Every BMW M3 and M4 owner who has researched the single turbo conversion has read the same basic comparison. Single turbo makes more peak power. Twin turbo spools faster. Single turbo has more lag. Twin turbo has a lower ceiling. Pick one based on your goals.
That comparison is technically correct and almost completely useless for making an actual decision.
We built an F82 M4 from stock S55 twin turbos to a 4N Motorsport single turbo setup and drove both versions of the car. Here is the real comparison of what actually changes when you go from the stock S55 twin turbo configuration to the 4N Motorsport single turbo kit, based on having experienced both on the same car.
Power Delivery, The Difference You Feel Before You See the Numbers
The first thing anyone notices when driving the 4N Motorsport equipped S55 after driving the stock twin turbo version is not the power number. It is the shape of the power.
The stock S55 twin turbo setup is designed to deliver power immediately and consistently across a wide RPM range. The two turbos are relatively small, which means they spool quickly and build boost early. The result is a car that responds to throttle inputs with an immediacy that makes every driving situation feel controlled and accessible. The power is always there, always immediate, always predictable.
The 4N Motorsport Borg Warner single turbocharger operates differently at every point in the power delivery curve. Below the boost threshold, the car feels like a naturally aspirated engine. There is power, but it is the engine's own power rather than boosted power. Then the 4N Motorsport turbo spools. And when it does, the power does not arrive gradually. It arrives in a way that rewards the driver for waiting and punishes the driver who was not expecting it.
This is what enthusiasts mean when they describe the single turbo character as violent. It is not that the power is uncontrollable. It is that it arrives in a shape that requires a different kind of driver engagement than the stock twin setup. The stock S55 is always fast. The 4N Motorsport S55 builds to something different.
Sound, The Most Immediate and Obvious Change
Before the power difference becomes apparent, the sound difference is already unmistakable.
The stock S55 twin turbo setup sounds excellent. BMW engineered the exhaust system to allow enough of the S55 character through to satisfy enthusiasts while keeping the car within the noise requirements of global markets. The result is a sound that is genuinely impressive for a factory configuration.
The 4N Motorsport single turbo setup sounds like a different engine in a fundamentally different category. The equal-length tubular manifold that is the centerpiece of the 4N Motorsport kit is specifically engineered to deliver exhaust pulses to the turbocharger consistently and efficiently. The side effect of that engineering is an exhaust note that is immediately recognizable as something different from any stock S55 configuration.
The single turbo spool is audible in the cabin during acceleration. The sound builds with the boost rather than being present immediately the way the stock twin turbo setup sounds. The character of acceleration becomes an audio experience in addition to a physical one, with the rising sound of the 4N Motorsport Borg Warner building toward boost serving as a preview of what is about to happen when it arrives.
We described the goal of this build as turning the M4 into a monster with a sound that would make other cars weep. After completing the build, that description is accurate.
The Upper Range, Where the 4N Motorsport Kit Creates a Completely Different Car
This is where the numbers stop being abstract and start being meaningful. The stock S55 twin turbo setup runs out of capability at a certain point. The turbos are at their physical limit and additional boost does not produce additional power because the turbos cannot flow more air than their compressor maps allow.
The 4N Motorsport Borg Warner turbocharger is designed for a substantially higher airflow capacity. This is a larger turbocharger than the stock units combined and it is sized for the upper power levels rather than for the specific combination of quick spool and moderate peak flow that the stock twin setup was designed for.
On our F82 M4 build, the first dyno pull before serious tuning work produced 667 wheel horsepower. The stock S55 at its most aggressively tuned twin turbo configuration produces peak numbers that fall significantly short of that starting point. The 4N Motorsport kit was already beyond what the stock twin turbos can achieve on the first pull, before the tune was optimized.
The final result after the tuning session was 710 wheel horsepower with additional room identified by the tuner for future gains. This is the power output that separates the 4N Motorsport conversion from every other modification available for the S55 engine. No combination of bolt-on modifications on the stock twin turbo setup reaches 710 wheel horsepower on pump gas with the same engine.
