New BMW M3 and M4 engine secrets laid bare

BMW's M division has lifted the veil of secrecy on the inline six-cylinder engine bound for its upcoming M3 sedan and new-to-the-range M4 coupe and convertible, revealing a potent twin-turbocharged 3.0-liter direct injection gasoline unit that produces an official 424 hp at 7,500 rpm and what officials describe as "significantly more than 369 lb-ft" of torque.

The newly developed engine, known under the internal codename S55 B30, shares its 84.0 mm bore and 89.6 mm stroke with BMW's standard turbocharged 3.0-liter in-line six-cylinder direct-injection gasoline mill, the M55 used across the German car maker's lineup, endowing the aluminum block unit with an overall swept volume of 2,979 cc.

This represents a sizeable 990 cc reduction on the engine BMW's M division's new unit replaces, the S65. The 4.0-liter V8 direct-injection gasoline unit, the last in an illustrious line of traditional naturally aspirated engines from BMW's revered M division, has been resigned to the history books after just six years of service in the outgoing fourth-generation M3 coupe, sedan and convertible -- a continuation of an engine downsizing program that has taken root throughout BMW M division's lineup in recent years.

Significantly, the new S55 B30 is the smallest capacity engine to be used by the M3 since the discontinuation of the first generation model and its naturally aspirated 2.3-liter four-cylinder multipoint-injected gasoline unit, the S14 B23, back in 1991.

Yet despite the 25 percent reduction in capacity and the loss of two cylinders, the new BMW M division engine delivers 10 hp more than its predecessor, thanks to patented induction process that is claimed to provide vastly improved levels of combustion efficiency, in the process preserving an illustrious 27 year tradition that has seen each incarnation of the M3, the ranks of which have now been reduced to a single four-door sedan model alongside two-door M4 coupe and cabriolet models, boast more firepower than its direct predecessor. But while the new S55 B30 develops its maximum power of 424 hp at 7,500 rpm, the old S65's 414 hp arrived at 8,300 rpm.

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We checked out the BMW M4 in Munich.

With 142 hp per liter, the new twin-turbocharged six-cylinder engine betters the specific output of the old naturally aspirated V8 by a substantial 38 hp per liter. As a measure of its overall efficiency, it also develops 16 hp per liter more than the twin-turbocharged 4.4-liter V8 direct injection gasoline engine used by the recently facelifted M5 as well as the M6 coupe, cabriolet and GranCoupe. The ignition cut out is yet to be officially revealed, although officials suggest it is pegged at 7,600 rpm.

Click here to find out why the new M3 and M4 engine doesn't quite cut it for us.

Among the technical highlights for the new BMW M division engine is its closed deck design for added rigidity. It also uses a spray coating instead of traditional cylinder bore liners for reduced weight. A forged crankshaft is also used for reduced rotating masses, which in combination with lightweight pistons, is claimed to bring about a significant improvement in throttle response compared to BMW's standard turbocharged 3.0-liter inline six-cylinder gasoline engine.

With BMW's M division claiming an overall weight of "under 3,307 pounds" for the new M4 coupe, which uses selected carbon-fiber-reinforced plastic panels and bracing components as part of a weight saving initiative, the new two door promises to boast a weight-to-power ratio better than the discontinued fourth generation M3 coupe's 8.4 lb per bhp.

An even lighter version of the new car, possibly resurrecting the famed CSL nomenclature and fitted with lightweight seats as part of a pared-down interior and other weight saving measures, is under consideration for sale later in the new M4's model cycle. BMW M division boss Freidrich Nitschke suggests it could come in close to 3086 pounds if it receives a production greenlight.

Although BMW M division isn't prepared to go public on the official torque figure some three months prior to a planned public premiere for the new M3 and M4 at the 2014 Detroit auto show, it does confirm the S55 B30 delivers "significantly more than 369 lb-ft at well under 2,000 rpm". By comparison, the non turbocharged S65 delivered 295 lb-ft at 3,900 rpm.

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A screenshot of the animation video.

BMW M division development boss, Albert Biermann, describes the added torque as being "central to performance gains" for the new M3 and M4. He points to a 70 percent improvement in the torque loading of the new engine in comparison to its predecessor at 2,000 rpm. This leads to improvements in real world performance, with in-gear acceleration claimed to be greatly enhanced. There is no official 0-62 mph time yet, but Autoweek has been told the M4 coupe will undercut the official 4.8-second time of the old M3 coupe "by some margin," suggesting a time in the region of 4.5 seconds. Top speed, as with all M-cars, will be limited to 155 mph in standard guise.

