BMW AG 3L DOHC I-6

In 12 years of creating the Ward's 10 Best Engines list, probably nothing caused more consternation among judges than shutting the door on BMW AG's classic high-volume inline 6-cyl. for the past two years.

This was deeply troubling, as there is perhaps no finer volume-production engine, all things considered, than BMW's standard inline 6-cyl.

Yet, not to be ignored was the onrush of high-powered V-6s (mostly from Japan) and a raft of new power- and efficiency-enhancing technology, including direct injection gasoline (DIG) fueling, cylinder deactivation and gas-electric hybridization.

For the last two years, BMW's baseline I-6 architecture was out-powered by market-comparable V-6s making as much as 65 more horses. With 235 hp in its highest spec, the engine most closely associated with the Ultimate Driving Machine had fallen off the pace, in terms of specific output and technical leadership.

Consider it but a brief interlude in BMW's onward march to perfect its brand-defining inline 6-cyl. layout: The new, 12th-generation (N52) '06 3L DOHC I-6 is an unqualified gem.

First, the numbers: 255 hp at 6,600 rpm and 220 lb.-ft. (280 Nm) of torque at 2,750 rpm. The figures represent a jump of 30 hp and a slight torque increase of 6 lb.-ft. (8 Nm) over the previous-generation M54 3L inline mill.

More telling, in relation to those previously mentioned V-6s, the new BMW I-6's specific output of 85 hp per liter now places it in the top tier of 6-cyl. power density, particularly now that some V-6s have endured a chopping of their output in the wake of new Society of Automotive Engineers horsepower testing procedures.

Most of the new N52's power gain comes thanks to the fitment of BMW's outstanding Valvetronic variable valve-lift system.

Complimenting the company's longstanding VANOS infinitely variable valve timing, Valvetronic – in use for some time on BMW V-8s and V-12s – increases the range of variable valve lift such that the engine no longer needs a conventional throttle.

Apart from the clearcut power gains, BMW engineers say Valvetronic cuts emissions and markedly improves efficiency – as much as 12%.

However, in some driving modes, Ward's testers noted much greater gains – as much as 34 mpg (6.9 L/100 km) in steady-state highway cruising.

From a driveability standpoint, Valvetronic has extended the available engine speed range. Redline now is a heady 7,000 rpm, a surprisingly satisfying expansion from the repressed 6,500-rpm limit of the M54, and Valvetronic also pulled the torque peak down from 3,500 rpm to an ultra-flexible 2,750 rpm.

But the headline-making technical leap for this new 3L I-6 is the move to an aluminum-magnesium hybrid engine block.

BMW says the bedplate and upper crankcase are magnesium alloy. Magnesium for the bedplate is a trick borrowed from racing, and the magnesium upper crankcase forms a jacket around the structural aluminum block. The previous 3L inline 6-cyl. had cast iron cylinder liners.

It probably does little as far as your right foot is concerned, but this new magnesium-intensive structure makes this fabulously performing 3L DOHC I-6 22 lbs. (10 kg) lighter, not to mention stronger.

And space doesn't permit a list of the other worthy improvements to engine internals, or the trick new electrically driven water pump.

It all adds up to a spectacular update to an outstanding layout. The new Valvetronic system intensifies the already thrilling throttle response and acceleration rip for which BMW inline 6-cyls. long have been glorified.

The additional revs are welcome, too, as we always believed BMW's ultra-smooth sixes were good for way more rpm that we were getting.

Best of all, these improvements come with a general increase in fuel economy. And of course, that spectacular smoothness and lack of vibration is even more enjoyable.

It's simple: BMW's latest inline 6-cyl. uncategorically is its best yet.

Judges' Comments

McClellan: I drove the 330i. Therefore, I am. Wow!

Murphy: Silky smooth in casual driving, yet always eager to grunt about.

Winter: It just doesn't get any better than this!

Banks: This engine is as close to perfect as an engine can be.

BMW AG 3L DOHC I-6

Engine type: 3L DOHC I-6

Displacement (cc): 2,996

Block/head material: magnesium-aluminum/aluminum

Bore x stroke (mm): 85 x 88

Horsepower (SAE net): 255 @ 6,600 rpm

Torque: 220 lb.-ft. (298 Nm) @ 2,750 rpm

Specific output: 85 hp/L

Compression ratio: 10.7:1

Fuel economy for tested vehicle (EPA city/highway mpg): 20/30

Application tested: 330i

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