The Mid-Range Improvement That Most Reviews Do Not Cover
Every comparison between single turbo and twin turbo setups focuses on peak power and spool response. The mid-range behavior is less discussed and arguably more important for the actual driving experience on the road.
Our dyno session captured the mid-range improvement directly. The comparison between the first pull of the day and the final pull showed that at 5,500 RPM, the car improved from 564 horsepower to 682 horsepower through the tuning session. That 118 horsepower gain in the mid-range at a single RPM point illustrates what the 4N Motorsport kit does where most driving actually happens.
The stock S55 is strong in the mid-range. The 4N Motorsport equipped S55 is extraordinary in the mid-range once the tune is properly developed for the kit. The power that arrives after the turbo spools does not stay at the spool point. It carries through the entire RPM range above it in a way that makes the upper range of every gear feel like a completely different engine.
Reliability After the 4N Motorsport Conversion
This question comes up in every single turbo conversion discussion and deserves a direct answer rather than the vague reassurances that often fill this part of the conversation.
The 4N Motorsport single turbo kit is a properly engineered system built around a Borg Warner turbocharger that has a proven track record in high-power applications. The kit itself is not where reliability concerns arise in single turbo conversions.
Reliability on a 4N Motorsport equipped S55 is entirely dependent on whether the supporting build was done correctly. The upgraded fuel system, the proper tune, the correct spark plug gap, the appropriate oil and maintenance schedule for an engine producing this level of output. An S55 with the 4N Motorsport kit running a proper supporting build, appropriate fuel, and a professionally developed tune is a reliable performance car. The same engine running an inadequate tune, wrong fuel, or insufficient fuel system support is not.
This is not unique to the 4N Motorsport kit. It is true of every serious single turbo conversion on any engine. The reliability outcome is determined by the quality of the complete build rather than by any single component including the turbocharger itself.
The Decision Framework, When the 4N Motorsport Kit Makes Sense
Based on the experience of building and driving this car, the 4N Motorsport conversion makes sense in specific situations and does not make sense in others.
It makes sense when the power goals genuinely exceed what the stock twin turbo setup can achieve. If 700 plus wheel horsepower from the S55 engine is the goal, the 4N Motorsport kit is the path. There is no combination of modifications on the stock twin turbos that reaches this output level reliably.
It makes sense when the driving character of the single turbo experience is specifically what the owner wants. The build to boost, the pull to redline, the audible spool of the large Borg Warner. These are not incidental side effects of the conversion. They are central to what the 4N Motorsport equipped S55 is.
It makes sense when the complete build budget is available. This is not a budget modification. The 4N Motorsport kit, the supporting fuel system, the tune, and the maintenance items that should be addressed simultaneously represent a serious financial commitment. Planning with accurate numbers before starting the build is essential.
It does not make sense when the instant response and accessible power of the stock twin setup is the primary thing the owner values about their M4. The 4N Motorsport conversion changes that character permanently. If the stock twin turbo S55 is delivering the experience the owner wants and the goal is simply more of it, aggressive tuning and supporting modifications on the stock twins produce a better result than the personality change the 4N Motorsport kit creates.
What the 4N Motorsport Build Proved
The four-day build and dyno session answered the question that drove the decision to do it in the first place. The S55 in the F82 M4 is an extraordinary engine with performance capability that the stock twin turbo configuration never fully reveals.
The 4N Motorsport single turbo kit removes the ceiling. It changes the personality. It produces a car that sounds different, accelerates differently, and creates an experience that the stock M4 can suggest but never fully deliver.
710 wheel horsepower. First pull at 667. The goal of 700 cleared. These are the numbers. The experience of driving the car after the build is what the numbers represent and what the 4N Motorsport kit is actually designed to produce.
Subscribe to Tysautoworks Performance on YouTube for the complete build series and every real BMW build we document.