The stock S55 in the BMW F82 M4 is one of the greatest turbocharged inline-six engines BMW has ever produced. Every BMW enthusiast who has driven one knows this. The instant response. The linear power delivery. The way it pulls from any RPM and delivers every one of its factory horsepower without asking for anything in return.
So why would an M4 owner take that engine apart, remove both turbos, and replace them with a completely different turbocharger setup from 4N Motorsport?
We asked ourselves the same question before we started the build. By the time the car came off the dyno at 710 wheel horsepower, the answer was obvious. Here is the full explanation of why the 4N Motorsport single turbo conversion has become one of the most compelling modifications available for the F82 M4 and F80 M3, and what it actually changes about the car.
The S55 Is Great. The 4N Motorsport Makes It Different.
This is the distinction that most people miss when they research the single turbo conversion and it is the most important thing to understand before deciding whether the 4N Motorsport kit is right for your M4.
The conversion does not make the S55 better in the way that an exhaust makes it sound better or a tune makes it faster. It makes the S55 different. It changes the fundamental character of how the engine delivers its power and how the car behaves when you ask for it.
The stock S55 twin turbo setup produces power immediately. The two small turbos spool quickly, the power arrives early in the RPM range, and the car responds to throttle inputs with a directness that makes it easy to use every bit of the factory 503 horsepower at any speed. This is exactly what BMW engineered the S55 to do and they did it extraordinarily well.
The 4N Motorsport single turbo setup produces power differently. The single large Borg Warner turbocharger at the heart of the 4N Motorsport kit builds boost progressively as engine speed climbs. There is a brief moment between the throttle input and the full arrival of boost that the stock twin turbo setup does not have. And then the power arrives completely, pulling through the RPM range in a way that the smaller stock turbos are not capable of because they are out of breath well before the 4N Motorsport unit reaches its stride.
The 4N Motorsport build on our F82 M4 transformed the car's personality exactly as described before the first bolt was turned. The instant hit of the twin turbo setup was replaced by the classic pull to redline thrill of a big single. The car changed from a car that is fast and accessible to a car that builds and then explodes.
Whether that exchange is worth making depends entirely on what the M4 owner wants from their car. For the owner who bought an M4 specifically for the daily drivability and instant response, the 4N Motorsport conversion may change things in a direction they do not actually want. For the owner who wants a completely different kind of performance experience and a power output that the stock twin turbo setup cannot achieve regardless of how aggressively it is tuned, the 4N Motorsport kit is the answer.
What the Stock S55 Twin Turbos Cannot Do That the 4N Motorsport Kit Can
The stock S55 twin turbo setup has a practical power ceiling. The two turbos are sized for the specific operating envelope BMW designed for the M3 and M4 from the factory. Aggressive tuning and supporting modifications push them meaningfully beyond stock output, but there is a point at which the stock turbos are working at their physical limit and more boost does not produce more power because the turbos cannot flow more air than their design allows.
The 4N Motorsport Borg Warner turbocharger is designed to flow significantly more air than the stock twin setup at elevated boost levels. This larger flow capacity is what produces the 710 wheel horsepower result on our build and what creates the room for further gains that the tuner identified at the end of the dyno session.
A properly built S55 on an aggressive tune with the stock twin turbos can reach impressive power numbers. The 4N Motorsport single turbo kit makes those numbers look modest.
This upper-range capability is the second major reason M4 owners choose the 4N Motorsport kit beyond the personality change. If the goal is maximum power from the S55 engine without replacing the engine itself, a properly supported 4N Motorsport single turbo conversion is the path that gets there.
What the 4N Motorsport Kit Does to the Sound
The equal-length tubular twin scroll manifold that comes with the 4N Motorsport kit is engineered as precisely as the turbocharger it feeds. The runners are designed to deliver exhaust pulses from each cylinder to the turbo as efficiently as possible, and the equal-length design produces a consistency of exhaust sound character that a non-equal-length manifold cannot replicate.
The S55 with the 4N Motorsport kit sounds like a different engine. Not a louder version of the same sound. A genuinely different sound with a different character. The build and spool of a large single turbocharger is audible in a way that the smaller, faster-spooling stock turbos are not. Every pull carries the sound of boost building rather than boost already being present.
We described it during the build as a sound that would make other cars weep. That description holds after driving the car. The 4N Motorsport kit produces an S55 sound that is immediately recognizable as something different from any stock M4 and different from most single turbo conversions on other platforms.
The Build Commitment This Conversion Requires
This is the section that needs to be read by anyone considering the 4N Motorsport kit seriously. The conversion is not a weekend modification. It is a serious build that requires genuine mechanical experience with the S55 engine, proper tooling, and the specific knowledge of the 4N Motorsport kit's installation requirements that only comes from having done it or from research as thorough as this series of videos and articles.
The S55 engine bay is among the most tightly packaged in modern BMW history. Getting the stock twin turbos out requires removing structural braces, the charge cooler plumbing, the motor mount and post mount, and working in positions that do not look possible in photographs but apparently are possible given that this build was completed.
The 4N Motorsport kit goes in with specific installation requirements that are not all covered in the kit instructions. The wastegate actuator rod alignment is the most critical step. The exhaust stud removal before the manifold installation saves significant time. The turbine fin protection during installation is not optional. These are the details that separate a smooth installation from an expensive mistake.
The supporting components beyond the 4N Motorsport kit itself add both cost and complexity. The port injection system, upgraded injectors, J-pipe, low pressure fuel pump, and ethanol fuel system components are all part of a complete build at the power levels this kit is capable of supporting.
The total investment in this build was referenced at $15,000 plus in parts alone before labor. This is not a figure that should surprise anyone who researches seriously before committing to the conversion.
The Wedge Performance Tune and Why It Matters
A 4N Motorsport kit installed without professional tuning produces a car that is dangerous to drive and potentially damaging to the engine. The tune is not an optional finishing touch. It is a fundamental requirement of the conversion.
The tuning on our build was handled by Steve from Wedge Performance. The difference between the first dyno pull at 667 wheel horsepower and the final pull at 710 wheel horsepower represents the improvement a skilled tuner extracts through systematic calibration of boost targets, ignition timing, fueling, and the various parameters that govern how the engine management system uses the 4N Motorsport kit's capability.
The mid-range gain demonstrated during the tuning session, from 564 horsepower to 682 horsepower at 5,500 RPM between the first and final pulls, illustrates what professional tuning does for a properly built 4N Motorsport equipped S55 that a less experienced tuner or an off-the-shelf map cannot fully replicate.
Anyone considering the 4N Motorsport conversion should plan for a professional calibration by a tuner with specific experience on S55 single turbo builds. This is not the application for a generic tune.
Is the 4N Motorsport Conversion Right for Your M4
It is if you want a different kind of performance experience from your S55 engine rather than simply more of the same. It is if your power goals exceed what the stock twin turbo setup can realistically support on an aggressive tune. It is if the sound transformation of the S55 with a large single turbo is something you have heard and want to experience from your own car. It is if you are committed to doing the supporting work correctly rather than treating the kit as a standalone modification.
It is not if you primarily value the instant, accessible power delivery of the stock S55 twin turbo setup and would trade that character for nothing. It is not if the build commitment, both financial and mechanical, is not something you are prepared for completely.
The 4N Motorsport kit on an F82 M4 is one of the most complete single turbo conversion options available for the S55 platform. Our build proved that at 710 wheel horsepower on a four-day build. What the numbers do not capture is that the car became genuinely, immediately, unmistakably different to drive from the moment it came off the dyno. That difference is what the 4N Motorsport conversion is actually about.
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