New M4, M3: A Light(er)weight Technical Turbo Masterpiece

How did BMW M go about, as Colin Chapman so famously put it, “adding lightness”? In a synergistic process of engine and chassis development that saw tiny gains over here, a few kilos over there—but it was never a matter of just grabbing a bale of carbon-fiber and sticking it here and there. Yes, the car does employ some carbon-fiber components, and those are lighter than their counterparts in aluminum or steel. But in the M4, they are also better solutions to various tasks. The trunk lid, for example is a composite of carbon-fiber and plastic shield-molding compound; not only does it provide a built-in aerodynamic element to reduce lift at the rear, it can be painted by conventional assembly-line processes. Oh, and it saves five kilograms.

Carbon-fiber is used in the roof of the M3—now you can get the M3 sedan with a carbon-fiber roof, too—and in the basement: The driveshaft, or torque tube, is wound of the stuff. That saves a few kilograms, sure—but it also creates a much stiffer driveshaft, with such torsional rigidity that it doesn’t need the center bearing of its predecessor. Since it has less rotational mass, it has less inertia to resist acceleration.

While the BMW M3 CRT—for Carbon Racing Technologies—provided at least some of the inspiration for the M4, the hood of the M4 is made of aluminum, not carbon-fiber, because pedestrian-safety standards demand a certain crushability that is just not in the nature of carbon-fiber. Under the hood, however, you’ll see a strut-tower brace made of tubular- braid CFRP. (That’s not the only stiffening brace, mind you; the structural rigidity of the M4 chassis is such that Albert Biermann, vice-president in charge of technical development at M GmbH, considers the body now to be an integral element of the suspension system.)

The electric steering rack saves some weight as well. But the main purpose of the use of computer-controlled electric assistance is simply the fact that finally an electrical unit lives up to the promise implied by its 21st-century technology: to provide appropriate road feel to an M car. The benchmark was high: the hydraulic steering system of the E92 M3. But M engineers are confident that they have matched—or exceeded—the precise road feel of that car.

The weight of the M4 is also affected by your choice of options, of course. If you absolutely have to have a sunroof, well, I suppose it’s your choice throw a pile of weight at the car and put the poundage up as high as you can, to screw up the center of gravity. And you won’t make that 3,300-pound fighting weight if you opt for the seven-speed dual-clutch gearbox; it’s heavier than the standard six-speed—which itself is derived from the six-speed manual of the 1 M coupe, although obviously reworked to stand up to the increased power and torque of the M4.

Important Fact Number Two: This car will make every previous M3 feel like a Flexible Flyer.

How is the differential attached to the body of your car? Most likely, it’s bolted into a cradle or subframe of some sort, and that subframe is attached to the body with rubber bushings. Oh, but wait: Suppose (said M engineers as they designed the current M5 and M6) we mounted that carrier assembly directly to the body? Would that not stiffen the chassis? And then we could isolate the differential within this subframe by some kinelastic bushings that would hold it securely in place, minimize noise and vibration from the Active  M differential? That’s another page from the M5 book of handling.

And we’d better make most of the rear suspension out of forged aluminum while we’re at it. Adding lightness, remember? Come to think of it, the front end could use aluminum forgings, too—along with a variety of stiffeners. That way, when you turn the wheel, the chassis will react with astounding speed and precision. And since we’re eliminating so much of the flex of the body, keeping the suspension much more predictably in line, we can widen the wheels and tires without fear of their hitting anything: 255/35-19s on the front, 275/35-19s on the rear.

Yes, you’ll need nineteens to clear the carbon-ceramic brakes that surely you will want as an option. And you want these brakes, not only because they will sop you with more confidence and aplomb than you deserve but also because they are so much lighter than iron.

See that synergy yet?

Important Fact Number Three: Forget what you think you know about the engine. Yes, it’s a twin-turbo straight six, and that’s a familiar refrain; the 335i started singing that song in 2006 with the N54 engine. But while that engine and the new engine in the M4—we might as well refer to it as an S58 until M GmbH delivers the proper nomenclature—both have an aluminum block with six holes, that’s about where the resemblance ends. First, this closed-deck (rigid) block has no steel liners; cylinder bores are treated to an arc-spray coating process and slipper pistons are dished, not domed, forming an efficient firing chamber. Intake air is controlled by the Valvetronic sytem for precise and instantaneous throttle response.

The turbochargers mounted low on the right side of the engine, each taking exhaust pulses from three cylinders, are eentsy-beentsy little things; not only are they smaller and lighter, but a smaller turbo has less rotating mass to spin up to speed. These puppies twirl up to about 200,000 rpm, and they push pressurized air through a huge water-cooled intercooler atop the engine; at last, a reason for that dome in the hood!

A new ultra-precision direct fuel-inject system not only powers the beast but ingeniously solves an inherent glitch in turbocharged systems. Ordinarily, if you lift your foot off a turbocharged car, the revs die, so when you put your foot back in it, the turbo has to spool up again. But the M4 does an amazing thing: In Sport or Sport Plus modes, when you lift, the computer quits sending fuel pulses to some—but not all—of the cylinders. The remaining cylinders continue to keep the turbines spinning, dropping to perhaps 100,000 rpm. So when you give it some gas again—literally—there is no turbo lag. Repeat: There is no turbo lag. Or at least none that I could detect from the right-hand seat while DTM racer Agusto Farfus demonstrated his confidence in such heresies as electric power steering.

The crankshaft is a forged and lightened unit, too—all the better for that higher torque, my dear, some 30% more than is generated by the S65 engine in the E92 M3. The oil pan is cast of magnesium, and features fore-and-aft scavenging pumps to make sure oil is delivered where it has to go at all times. Remember, oil cools those little turbochargers; the M4 engine continues to pump oil through them even after the car has been parked, avoiding “coking” the turbos, a malady often afflicting turbocharged cars of the last century.

Yes, the S65 V8 of the prior generation is a magnificent engine, perhaps the pinnacle of M GmbH’s naturally spirited engine technology. But even the most ardent lover of that car will admit to a grimace of pain when it comes time to fill it up. The new engine delivers a significant increase in power, a whopping increase in torque, and about a 25% increase in fuel efficiency. You already understand the magic of double-VANOS variable valve timing, the advantages of precision fuel injection and an intake system that uses the intake valves themselves to control incoming air. All the evolutionary developments of the last twenty years have apparently found their way into the new M3 and M4.

Which is exactly as it should be—because the M3 has never been just a driver’s car. For BMW, the M3 has always been the driver’s car, the touchstone, the reminder of what M GmbH has always been about. Yes, they build M Performance cars now—with diesels, even—and M is where you go when you want something by BMW Individual. The M5 is a fabulous flyer, and the M6 Gran Coupé is a magnificent statement of luxury and power.

But the M3—and now the M4—is for the nut-job gearhead drivin’ fool who will grab the car and wring the hell out of it, just because that’s what we were put on Earth to do. And holy mother of Stroker Ace, the M boys have given us the tool to do that job.—Satch Carlson

Video: DTM boys whoopin’ it up on the Nürburgring in the M3 and M4.