kit includes braided lines; if not, you’ll need to source compatible AN fittings
  • Coolant lines – if routing coolant to the turbo (applications vary)
  • Turbo blanket or heat shield (optional but recommended)
  • Anti-seize compound and high-temp thread locker
  • Jack and jack stands or a lift
  • Battery disconnect tool
  • A laptop or handheld device for reflashing ECU (post-install tuning)

Step-by-Step Installation

Always disconnect the battery negative terminal before starting. The engine should be cool to avoid burns. It’s also wise to have a second set of hands for removing the heavy turbo.

  • Pre-Removal: Drain the engine oil and coolant to prevent spills. Remove the engine cover, air intake box, and any intercooler piping that blocks access to the turbo. You may also need to remove the heat shield around the exhaust manifold.
  • Remove the Downpipe: Unbolt the downpipe from the turbo exhaust housing and separate it from the mid-pipe. Support the downpipe so it doesn’t hang on the oxygen sensor wiring.
  • Disconnect Oil and Coolant Lines: Using the flare nut wrenches, carefully loosen the oil feed line from the top of the turbo and the drain line from the bottom. Have a catch pan ready. Coolant lines (if present) also need to be disconnected at the block or at the turbo.
  • Unbolt the Turbo from the Manifold: There are typically four or five bolts securing the turbo to the exhaust manifold. Use penetrating oil if they are stubborn. Place a jack under the turbo to support its weight once the last bolt is removed.
  • Remove the Stock Turbo: Carefully lower the turbo down and out from under the car or from the top (depending on clearance). On the A45, removal from the bottom is often easier.
  • Prepare the Precision Turbo 5857: Install the oil drain flange and any supplied adapters. Apply anti-seize to threads. Some installations require rotating the compressor housing to align with your intercooler piping. Precision Turbo uses a V-band clamp for the compressor cover; loosen it and rotate as needed.
  • Install the New Turbo: Mount the new turbo onto the exhaust manifold using new gaskets. Torque the bolts to the specified value—typically around 25-30 Nm. Reconnect the oil feed and drain lines, ensuring no cross-threading. Use new copper washers on the banjo fittings.
  • Reconnect Coolant Lines (if applicable): Attach the coolant lines, then purge air from the system after filling.
  • Reinstall the Downpipe and Intercooler Pipes: Use new gaskets at the downpipe joint. Tighten all V-band clamps and pipe couplers securely.
  • Final Assembly: Reinstall the heat shield, engine cover, and intake components. Double-check all connections. Refill engine oil and coolant. Reconnect the battery.
  • Initial Start-Up: Start the engine and let it idle for several minutes. Check for oil leaks around the turbo lines and drain. Ensure no coolant leaks. Listen for unusual noises. Turn the wheels full lock to check for clearance issues with the downpipe.

Post-Installation Checks

After a short test drive, re-check all bolts and fittings. The turbo will have heat-cycled, and some fasteners may need re-tightening. Avoid full-throttle runs until you have a proper tune loaded.

Power Gains and Performance Improvements

With the Precision Turbo 5857 A/R .63 installed and a professional custom tune, the A45’s character changes dramatically. The following figures are based on real-world dyno results from M139 builds using 93 octane (or 98 RON) with supporting modifications such as an upgraded intercooler, catless downpipe, and high-flow intake.

  • Horsepower Gains: Stock A45 (Stage 1 tune) typically makes around 400-420 whp. With the 5857 and supporting mods, owners report peak wheel horsepower between 480 and 520 whp on pump gas. With ethanol blends (E85), numbers can exceed 550 whp.
  • Torque Delivery: The torque curve shifts higher in the rev range but remains flat between 4,000 and 6,500 RPM. Peak torque can reach 500-550 Nm at the wheels, providing strong mid-range pull that doesn’t fall off as aggressively as the stock turbo.
  • Boost Pressure: The 5857 can sustain boost levels of 28-30 psi all the way to redline, compared to the stock turbo which begins to taper after 6,000 RPM. This increases the usable powerband by approximately 1,000 RPM.
  • Spool Characteristics: Full boost arrives around 3,500 RPM under load—roughly 500-700 RPM later than stock. However, the turbo still spools quickly enough for daily driving, and the payoff on the top end is substantial.

Supporting Modifications Needed

To safely realize these gains, you’ll need more than just the turbo. Here are the essential supporting modifications:

  • Fuel System Upgrades: Larger fuel injectors (often 1,000 cc or more) and a higher-flowing fuel pump are required to supply enough fuel at high boost levels.
  • Intercooler: An upgraded front-mount intercooler reduces intake air temperatures, preventing heat soak and allowing consistent power.
  • Exhaust System: A full 3-inch downpipe and cat-back exhaust reduces backpressure, helping the turbo spool and flow.
  • Intake System: A high-flow intake with a larger filter reduces restriction on the compressor inlet.
  • Engine Management: A custom ECU flash is mandatory. Most tuners use tools like ECUtek, Bootmod3, or a standalone ECU to calibrate fueling, timing, and boost control for the new turbo.

Tuning Considerations

Simply bolting on the 5857 without tuning is dangerous. You will likely run lean and cause detonation. Seek a reputable tuner with experience on the M139 platform. The tuning process involves:

  • Base Mapping: The tuner will start with a conservative boost target (e.g., 18-20 psi) and gradually increase while monitoring knock, exhaust gas temperatures, and lambda.
  • Boost Control: The stock electronic boost solenoid may need to be replaced with a high-flow unit to control the larger wastegate actuator effectively.
  • Fuel Correction: Expect to run high-octane fuel or ethanol blends to keep cylinder temperatures in check. Many tuners recommend at least 93 octane (98 RON) for pump gas setups.
  • Dyno Tuning vs. Street Tuning: A dyno session is highly recommended for accurate power figures and safe AFRs. If only street tuning is available, ensure data logging is used.

Reputable tuning solutions for the A45 include ECUtek and Bootmod3, both of which offer robust support for larger turbochargers.

Conclusion

Upgrading to the Precision Turbo 5857 A/R .63 is one of the most rewarding modifications for the A45. It transforms the car from a quick hot hatch into a serious performance machine capable of keeping up with much more expensive sports cars. The installation requires mechanical competence and patience, but the results—documented gains of 80-130+ wheel horsepower on pump gas—make it a worthy investment for those who live for the thrill of boost. Always work with a skilled tuner and support the turbo upgrade with proper fueling and cooling modifications. When done right, the A45 becomes a truly exhilarating car that punches well above its weight class. For more technical details and build logs, visit enthusiast forums like the A45 Owner’s Forum or consult with a certified Precision Turbo dealer for specific part fitment.