The new inline-6 engine displaces 3.0-liters and develops maximum output of 300 hp at 5,800 rpm, with peak torque of 300 lb-ft available all the way from 1,200–5,000 rpm. Redline is 7,000 rpm. This is the same level of performance as the previous 3.0-liter twin-turbo inline-6 but with the innovation of twin-scroll technology and the integration of Valvetronic, this new engine is even more fuel efficient.
Historically, BMW engineers were never satisfied with the most often-cited downside to turbocharging: the frustrating “turbo lag” that occurs between the time at which driver presses the accelerator pedal and when the turbocharger develops sufficient rotation to increase power. Turbo lag is usually worse at low engine speeds. Other drawbacks included relatively high fuel consumption and poor emissions output. At the time, turbocharger technology was not a reliable, practical or efficient was to make power. For these and other reasons, BMW put turbocharged gasoline engines on the shelf for more than 20 years.
Two recent developments caused BMW to reconsider turbocharging. While many luxury-performance carmakers began to achieve power gains by building engines of ever-increasing displacement, BMW looked for a more efficient way; smaller engines and turbocharging made sense. Also, the technology transfer from BMW’s turbocharged diesel engines could easily be transferred to their gasoline-fired counterparts. Accordingly, BMW articulated its new approach to developing high-performance engines as part of its EfficientDynamics initiative.
The new N55 is the first BMW inline-6 to combine turbocharging, High Precision direct fuel injection, and Valvetronic variable intake technology. It features a single, mid-sized turbocharger with a “twin-scroll” housing to boost performance and minimize the response lag. Thanks to its housing design which maintains proper separation between streams of exhaust gasses, the turbocharger builds up pressure much faster than previous-generation turbochargers, thus eliminating even the slightest tendency for lag.
A further advantage of turbocharging is that this is the most weight-efficient method to boost engine power and performance. The N55 turbocharged inline-6 weighs approximately 150 lbs. less than an equally powerful eight-cylinder engine displacing 4.0 liters. This lower weight means a significant advantage not only in fuel economy, but also in balancing the car's weight distribution.
Using Valvetronic for the first time on a turbocharged inline-6 allows the engine to “inhale” air for combustion with virtually no delay and with reduced pumping losses. As a result, the engine makes power more quickly than ever before, and this is proven by the N55’s ability to reach peak torque at 1200rpm, 200rpm earlier than its predecessor.
Turbocharging typically includes intercooling of the engine's induction air, that is, cooling the compressed air that emerges, very much heated up by the compression process, from the turbocharger(s). Sometimes it's done with coolant; in the case of the N55 engine, it's accomplished with outside air. Intercooling is necessary to reduce the temperature of the incoming air to preclude detonation or "knocking" that can reduce power or, in the extreme, damage the engine. Of course, the N55, like all other current BMW engines, is equipped with knock control as part of the Digital Motor Electronics (DME) engine management system. On the N55 engine, the DME is now mounted directly to the top of the engine for better packaging and weight savings.
The significant loads and cylinder pressures of the N55 300-hp engine required the use of an aluminum engine structure with cast-iron cylinder sleeves. Altogether, the N55 weighs about 427 lbs. At least as important is the fact that this engine can deliver its peak torque from 1200 rpm - not far above idling - to 5000 rpm. On the road, this power and torque result in outstanding performance. The N55 equipped 2011 335i Coupe with the manual transmission can accelerate from 0 - 60 mph in just 5.3 seconds.
Finally, the new N55 engine is able to achieve a more favorable emissions signature than its predecessor. The single turbocharger has only one exhaust path and feeds a single catalytic converter in place of the previous engine’s two. This means the exhaust gases are concentrated at the catalytic converter for better cold-start emissions performance, making the N55 a more environmentally friendly engine.
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