Overview of the Mercedes-Benz AMG CLA45

The Mercedes-Benz AMG CLA45 has carved a distinct niche in the compact luxury sedan segment since its debut. It combines the aggressive styling of the CLA-class with handcrafted AMG performance, delivering one of the most potent four-cylinder engines ever fitted to a production car. The 2.0-liter turbocharged inline-four, known internally as the M139, is not just powerful but also remarkably responsive, thanks to its twin-scroll turbocharger and advanced direct injection. In standard form, the CLA45 offers a spirited driving experience that rivals many six-cylinder competitors.

The base model produces 382 horsepower and 354 lb-ft of torque, enough to propel the sedan from 0 to 60 mph in 4.1 seconds. Its top speed is electronically limited to 155 mph. All-wheel drive, AMG Speedshift DCT transmission, and adaptive damping are standard, ensuring that the CLA45 handles corners with precision while maintaining everyday comfort. Yet for enthusiasts who crave even more, the aftermarket offers solutions like the Mountune Boost Kit, which promises to unlock significant additional performance without sacrificing reliability.

What Is the Mountune Boost Kit?

Mountune is a well-known name in the automotive tuning world, particularly recognized for its work with Ford performance models and now expanding into Mercedes-AMG territory. The Mountune Boost Kit for the CLA45 is a comprehensive package that combines hardware upgrades with custom ECU calibration. The kit includes a high-flow downpipe, upgraded intercooler, revised intake system, and a specially tuned engine control unit (ECU) map. The goal is to optimize airflow, reduce intake temperatures, and increase boost pressure safely.

Mountune’s philosophy focuses on delivering gains that are not only measurable on a dyno but also noticeable behind the wheel. The kit is designed to maintain the engine’s longevity by staying within safe limits of fuel and boost. Unlike some aggressive tunes that push components to the edge, Mountune emphasizes balanced performance. The result is a claimed 450 horsepower and 400 lb-ft of torque, representing a substantial increase of 68 horsepower and 46 lb-ft over stock.

Performance Gains: Stock vs. Mountune Boost Kit

To truly understand the impact of the Mountune kit, we need to break down the raw numbers. The standard CLA45 already sets a high benchmark. With the tuning, the improvements are not just about peak figures but also about how the power is delivered across the rev range.

Horsepower and Torque Curves

While peak horsepower rises from 382 to 450 hp, the torque increase from 354 to 400 lb-ft is equally important. Torque is what you feel during acceleration, and the Mountune kit provides a much flatter torque curve. In real-world driving, this means the car pulls harder from lower RPMs, reducing the need to wring out the engine. The standard car’s torque peak arrives around 4750 rpm, whereas the tuned version delivers near-maximum torque from 3200 rpm all the way to 5300 rpm. This makes the car far more responsive in everyday situations, such as merging onto highways or overtaking on two-lane roads.

0-60 mph and Quarter-Mile Times

Acceleration times drop notably. The stock CLA45 reaches 60 mph in 4.1 seconds; with the Mountune kit, that time shrinks to 3.8 seconds. More revealing is the quarter-mile trap speed. A stock CLA45 usually runs the quarter-mile in about 12.5 seconds at 112 mph. With the Mountune upgrade, expect numbers in the low 12-second range with trap speeds around 118-120 mph. This puts the tuned CLA45 in the same league as sports cars like the Porsche 718 Cayman GTS and the Audi RS3.

Top Speed and Stability

The factory electronic limiter holds the CLA45 at 155 mph. Mountune’s calibration raises that limit to 165 mph. This is not just a trivial gain; at high speeds, the increased power allows the car to reach and sustain that speed more easily. The all-wheel-drive system and chassis remain well up to the task, though it’s worth noting that additional cooling and aerodynamic stability measures are already part of the AMG package. Drivers will appreciate the extra headroom on racetracks where top speed is a factor, such as the long straights of the Nürburgring.

Hardware Upgrades Included in the Mountune Boost Kit

The Mountune kit isn’t just a software flash; it includes several physical components that work together to deliver the gains. Understanding these helps explain why the performance is both substantial and reliable.

High-Flow Downpipe

Replacing the restrictive factory catalytic converter with a high-flow unit reduces backpressure, allowing the turbocharger to spool faster and produce more power. Mountune uses a catalyzed downpipe that meets emissions requirements in most regions, though some users may opt for a sportier, uncatalyzed version for track use.

Upgraded Intercooler

The intercooler is a critical component for sustained performance. The stock intercooler works well for daily driving but can heat-soak during repeated hard runs. Mountune’s larger intercooler lowers intake air temperatures by up to 30%, which reduces the likelihood of engine knocking and allows the ECU to maintain more aggressive timing. This is especially beneficial in warm climates or during track sessions.

High-Flow Intake System

An improved air intake draws cooler, denser air into the engine. Mountune’s intake includes a larger filter and a smooth, insulated duct that reduces restriction. This alone can yield 10-15 extra horsepower at the top end, but the real benefit is in throttle response. The engine feels more eager to rev.

Custom ECU Calibration

The heart of the kit is the calibration. Mountune engineers have spent hundreds of hours mapping ignition timing, fuel delivery, and boost pressure curves. They use a handheld device that plugs into the OBD-II port to flash the ECU. The tune is designed to work with 93-octane fuel (or 98 RON) and includes safety measures such as knock sensors and temperature compensation. The calibration also locks out any error codes related to the downpipe or intake.

Driving Experience: How It Feels from the Driver’s Seat

Numbers only tell part of the story. Behind the wheel, the difference between stock and Mountune-tuned CLA45 is dramatic. Immediately upon starting the engine, the exhaust note deepens. The combination of a freer-flowing downpipe and revised engine mapping gives the engine a more aggressive sound, with pops and crackles on downshifts.

