Without question, the N54 six-cylinder ranks high among BMW’s finest powerplants of all time. By pairing two small turbochargers (each operating on three of the six cylinders) with variable valve timing and direct fuel injection, this 3.0-liter motor delivers performance on a level never before seen from any of BMW’s six-cylinder engines, barring those that hail from the workshops of BMW M. Unlike those harder-edged M motors, which require full use of their stratospheric rev range to extract maximum performance, the N54’s robust torque is accessible almost immediately and can be fully appreciated in the day-to-day conditions most drivers encounter.
After two years of first-hand experience with an N54-powered 135i, I can confirm that this is one of just a handful of modern internal combustion engines that’s both powerful enough to satisfy serious performance junkies yet docile and tractable enough to tackle the daily grind of even the most congested urban environments.
That said, the N54 is not without its vices. A significant percentage of owners have experienced fuel pump failure, often on a chronic basis. Even though my motor never skipped a beat in over 12,000 miles, I was not immune to the N54’s other significant drawback: dreadful fuel economy. How dreadful? A typical average of city and highway driving never yielded more than about 15.5 mpg which, coupled with a puny 14-gallon fuel tank, meant that the driving range hovered right round the 200-mile mark. I realize the 135i is hardly a Prius substitute, but 15.5 mpg? That’s downright embarrassing.
Swapping twins for singles
And so it wasn’t a complete shock when BMW recently announced that the N54 would be superseded throughout much of the 2011 lineup by the more fuel-efficient N55. Though the N54 and N55 have much in common, the N55 swaps the N54’s twin blowers for a single, twin-scroll turbocharger that promises to banish any remaining traces of turbo lag and also allows the use of a single exhaust catalyst for cleaner cold-start emissions. Furthermore, the N55 adds fuel-saving Valvetronic technology that is claimed to reduce gasoline consumption by around ten percent. And the best part? This is all achieved without any reduction in peak horsepower or peak torque compared to the outgoing N54.
Short of popping the hood and peering at the new (and confusingly worded) “TwinPower Turbo” inscription on the valve cover, there is absolutely no way to distinguish the latest 135i from a 2010 model—at least visually. Start up the 2011 135i, however, and the N55’s deeper and more assertive exhaust note is an instant giveaway. The resonant new rumble sounds superb and suits the feisty character of the 135i to a tee. Overlaid with this burbling exhaust note is a prominent “whoosh” from the wastegate, a sound that was barely audible in the N54 and which serves as one of the few reminders that the N55 is indeed turbocharged.
Just as BMW promised, any vestiges of turbo lag have been diminished to the point of being inconsequential. Sure, you can still catch the blower off-guard if you select fourth gear at 1,500 rpm and nail the throttle, but generally engine response is more or less instantaneous and feels appreciably stronger in the lower rev range than even the torque-rich N54.