The Four-Cylinder Solution to BMW's Future
It’s been 12 long years since BMW sold a four-cylinder engine to the U.S. market — a market that in 1999 saw gas prices well under $2 per gallon, with burgeoning tech and telecommunications industries, and was generally considered to be experiencing boom times. In 1999, BMW thought the slower performance (and slower sales) of its four-cylinder option made little to no financial sense for lead-footed Americans. At the time, it was probably right.
But by now, things have changed. A gallon of premium gasoline routinely roller-coasters between the cost of a gallon of milk ($3.50 or so) and a cheap six-pack of beer ($4.99). Looming (and some say draconian) CAFÉ standards have every automaker scrambling to lower emissions and boost fuel economy — and to do it quickly. According to BMW, that makes the time right again in the U.S. for a four-cylinder solution to an age-old problem — offering acceptable performance while maintaining acceptable fuel mileage.
Fortunately, technology also has changed with the times. Thanks to tricky forced induction options, today’s small-displacement turbocharged engines can pack a wallop normally reserved for larger, naturally aspirated six-cylinder units. BMW’s small 2.0-liter turbocharged four-cylinder engine starts with an all-aluminum crankcase, adds direct-injection and variable valve timing (Valvetronic in BMW speak), and caps it all off with a twin-scroll turbocharger. The twin-scroll turbo is fed by two separate, spiral flows of exhaust gas, one from each of the two pairs of cylinders. The setup is said to optimize rotation of the turbine, producing more power while sharpening throttle response and reducing turbo-induced lag.
The end result is 240 horsepower and 260 lb-ft of torque — 15 fewer horses than the outgoing 3.0-liter I-6 it replaces, but 10 more lb-ft at significantly lower rpm. While the turbo-four is appreciably more compact than the inline-six it replaces, you might be surprised to hear the weight difference isn’t all that great. By the time the turbocharger and its associated plumbing and cooling systems have been added, BMW says the four-cylinder engine is just 22 pounds lighter than a six-cylinder unit producing comparable power. Total weight of the car remains roughly the same as the sDrive 3.0i at just under 3300 pounds.
As an aside, it’s worth noting that this engine has longer legs than just the Z4 range. BMW will begin installing it in the 5 Series sedan this October, has already put it in the global X1, and will likely bring it to the all-new 3 Series when that car debuts later this year.
Goosing the throttle of a 2012 Z4 sDrive 2.8i with a six-speed manual transmission (an eight-speed automatic is also available) along California’s famed Pacific Coast Highway, the I-4 Z4 doesn’t disappoint in the torque department. The punch is noticeable from just below 2000 rpm, steadily powering the roadster out of the serpentine twists and turns that follow the jagged shoreline. It’s not an overwhelming punch, but it is noticeable and it does help the car feel quick in the cut-and-thrust world of real city driving (BMW estimates mid-fives to 60 mph). The exhaust note that accompanies the rush of torque is fairly mellow — a deep grumble that will pop and burble occasionally on the overrun. Nearly as loud as the exhaust is the whistle of the single turbocharger spooling up and doing its thing. To the enthusiast who appreciates such mechanical noises, it’s not obtrusive. But with the casual driver, we suspect ears will be perked and questions asked (“What’s that strange whistling sound, Betsy?”) — especially with the top down.
Start-stop functionality has been added to this Z4’s repertoire in the name of heightened fuel economy, and the system seems to work well enough. Our only gripe is that restarts (typically initiated by clutching in when it’s time to move off again) can be a bit rough, sending a shudder though the little roadster. It’s guessed that fuel economy will be increased by around 20 percent over the outgoing naturally aspirated six-cylinder car, putting highway figures into the low 30-mpg range. That’s something even sports car enthusiasts can get behind.
Unfortunately, it’s the casual drivers who will appreciate this engine the most. BMW has specifically tuned the turbo for that low-end punch it gives, but at the sacrifice of the top end. Sadly, as we previously experienced with this engine in the small X1 crossover, the little 2-liter turbo runs out of steam at anything much above 5000 rpm. While that’s well and good for keeping emissions low and fuel economy high, drivers who would otherwise revel in running the little mill out to its 7000-rpm redline will be rewarded only with a harsher, almost strained, exhaust note and no extra power.
That said, most buyers shopping for an entry-level Z4 won’t care about any of that. They’ll be too busy cruising around town, top dropped, chatting to friends via the new standard Bluetooth connection, listening to music via the new standard iPod connection, and appreciating the increased fuel economy their new sports car is getting at a price increase of roughly $1000 from the outgoing model. (Official EPA figures aren’t yet available.) And for the rest of us, the rocket-powered, spine-tingling Z4 sDrive 3.5is is still available — a consolation prize we’ll gladly accept. Sounds like BMW’s got a solid plan, then.
|2012 BMW Z4 sDrive 2.8i|
|VEHICLE LAYOUT||Front-engine, RWD, 2-pass, 2-door convertible|
|ENGINE||2.0L/240-hp/260-lb-ft turbocharged DOHC 16-valve I-4|
|TRANSMISSION||6-speed manual, 8-speed auto|
|CURB WEIGHT||3300 lb (mfr)|
|LENGTH X WIDTH X HEIGHT||166.9 x 70.5 x 50.8 in|
|0-60 MPH||5.5-5.6 sec (mfr est)|
|EPA CITY/HWY FUEL ECON||24/32 mpg (MT est)|
|ENERGY CONSUMPTION, CITY/HWY||140/105 kW-hrs/100 miles (est)|
|CO2 EMISSIONS||0.72 lb/mile (est)|
|ON SALE IN U.S.||October 2011|