Let us just say now that there’s nothing wrong with BMW’s N54 engine, the current twin-turbo unit that is, as of the 2010 Detroit show, offered from the bottom (135i) to the top (740i) of the company’s product range. In fact, we could give you a pretty long list of reasons why it’s one of the best engines in the world, and others agree—more than three years after its debut, the N54 is still winning awards left and right. But now there’s an N55, and supposedly, it’s even sweeter.
To believe that claim, you’ll have to wash your brain of the preconceived notion that more is better, because in place of two smaller turbochargers working alongside one another, BMW’s new turbo six goes to work with one single, albeit twin-scroll, turbocharger pumping in air. One thing that does carry over from the outgoing twin-turbo engine, though, is BMW’s attention to detail and the engineering team’s fight against turbo lag. As such, this new engine uses a very clever exhaust manifold and turbocharger design that is essentially a concentrated twin-turbo design. Inside the manifold, the front three cylinders are completely divided from the rear three, so while from the outside it appears that gas from all six cylinders is routed through one exit to the turbocharger, there’s actually a wall inside splitting the air in half. That splitting of air is continued right into the twin-scroll turbocharger housing.
The idea of this system is to split the pulses of exhaust gas so that regardless of where the engine is in its firing order, there’s always good pressure and as such, reduced lag. Additionally, the design allows for a lower exhaust gas counter-pressure buildup at low engine speeds, again reducing lag. It’s like having two turbochargers but without the complexity and like having one, only without the drawbacks of having to use one large, lagging turbo in place of two smaller ones. The best of both worlds.
Most of the engine’s efficiency gains don’t come from the turbo rethink, however. They instead come from the fact that this is BMW’s first ever combination of direct injection, turbocharging, and its Valvetronic system. The latter, which was used on the 3-series before turbocharging came into the story, is a fully variable valve management system. It allows for infinite control and adjustment of stroke on the intake side, making throttle butterflies unnecessary (though the throttle plate is not deleted completely, as it serves as a backup system in the event of Valvetronic failure.) As such, throttle losses are minimized while engine response is sharpened, since the exact air mass entering the cylinders can be measured more precisely. The result is better engine efficiency.
Valvetronic hasn’t been used on BMW engines with direct-injection thus far because there simply wasn’t room in the head to fit all of the components of both systems. This new generation of the system is more compact, allowing it to return. Additionally, this new Valvetronic uses a new adjuster with an integrated sensor that allows even quicker adjustments than before. According to BMW, the inertia factor of the adjustment process is just one-tenth of the former system.
For as much that has changed dramatically in the transition from N54 to N55, some things have been just slightly tweaked. This includes the power figures, which go from an even 300 hp and 300 lb-ft of torque to a increased 306 hp but a decreased 295 lb-ft. What the changes have done, though, is stretch the torque plateau down in revs. Instead of peak torque coming at 1400-5000 rpm, it is now served up from a low 1200 rpm on up to the same 5000. Peak horsepower of the twin-turbo engine came at 5800 rpm, and it still does, but it doesn’t drop until 6400 rpm. Displacement goes unchanged completely at 2979 cubic centimeters, but the new single-turbo layout makes this new engine about nine pounds lighter.
This is one of those have-your-cake-and-eat-it-too moments, because while the engine promises to have more responsiveness and a meatier power curve, it’s approximately eight percent more fuel efficient than its twin-turbo brother. The EPA hasn’t gotten its hands on anything for comparative purposes yet, but if we apply an eight percent increase to the manual transmission-equipped 2010 335i sedan’s numbers, they jump from 17 mpg city and 26 highway to 18.3 mpg and 28.1 mpg. Nothing dramatic, but still a good improvement. BMW would rather compare the efficiency to its old 4.0-liter V8, which made similar power but was 29 percent less efficient. In the N55’s first application, the new 535i Gran Turismo, that improvement in coupled with BMW’s new eight-speed automatic, which the company says is good for another six percent improvement in fuel efficiency versus its current six-speed.
Valvetronic, an advanced new turbocharger, and direct-injection. BMW is throwing every bit of its latest technology at this newest and greatest in a long line of award-winning inline sixes. We haven’t experienced it first hand yet, but we’re ready to call this one already—the N55 is a world-class engine. You’ll get your first opportunity to buy one in the coming months when the 535i GT hits our market, but expect it to trickle through the rest of the lineup slowly, as mid-cycle refreshes, or as BMW calls them, Life Cycle Impulses, take place. It will likely make it to the X5 next, and perhaps the 2011 3-series coupe and convertible. Last on the list will probably be the Z4 and the 740i, which won’t be due for improvements for some time still. But within a few years, don’t be surprised if the twin-turbo N54 disappears completely in BMW’s quest for higher efficiency.