At the same time, BMW's M division suggests the new M3 sedan and M4 coupe will be certified with combined fuel economy of over 27.5 mpg, thanks in part to the inclusion of a stop/start function and brake energy recuperation.

Despite the similarities in specification between the new M3 and M4's engine and that of the earlier 1-series M coupe, Biermann, reveals the two engines share little apart from their bore and stroke measurements. "They are completely different, not only in construction but internally and their respective induction processes," he says, adding, "They're worlds apart."

Click here for our ride-along test with the new M4 engine.

At the heart of BMW M division's new twin-turbocharged 3.0-liter inline six-cylinder, which is mounted longitudinally, is a patented induction process that uses two low-inertia turbochargers. Eschewing both twin-scroll and variable vane geometry, the IHI units provide a relatively conservative maximum boost pressure of just 18.9 psi.

"There's nothing particularly groundbreaking about the turbochargers themselves. But the induction system is significantly more free-flowing, with greater volumes and a faster transfer of air through the entire system, than with either the 1-series M engine or BMW's regular turbocharged 3.0-liter engine," says Biermann.

BMW's M division engineers have mounted the water-to-air interooler system for the turbochargers atop the engine instead of out in front, justifying the inclusion of a prominent power dome within the bonnet. Unique features such as a magnesium sump ensure the new engine weigh 22 pounds less than the naturally-aspirated 4.0-liter V8 direct injection gasoline unit it supplants at a claimed 452 pounds.

The engines heady reserves are channeled to the rear wheels via a standard six-speed manual gearbox. A development of the ZF-produced unit used in the 1-series M coupe, it uses a newly designed twin plate clutch to handle the added torque loading and is claimed to weigh 26 pounds less than the similarly specified gearbox used in the fifth-generation M3. A development of the existing Getrag-produced seven-speed dual clutch Drivelogic gearbox with remote paddle shifters will be available as an option, providing the M3 and M4 with automatic shift capability and a launch function. Described as a third-generation unit, it adopts a raft of changes that Biermann suggests provide it with "vastly improved shift quality."

A further development brought to the M3 and M4's driveline is a new single piece carbon-fiber reinforced driveshaft. Claimed to weigh 40 percent less than the all-steel unit used on the old M3, it brings a reduction in reciprocating masses and what BMW M division engineers are claiming is sharper response. It acts on a reworked M differential that now uses an electric actuator to constantly vary the locking effect. As on the M5 and M6, it is capable of delivering 100 percent lock-up. Biermann suggests the new driveline components bring about improved traction and on-the-limit adjustability.

Underpinning the M3 and M4 will be a common chassis boasting unique track widths, forged aluminum suspension components and a rear axle supported by a new steel carrier that, unlike that of the old M3, is bolted directly to the body, without the use of traditional rubber bushings. The new carrier is claimed to provide added lateral rigidity, leading to what Biermann describes as "extremely precise wheel control and a reduction in roll steer." Overall, some 17.6 pounds have been pared from the standard 3- and 4-series suspension – 11 pounds up front, and a further 6.6 at the rear, leading to lower unsprung masses at each wheel.

Official figures are not yet being made available, but the overall footprint is significantly larger than that of either the standard 3- or 4-series, requiring the use of widened fender panels to house forged aluminum wheels of up to 19 inch diameter wearing 255/35 ZR 19 front and 275/35 ZR19 rear Michelin Pilot Super Sport tires. The steering is an electro-mechanical system, which is a first for the M3. "Our mission is to provide the best electro-mechanical steering. There is no doubt: it is better than the hydraulic system of the old M3," boasts Biermann.

As with the latest M5, the driver can choose between three different driving modes: efficiency, sport and sport Plus. Each mode provides differing mapping characteristics for the steering, throttle, gearbox (on models equipped with the optional seven-speed dual clutch unit) differential and threshold of both the traction and stability control systems. The damping also boasts three modes: comfort, sport and sport plus.

Meanwhile, BMW has revealed both the new M3 sedan and M4 coupe will a carbon-fiber-reinforced roof panel. The hood and front fenders are aluminum pressings with the doors and rear fenders made of steel.

Following their unveiling at Detroit auto show in January, the M3 sedan and M4 coupe are planned to go on sale in North America in May 2014.

By Greg Kable