Out With the Six, In With the Four
Published: 08/24/2011
-
by
Mike Monticello, Road Test Editor
It took about 20 seconds after we got onto California's Pacific Coast Highway to realize why BMW's all-new turbocharged four-cylinder is a good fit in the Z4 two-seater. That's the time it takes to lower the 2012 BMW Z4 sDrive28i's two-piece power-retractable hardtop, and doing so lets you properly experience its mechanical soundtrack.
Suddenly, we could hear the whistle from the turbo as it spooled up and down, not to mention some cool decelerative pops from the exhaust pipes. With the windows and top up, all you can hear is a small four-cylinder working away, one that's not particularly sonorous high in the revs, either.
Then again, 7,000 rpm is not where this engine does its best work. Nope, BMW's first four-cylinder in 12 years is better at delivering midrange power. And although the N20 (its internal code name) doesn't pack the supple character of an inline-6, the new turbo-4 packs something the normally aspirated engine doesn't — personality.
What's in a Name
In BMW-speak, the "TwinPower Turbo" label on the 2012 BMW Z4 sDrive28i's engine shroud refers to the combination of direct injection and Valvetronic intake control, not a statement that the N20 has twin turbos. Further muddying the BMW naming waters, the car is labeled "28i," not 20i, because the engine "provides the power of a 2.8-liter six-cylinder," or so says BMW. We'll tackle the German reasoning for the wordy "sDrive" tag another day.
Confusion and misleading labeling systems aside, the N20 turbo-4 produces 15 fewer ponies than the outgoing inline-6 Z4 base engine, although it reaches its peak of 240 horsepower at 5,000 rpm versus 6,600 rpm.
But torque is what the N20 is really all about, generating 258 pound-feet from 1,250-4,800 rpm. The old inline-6? Just 220 lb-ft at 2,600. And you feel that torque hit. True, there's a little bit of turbo lag, but by 2,500 rpm things are really humming along. There's no need to rev the turbo-4 to its 7,000-rpm redline either, as anything above 6K is mostly just noise with little reward. That said, having an actual spike of motive force makes this engine more exciting to drive versus the nondescript delivery of the N52's silky-smooth inline-6 power.
The 2012 BMW Z4 sDrive28i comes standard with a precise, if slightly notchy, six-speed manual transmission with reasonably short throws. But the car we spent the most time in was fitted with the no-extra-cost automatic, now with eight forward gears (replacing the previous six-cogger). Our tester also had the benefit of the $3,900 M Sport package, which — besides driver-controlled Adaptive M suspension, 18-inch light-alloy wheels, an aero body kit and sport seats — adds an M steering wheel with paddle shifters. The tranny's shifts are always smooth regardless of mode; manual downshifts (via the paddles or the console lever) are aided by computer-controlled throttle blips.
Our seat-of-the-pants impression tells us the Z4 with the turbo-4 has a significantly stronger midrange than the outgoing six. In terms of pure acceleration numbers, BMW claims the six-speed manual-equipped Z4 sDrive28i hustles to 60 mph in 5.5 seconds, a tenth quicker than the three-pedal Z4 sDrive30i. But it's with the new automatic where the biggest gains are seen, 60 mph arriving in just 5.6 seconds, a good 0.4 second quicker than the previous six-speed auto/inline-6 Z4.
BMW expects the Z4 sDrive28i with the eight-speed automatic will see about a 20 percent improvement in fuel mileage over the auto-equipped Z4 sDrive30i it replaces, which should put its EPA figures somewhere around 21 city/33 highway mpg. The six-speed manual Z4 will also get the benefit of start/stop technology.
A Weighty Matter
Besides the superior power curve characteristics of the new turbo-4, it's also smaller and lighter. A BMW spokesperson told us the new engine with the manual gearbox weighs "about" 33 pounds less than the outgoing inline-6, while the automatic version sees the fat trimmed by about 44 pounds. As such, curb weight on the manual model went from 3,252 pounds for the 2011 car to 3,263 pounds.
Wait, say what? Yes, you read that correctly, the 2012 BMW Z4 actually weighs 11 pounds more than the previous car due to "a substantial increase in standard equipment," according to BMW. Apparently floor mats are now standard on the Z4. Floor mats, folks. Truth be told, the eight-speed automatic model does, in fact, weigh 22 pounds less than before.
Since the Z4 sDrive28i's weight has hardly changed, we can't report any drastic updates in the way this roadster handles, although the shorter engine does shift some weight rearward. Still, drive the Z4 hard on a twisty road and you realize it offers nowhere near the precision of an M3, especially when it comes to the Z4's electric-assist steering. The responses coming through the perfectly thick M steering wheel remain on the vague side, although the three-way-adjustable Adaptive M suspension and grippy, staggered-width 18-inch Bridgestone Potenza summer tires deliver plenty of stick.
It Prefers a Smoother Road
After a day spent in a Z4 equipped with the automatic, we spent a valuable hour flogging a six-speed manual test car on Carmel Valley Road near Monterey, a winding stretch of blacktop that could serve as a suspension test loop for just about anything. The stiffest Sport Plus setting actually proved a bit too harsh and bouncy, the Z4 skittering about over the seemingly endless bumps.
The smooth, new pavement of Highway 33 near Ojai was much more to the Z4's liking, where it could strut its high grip level and show off the turbo-4's plentiful torque. That being said, don't mistake the Z4 sDrive28i for a tool with which you can execute easy power-on-oversteer corner exit slides. It just doesn't have that kind of punch.
The brakes were impressive, especially during heavy use on the CVR torture test. The pedal remained firm throughout, and we saw no fade and minimal brake odor. It was also on CVR that we appreciated the sport seats' extra bolstering.
The interior is little changed from previous second-generation Z4s. The center stack is clean and largely devoid of excessive buttonry due to the iDrive controller, and materials are mostly of a high quality. This is classic roadster simplicity here, and we like it. There is, of course, the afterthought cupholder stabbing the passenger's left knee.
Blasphemy or Brilliance?
BMW doesn't shy away from controversy. There were the Bangle Years. Early generations of iDrive. A V8-powered M3. A turbocharged 1 Series with an M badge. And now BMW has replaced one of its ubiquitous inline-6s with a turbo-4.
In the case of the 2012 BMW Z4 sDrive28i, you're getting a slightly quicker, more fuel-efficient and ever-so-slightly more exciting two-seat sports car. Not a bad deal, right?
Well, sort of. Despite losing two cylinders, a liter of displacement and a few horsepower, the 2012 BMW Z4 sDrive28i will start at $49,525 (including $875 destination). That's actually more expensive than the sDrive30i it replaces. The extra cost comes from added standard features like Bluetooth, USB integration, an alarm system, a ski pass-through and those aforementioned floor mats. BMW says it's an actual savings of $500.
Yes, it is a lot of money for a four-cylinder. It's also a lot of money for a Z4. But the N20 is the way of the future, an enthusiast's engine that also makes sense from an efficiency standpoint. So much sense, that BMW will plop it into the 2012 528i when that car starts production in September, surely followed by the X3 and 3 Series soon after. We expect few will miss the six it replaced.
Edmunds attended a manufacturer-sponsored event, to which selected members of the press were invited, to facilitate this report.
