Before purchasing a turbocharged 135i, 335i or 535i it is important to consider the differences between the N54 and N55 powered BMW’s. The N54 began production in 2006 and first appeared in the 2007 BMW 335i; in 2008, the N54 was introduced along with the 135i and 535i. BMW’s N55 engine began production in 2009 and started replacing the N54 in 2010. However, most applications built in 2010 still carried the twin turbo N54. The N55 officially replaced the N54 in 2011 (with the exception of the 1M and Z4 35i). Although the engines have many similarities this post will highlight the key differences between the two.
N54 vs. N55: Single vs. Twin Turbochargers
Arguably, the most significant difference between the N54 and N55 is the turbochargers; the N54 bolsters true twin turbochargers while its N55 counterpart carries a single twin-scroll turbocharger. A twin-scroll turbocharger is a single turbo where the exhaust housing of the turbocharger is split into 2 “scrolls”. Each 3 cylinders on the same firing cycle feed an individual scroll of the turbocharger which reduces exhaust reversion. Reversion is the process of exhaust gases entering the combustion chamber as the gases interact with each other; this creates higher temperatures, less power, and higher emissions. So what does all of this mean? A twin scroll turbocharger increases efficiency thereby decreasing turbo spool time, increasing power, and allows the cylinders to run cooler. Despite the benefits of a twin-scroll turbo as compared to a single single-scroll turbo, a true twin turbo setup mirrors the benefits of a twin-scroll turbo. As with the twin-scroll, 3 cylinders feed each of the two turbos thereby reducing reversion.
Out of the factory, performance doesn't differ much
From the factory, both the N54 and N55 engines produce similar torque and power bands. The twin-scroll design allows the turbocharger to spool slightly faster on the N55, delivering peak torque 100 RPM’s sooner than the N54. Although, this does not lead to a significant difference – the N54 and N55 each produce minimal turbo lag and feel similar to a naturally aspirated V8. On stock applications both engines perform roughly the same despite the difference in turbochargers. However, when comparing the N54 and N55 mod-for-mod the benefits of a true twin turbo set up become evident.
The N54 is more tuner-friendly
BMW’s N54 twin-turbocharged engine really comes to life when modified; I do not intend to suggest the N55 is anything but impressive when modified. The N54’s twin turbo design simply moves more air and does so more efficiently. Mod for mod the N54 will outperform the N55. A large reason for the switch from the twin-turbo to twin-scroll turbo is cost and complexity. Two turbochargers with all of the associated hard-ware are more expensive and require additional space. Even though the N54 is more efficient when modified, each engine and its respective turbocharger set up will produce well into the range of 400+ wheel horsepower and torque.
N54 vs. N55 Reliability: Which Engine is More Reliable?
This is always a touchy subject when it comes to discussions about the N54 and N55. Simply put, the N55 is more reliable. BMW’s N54 was the first mass production turbocharged gasoline engine produced by BMW in decades. Looking at BMW’s current line-up in 2017, which is mostly turbocharged, I believe it is fair to say the N54 was in many ways an “experiment”. The N54 was notorious for its common and troubling issues early on. Fuel injectors, the HPFP (high pressure fuel pump), and waste-gate rattle plagued the N54 for the first several years of its production. These are among several other issues that have convinced some BMW owners to avoid the N54. However, BMW offered many recalls and extended warranties to help remedy the reliability issues. You can read about all of the most common N54 problems here.
Since the N55’s introduction many of the issues with the N54 have been resolved as BMW introduced new HPFP’s and fuel injectors. Waste-gate rattle is still a relatively widespread problem on the N54, but this typically does not affect the life or longevity of the turbos. Additionally, BMW offers an 8 year, 82,000-mile extended warranty for waste-gate rattle on the N54. I actually had my turbochargers replaced on my 2007 335i under the extended warranty (a story for another day). Long story short, I am impressed with BMW’s willingness to “make things right” and ensure the issues were resolved with the N54. While the N55 was produced to be more reliable than the N54, it still has it's problems. Read about the most common N55 problems.
At the end of the day, the N55 is the more reliable engine
BMW got things right from the start with the N55 – something that can’t be said for the N54. Putting aside the early issues the N54 and N55 are both relatively reliable engines. Some of the supporting hardware such as valve cover gaskets and water pumps are common issues on each, and will likely be for the distant future. After all, BMW isn’t targeting customers looking for the most reliable cars on the road. BMW builds “The Ultimate Driving Machine” which the two engines live up to and then some.
N54 vs. N55 Engine Internals
I won’t spend long on this subject as the engine internals can be another controversial topic, and for those looking to stay with stock turbos the internals on each engine are plenty strong. The N54 comes stock with a forged crankshaft and rods. The N55 features a cast iron crankshaft and rods. Both the N54 and N55 have cast irons pistons. Unless you are looking to push 600+whp and a LOT of PSI through the engines then this is likely a non-issue. Both engines are incredibly strong factory motors than can handle plenty of abuse on the stock blocks and internals.
So...Twin-Turbochargers or Single Turbo?
BMW’s N54 and N55 engines are similar in many aspects and stock-for-stock perform almost identically. The N55 twin-scroll turbo offers slightly faster turbo spool and delivers peak torque 100 RPM’s sooner than the N54; a characteristic that is not noticeable as each turbo set up spools quickly and delivers impressive torque through-out the low end and midrange. Thanks to the N54’s twin turbos and forged internals, it is the more capable engine mod-for-mod. However, the N55 has proven to be the more reliable engine. N54’s are notorious for their common problems, especially early on, but these issues are currently resolved. If you are looking for an all-around reliable car, chances are, neither the N54 nor N55 will be at the top of your list.
Read our guide on the 8 Most Common N54 Engine Problems to learn what to watch for, and how to minimize N54 reliability issues!
Our Pick: N54 (although I’m sure you assumed that before even clicking this post)