Everything You Wanted to Know About the 2015 BMW M3 and M4's New Turbocharged Inline-Six Engine

Giving customers lots of choice today’s M3 has been offered in a trio different body styles: coupe, sedan and convertible. But going forward the company is changing things; the two-door has been spun-off as the new M4. Of course the M3 will continue, but only in sedan form.

The 2015 iterations of this legendary performance car are reaching back into M3 history by eschewing today’s high-winding V8 in favor of an engine layout that’s quintessentially BMW. The Bavarian automaker is basically synonymous with the inline-six configuration the way Leonardo da Vinci equates to visionary genius or the NSA to unscrupulous privacy violation. These two-cars are powered by an exciting new twin-turbocharged powerplant, and here’s an in-depth look at this exciting new M engine.

BACKGROUND

Before wading neck-deep in engineering details let’s get a few of the basics out of the way first. Called the S55, this straight-six displaces 3.0-liters and features a duet of single-scroll turbochargers hanging off its cylinder head. These blowers multiply atmosphere like bacteria in room-temperature lunchmeat and help the engine cut a check for 425 hp and 406 lb-ft of torque.

When matched to a seven-speed dual-clutch automatic transmission it can propel the new M cars from a standstill to 60 miles an hour in a scant 3.9 seconds. True car connoisseurs that opt for the traditional six-speed manual gearbox will get there a whisker slower, hitting the mile-a-minute velocity in about 4.1 seconds.

If you remember back to the E46 M3, which was produced from roughly the year 2000 until 2006, it featured a naturally aspirated, iron block, 3.2-liter inline engine that was referred to as the S54. Today’s new turbo unit sounds like the spiritual successor to this heroic powerplant.

THE BASICS

BMW offers several different flavors of turbocharged six, but the unit found in their new M cars is totally unique. Dave Buchko from the company’s product and technology communications department said “some people thought it would just be a modified version of our regular Twin-Power turbo engine,” but that is not the case.

This M powerplant is a totally different architecture. To support its massive output Buchko said “the block and internals are unique to this engine.”

Like most modern powerplants the S55 features a weight-saving aluminum block and head. Helping trim additional grams – and engineers fight for each one they can these days – the oil pan is constructed of exotic-sounding and feather-light magnesium, though it feels like plastic when you drum your fingernails on it.

This engine has all the ingredients of a truly great powerplant, but the 4.0-liter V8 in today’s M3 is equally stellar. It delivers 414 hp and revs to the moon, topping out at 8,400 RPM. Why has the company decided to ditch this impressive piece of kit?

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“High-revving normally aspirated engines are not known for their efficiency” said Buchko, noting that the new turbo-six delivers “more horsepower (and) definitely more torque” than the outgoing V8. In fact it delivers the full 406 LB-FT of twist from 1,850 all the way to 5,500 RPM meaning its torque curve is about as flat as a machinist’s granite surface plate.

It’s exactly the same story when you’re counting horses instead of torque, though the equine critters manifest much higher up the tachometer. The S55 cranks out a full 425 ponies between 5,500 to 7,300 RPM.

Even performance cars aren’t immune to fuel-economy improvements and the 2015 M3/M4 combo should be nearly 25 percent more efficient than their direct predecessors. According to the EPA, today’s coupe averages 16 miles per gallon in mixed driving. If engineers can deliver on their efficiency promise the 2015 models could hit an average 20 mpg, which is a huge improvement.

DEVILISH DETAILS

Further aiding fuel economy the new turbo-six is roughly 22 pounds lighter than the outgoing V8. This mass reduction is part of an overall theme. The 2015 M3 and M4 feature quite a bit of carbon fiber; most notably their roofs are constructed of this advanced material.

The S55 inline-six features direct fuel injection, which sprays a fine mist of gasoline right into the center of its combustion chambers. Staying in the cylinder head it also features variable valve timing on both intake and exhaust camshafts as well as the company’s Valvetronic system.

With forced induction variable valve-lift technology hardly seems worth the cost and complexity but Buchko said “it makes a difference.” Valvetronic cuts pumping losses by eliminating the throttle plate and helps improve the engine’s responsiveness. “Issues like turbo lag are a thing of the past” noted Buchko.

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Moving from head to block, the S55 features a closed deck for maximum strength. Additionally, its cylinder bores have a high-tech wire-arc spray-coating instead of traditional liners, which saves significant weight. Are you starting to see a pattern here?

As for the lower end a super-rigid forged crankshaft supports the engine’s rotating components from idle to redline. And don’t worry, this baby still screams, if not as frantically as the outgoing V8. It winds up to 7,600 RPM, which is unusually high for a turbocharged engine.

When asked whether or not the S55 will see service in other applications Buchko said “Who knows? Time will tell.” And that right there is a definite maybe, folks.

AVAILABILITY

The 2015 BMW M3 and M4 are set to arrive at dealerships early this summer. With an intoxicating blend of performance and curb appeal we cannot wait to sample these high-performance machines and put their brand-new engine through its paces.