Throttle Response

In the stock car, there is a slight hesitation when you floor the throttle from a standstill, typical of modern turbo engines with torque management. The Mountune calibration sharpens the pedal mapping, reducing lag significantly. Power comes on almost instantly. In Sport+ mode, the response is telepathic, making heel-toe downshifts effortless.

Mid-Range Punch

This is where the Mountune kit truly excels. Between 3000 and 6000 rpm, the tuned CLA45 feels like a completely different car. It pushes you back into the seat with a relentless surge that continues all the way to redline. Overtaking on country roads becomes a brief, safe affair—a glance, press the pedal, and you are past. The additional torque transforms the car from a quick compact to a genuinely fast machine.

Cornering and Chassis Behavior

With more power, the chassis balance changes slightly. The standard CLA45 is already very neutral, with slight understeer. The extra horsepower can rotate the car more easily if you use the throttle mid-corner. The all-wheel-drive system, which can send up to 50% of power to the front wheels, helps maintain traction. However, drivers should note that on damp roads, the front tires may break loose under heavy acceleration, requiring careful throttle modulation. Mountune does not alter the suspension, but owners can optionally upgrade to stiffer anti-roll bars or coilovers to match the increased performance.

Reliability and Daily Usability

One of the biggest concerns with tuning is whether it will compromise reliability. Mountune has designed the kit with a safety margin. The M139 engine has forged connecting rods and pistons from the factory, making it inherently strong. The Mountune tune stays within the limits of the fuel system and turbocharger efficiency. Many users have reported tens of thousands of trouble-free miles after installation.

Drivability on a daily basis remains excellent. The car can be driven sedately in Comfort mode with no issues, and fuel economy in mixed driving drops only by about 1-2 mpg (amortized over a full tank). Cold starts and idle quality are normal. The only potential annoyance is a slight drone at highway cruising speeds due to the less restrictive exhaust, but this is subjective and can be mitigated by choosing a resonated downpipe.

Warranty coverage is a consideration. While Mountune provides a limited warranty on their components, the ECU flash will void your factory powertrain warranty on the affected parts. It is advisable to have a relationship with a dealership that is aftermarket-friendly, or to retain the ability to revert to stock software before dealer visits.

Comparing the Mountune Boost Kit to Other Tuning Options

The CLA45 tuning market is crowded. Competitors include Renntech, Eurocharged, Unitronic, and JB4 (Burger Motorsports). Each has a different approach. Renntech, for example, offers a 450 hp package similar to Mountune’s but at a higher price point. Eurocharged provides a flash-only tune that yields 430-440 hp without hardware changes. The JB4 is a piggyback system that can add 30-40 hp but does not address intake or exhaust restrictions.

  • Mountune offers a complete hardware+software package, optimized for reliability and drivability.
  • Renntech has a comparable package but focuses more on peak power and may require higher-octane fuel.
  • Eurocharged is a good budget option for mild gains, but lacks the extra cooling of the Mountune intercooler.
  • JB4 is user-adjustable but cannot match the low-end torque improvement of a full ECU reflash.

For enthusiasts who plan to track their car or live in hot climates, the Mountune kit’s intercooler upgrade is a clear advantage. For those who simply want a street-friendly daily driver with more punch, the kit is hard to beat.

Cost and Value Proposition

The Mountune Boost Kit for the CLA45 retails for approximately $3,500 to $4,000 depending on options and installation. While this may seem steep, consider that you are gaining nearly 70 horsepower and a comprehensive set of hardware. Compared to the price of upgrading to a CLA45 S (which offers 421 hp from the factory), the Mountune kit provides better performance for a fraction of the cost. The CLA45 S also has a higher purchase price and may still require tuning to match the Mountune numbers.

Installation can be done at home by a confident mechanic, taking about 4-6 hours. Many owners report that the driving thrill outweighs the cost, and the resale value of a tuned car can hold up if you keep all stock parts to revert later.

Real-World Results and Owner Testimonials

Data from forums such as MBWorld and CLA45-specific groups shows consistent results. One owner achieved a verified 3.79-second 0-60 mph on a dragy, with a quarter-mile of 12.2 seconds at 119 mph. Another reported a peak gain of 450 horsepower and 405 lb-ft on a Mustang dyno (which reads lower than Dynojet). The consensus is that the kit performs as advertised.

Owners also note improved autocross times due to the better responsiveness. The only minor complaint is that the kit can make the car feel almost too quick for its chassis, especially on tight tracks. Some have upgraded to wider tires (from 235 to 245 front) to cope with the extra torque.

Conclusion: Is the Mountune Boost Kit Worth It?

The Mercedes-AMG CLA45 is already an exceptional car. The Mountune Boost Kit elevates it to a level that rivals far more expensive machines. The gains in horsepower, torque, and throttle response are not just numbers on paper; they translate into a daily driving experience that is more engaging, more responsive, and simply more fun. The kit addresses the main weaknesses of the stock car—slightly lazy low-end response and heat soak during hard driving—while maintaining the factory’s refinement.

For the performance enthusiast who wants to extract the full potential of the M139 engine without pushing into the realm of unreliability, the Mountune Boost Kit is an outstanding choice. It offers a balanced, well-engineered upgrade that should satisfy even the most demanding drivers. Combined with the CLA45’s luxurious interior and all-wheel-drive grip, the result is a true pocket rocket that punches well above its weight.

For more information, visit the official Mountune website and Mercedes-AMG CLA45 specifications. Additional performance data can be found on Car and Driver and MBWorld forums.