The BMW N55 TwinPower Turbo vs. the BMW N54 Twin Turbo
Birth of the Turbo
Let’s start from the beginning and talk a little about where the turbo came from. In 1905, Swiss engineer Alfred Buchi applied for a patent on what would become the modern-day turbocharger. His patent was specifically for an exhaust driven compressor wheel that forced compressed air into the combustion chamber of an internal combustion engine. Alfred Buchi wasn’t the first engineer to realize that forcing air into an engine would increase its power but he was the first patent holder for a compressor driven by exhaust gasses. Twenty years earlier Gottlieb Daimler patented the modern supercharger by designing an air pump (compressor) driven by a gear.
The turbo started appearing in aircraft and marine applications in the 1920’s. Because of the increased horsepower output of turbocharged engines, turbo’s started appearing on race cars in the 1930’s. Turbochargers eventually made their way into production automobiles and today the industry has heavily adopted the use of forced induction internal combustion engines via turbos.
How a Turbo Works
I’m going to keep this section as simple as possible. There’s plenty of great resources out there that get into great technical detail on how a turbo works. Here’s a couple for you on the World Wide Web and you can always look on Youtube. Turbos compress the air going into the engine. Compressed air is more dense and therefore contains more oxygen per square inch (PSI.) More oxygen means you can also add more fuel and that results in more power. More power = 🙂
What’s the difference between a Twin-Turbo BMW N54 and a Twinpower Turbo Like the BMW N55 in the X1 35i?
For starters the BMW N54 is a true twin-Turbo. It has two separate turbos, each powered by 3 of the engines six cylinders. Both of the turbos on the N54 are of equal size and configuration. The BMW N55 TwinPower turbo is a single Turbo engine with two separate exhaust runners powering the hot side of the turbo. Because one exhaust runner dumps its exhaust on the smaller, inner part of the exhaust turbine it can spool up faster.
The result is less turbo lag and better low-end torque. The second exhaust runner is focused on the larger, outermost part of the turbine. This allows the compressor to spin faster at higher RPM’s without additional moving parts. The result; faster spooling times like a smaller turbo without sacrificing top-end power. BMW calls this setup Twinpower.
Why do tuners like the N54 over the N55? Although BMW posted the same 300 horsepower, 300 ft/lbs of torque for both of these engines in stock trim, the tuning capabilities of the N54 far exceed that of the N55. Both engines have the same displacement of 2,979 cc (181.8 cu in) and a 10.2:1 compression ratio but the turbo setups make the difference. The N55’s single turbo has its limitations at higher RPM’s. In stock use, BMW programmed both engines to run at a max boost of between 8-9 PSI. Increasing boost pressure, or “tuning” increases horsepower and torque in either engine.
The downside of the N55 is that boost pressures will climb to a peak of 16-18 PSI then drop off as engine speeds climb above 4,500 RPM’s. On the other hand, the twin-turbo setup on the N54 will climb to as much as 20 PSI (depending on supporting mods) and does not fall off as dramatically as the N55.
The Business Case for an N55 Turbo Upgrade
Numerous turbo upgrades are available for the BMW N55 engine but Pure Turbos is the way to go. Their Stage 1 upgrade uses the stock turbo housing which is a Borg Warner B03 Twin Scroll. Pure Turbos machines the housing to accept a larger compressor wheel and they modify the turbine wheel to improve flow. The results are noticeable and with supporting mods bring another 70hp to the party in the upper RPM’s. Spool times are similar to stock but above 3,000 RPM’s you know something is different, and it’s good!
Pure Turbos also makes a Stage 2 turbo upgrade for the N55. We’ve installed both S1 and S2’s on our previous BMW E84 X1 35i sleeper projects and currently have a Stage 1 on our Sleeper X1 35i project build. Stage 2 is capable of 550 horsepower but requires more supporting mods. Reliability started to suffer a bit for us after moving to S2 and that’s why we went back to a stage 1 on our current X1 35i project.
Fuel becomes an issue when you get north of 20 psi and we were plagued with check engine lights under racing conditions. During a few 1/2 mile roll races our Stage 2 X1 35i would go into limp mode. We later found that the car was producing more torque than the DME would allow and we kept getting a transmission fault code.
The Borg Warner B03 Twin Scroll Wrap-up
At the end of the day, the BMW N55 TwinPower turbo engine may not be as robust as the N54 in the mind of a tuner but it’s one helluva motor. The Borg Warner B03 Twin Scroll turbo is tune-able in stock form and incredibly reliable. If you’re looking for a BMW E84, make sure you get the 35i and read our article on why you should buy an E84 X1 35i.
BMW X3M Competition – Pocono Raceway – Powerful & Hits 157mph