The temptation may have been great forAG to rest on its laurels and feel confident its inline 6-cyl. engines had secured their place in the pantheon of powertrain excellence over the past 40 years.
After all, the Bavarians have amassed more Ward’s 10 Best Engines honors than any auto maker on the planet – 25, and counting, since the competition began in 1995. And 20 of the awards were for I-6 gasoline engines.
Distill the data further, and nine of those honored 6-cyl. engines displaced 3.0L, give or take a cubic centimeter.
The latest honoree is – you guessed it – a 3.0L I-6 that produces 300 hp and 300 lb.-ft. (407 Nm) of torque with ridiculous ease and efficiency. Its place on this year’s list was locked as soon as the first Ward’s editor stepped on the accelerator during our test drive of the ’11 335i sedan.
This turbocharged, direct-injection N55 engine carries over’s outstanding I-6 all-aluminum architecture from the N54, which earned 10 Best Engines honors in 2007, 2008 and 2009 with twin turbochargers producing the identical 300 hp and 300 lb.-ft. of torque.
The key difference with the N55 is that it employs a single twin-scroll turbocharger that reduces weight and cost, improves packaging and minimizes turbo lag – not that the N54 ever suffered from it.
The new turbocharger builds up pressure much faster than its predecessor because the housing design provides a more direct route for exhaust gases. The torque peak arrives earlier, at a mere 1,200 rpm, compared with 1,400 rpm for the twin-turbo.
Another unique addition for the N55 is BMW’s third-generation Valvetronic intake system, which uses an electric motor built into the cylinder head to continuously vary the amount of valve lift. As a result, there’s no throttle to regulate engine power. Valvetronic allows the N55 to “inhale” air for combustion with virtually no delay.
Why put your engineers through such an exercise merely to achieve the same output as an older engine?
Because the N55 delivers up to 15% better fuel efficiency, and every auto maker, particularly luxury-oriented BMW, is under the gun to achieve a 35.5-mpg (6.6 L/100 km) corporate average fuel economy by 2016 in the U.S. and meet tough carbon-dioxide emissions limits in Europe.
During our test drive, Ward’s editors didn’t baby the 335i but managed 23 mpg (10.2 L/100 km), impressive for a 6-cyl. luxury sport sedan.
The single turbocharger has only one exhaust path and feeds a single catalytic converter instead of two in the N54. This concentrates the exhaust gases at the catalyst for better cold-start emissions performance, making the N55 more environmentally friendly.
So confident is BMW in this new N55 3.0L I-6 that it appears as the base engine in the new 5-Series sedan and even larger Gran Turismo.
But don’t discount the much-heralded N54. Instead of sending it out to pasture, engineers in Munich have bestowed the twin-turbo I-6 with an extra 20 hp and 33 lb.-ft. (45 Nm) of torque in the new 335is coupe. The extra power and sporty exhaust note make the 335is a poor man’s M3.
Consider the N54 and N55 as BMW’s double-barrel approach to a fiercely competitive luxury segment.
Either way, the single-turbo N55 is no patsy: It sprints to 60 mph (97 km/h) in 5.4 seconds.
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