Cutaway view of the direct-injection inline 6. Click to enlarge.

The 335i’s new inline-6 engine (designated N55) displaces 3.0-liters and develops its maximum output of 300 hp at 5,800 rpm—70 hp up on the 2011 328i models—with peak torque of 300 lb-ft (407 N·m) 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 application of twin-scroll technology and the integration of Valvetronic, this new engine is more fuel efficient.

The 335i Coupe and Convertible can accelerate from 0 - 60 mph in 5.3 (5.5) seconds and 5.5 (5.7) seconds, respectively, when equipped with the manual (automatic) transmission.

The twin-scroll charger. Click to enlarge.

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.

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 air intake combustion with virtually no delay and with reduced pumping losses. As a result, the engine makes power more quickly than ever before—shown 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 knocking that can reduce power or, in the extreme, damage the engine. 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 (194 kg). The 335i Coupe and Convertible dual exhaust system runs at both sides of the vehicle. At low loads, a flap channels most gas through one side to reduce low-frequency exhaust rumble. The 335i Coupe and Convertible feature an air-to-oil external oil cooler mounted in one wheel well as opposed to the N52’s coolant-to-oil unit.

The new N55 engine is also 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.

The N52. The 328i Convertible is powered by BMW’s 230 hp, 200 lb-ft, 3.0-liter inline-6 engine, known internally as the N52. Its magnesium/aluminum construction and Valvetronic variable valve lift are features found only on BMW engines. The N52 achieves impressive progress on all performance and technology fronts, especially in its remarkably light weight of 357 lbs. An aluminum/magnesium engine block, hollow camshafts, plastic camshaft cover, improved combustion chambers, a further evolved Double VANOS (VAriable NOckenwellen Steuerung = variable camshaft control, or variable valve timing), higher fuel injection pressure, sophisticated engine electronics, an electric coolant pump, a variable-volume oil pump and an oil/coolant heat exchanger are the other weight-saving features and improvements of this engine over previous generations.