2007 BMW M6
--> Article summary:
Once again, BMW M has blazed new trails in powertrain technology and performance. Model year 2000 saw a new V-8 engine for the then-current M5, a 5.0-liter unit developing 400 hp and thrusting that luxury sports sedan to 60 mph in 4.8 seconds. Then, in ‘01, M launched an inline 6-cylinder unit employing advanced low-mass valvetrain technology to achieve stunning high-rpm performance: With its “redline“ of 8000 rpm, this engine gave the new-generation M3 a nearly 100-hp increase from the former M3‘s 240 hp to 333 hp. This time, it’s M’s first V-10 engine for a roadgoing vehicle – introduced in the new M5 last fall and now powering the new M6 as well. In both models, it achieves essentially the same dramatic results.
Let’s begin a description of this amazing powerplant with some key data:
• 0-60 mph in 4.5 seconds
• 500 hp @ 7750 rpm, 383 lb-ft. of torque at 6100 rpm
• Redline 8250 rpm.
This new V-10, called S85 in BMW engine parlance, is yet another masterpiece of power from BMW M, setting a new milestone for the performance that can be achieved in a practical and luxurious automobile, with generous trunk space and the range of safety features one expects from BMW. So let’s look at the vital attributes of this monumental new engine, and see how they contribute to the impressive outcomes just listed.
Why 10 cylinders? Elmar Schulte, head of engine development at BMW, has a disarmingly straightforward explanation. “We wanted 5 liters. The ideal cylinder displacement is 0.5 liter. To get 5 liters, we needed 10 cylinders.”
In its general layout, the new engine relates to, and was inspired by, BMW’s Formula 1 racing engine – also a V-10. (Both engines’ major castings are done at the same BMW plant.) Though unusual, a V-10 is also a satisfactorily balanced configuration, requiring no balance shafts to make it acceptably smooth. Instead, the crankshaft incorporates two large counterweights. In its February ‘06 issue, Road & Track had this to say about it: “one of the most free-revving engines ever“…“like a Formula 1 car with mufflers. It plays such a ripplingly beautiful song up to its 8250-rpm redline that you find yourself running up and down through the gears just to hear it one more time.“
Even when idling, the V-10 sounds exotic. And, according to Automobile (July ‘05), “It sounds stunning, with a hard-edged snarl that is utterly distinctive and deeply intoxicating.“
High-rpm concept. Like the 6-cylinder M3 engine, the M6’s V-10 was conceived to exploit high engine speeds in the quest for the highest performance. Its maximum power of 500 hp is achieved at 7750 rpm, its maximum torque of 383 lb-ft. at 6100 rpm. This strategy, which avoids extreme torque and instead lets the driver extract maximum results by “revving,” allows the use of relatively light, low-mass reciprocating components inside the engine; in turn, this helps keep overall vehicle weight in check and optimize front/rear weight distribution.
Current M engines do not employ the Valvetronic system now found in BMW’s regular-production V-8 and V-12 engines as well as the new N52 6-cylinder unit in the 3, 5 and Z4 Series. Though Valvetronic eliminates throttles and their throttling effect, it is not (yet) suitable for a very high-rpm engine like this. Instead, the V-10 employs a typical BMW M valvetrain, with its 4 valves per cylinder actuated via “box-type” hydraulic lifters derived from racing practice. These are small, light and extremely rigid, as they must be for 8250 rpm; they are also specially shaped for efficient valve actuation, with an oblong cross-section (not round like bucket tappets), slightly curved contact surface and guiding tab to ensure a consistent position in their bores.
Also light in mass are the valves themselves, with stems of only 5 mm/0.2 in. And as on the new 6-cylinder, the camshafts are hollow, further reducing valvetrain inertia and enhancing engine response. Altogether, the valvetrain’s reciprocating mass has been reduced 17.5% from the former M5’s V-8 engine; this is an important facet of the high-rpm concept. So are light but ultra-strong pistons and connecting rods.
As in all recent M engines, a special High-Pressure Double VANOS system varies the intake and exhaust valves’ timing steplessly and ultra-quickly. Here the VANOS system has its own hydraulic pump; this contrasts with BMW’s regular-production engines, whose VANOS draws its pressure from the main oil pump. The resulting very high pressure (up to 115 bar/1690 lb./sq in.) enables the valve timing to be varied more quickly than on the regular engines – yet another factor in the high-rpm concept.
Four overhead camshafts actuate the valves. Each of the two intake camshafts, positioned inboard, is driven by a simplex chain; from the camshafts’ chain sprockets, the exhaust camshafts are driven by gears. The system is extremely rigid – again, as it must be for this engine’s level of rotational speed. Each chain is hydraulically tensioned and needs no periodic adjustment or replacement.
A throttle for each cylinder. Maintaining a key BMW M tradition, the V-10 has an individual, electronically controlled throttle for each cylinder.
“Breathing” – an engine’s ability to ingest and combust air – is naturally a top priority in a super-performance engine. Air is taken in at the vehicle’s front, passes through two large intake silencers and two air-mass meters, and then flows into the voluminous plenum (made of a lightweight compound material) atop the engine. From there, air passes through equal-length intake trumpets and the 10 throttles to the individual cylinders. The entire assembly of plenum and trumpets is attached to the throttle housings via a flange (one per bank) that acoustically and thermally decouples the plenum from the engine itself.
Compared to most engines’ single throttle (or even to the throttle-less Valvetronic system of contemporary BMW engines), these are a costly feature, reserved for the highest-performance engines. Positioned much nearer the cylinders than a single throttle can be, they eliminate a “lag time” inherent in airflow and foster lightning-fast response to throttle movements. Motor Trend characterized this engine’s throttle response as “blazingly urgent.”
Actuation of the throttles is electrically driven and electronically controlled. Each cylinder bank’s five throttles are actuated from a single shaft. Via the accelerator pedal, the driver gives the commands. In turn, these commands are processed by the engine control module and received by a DC servo motor (also one per cylinder bank). The motor drives the throttle shaft through a tiny gearbox.
Via the MDrive system (described later), the engine’s maximum power and the throttles’ response to the accelerator pedal can be set to different levels; see MDrive.
Ultra-high compression ratio. At 12.0:1, the V-10 notches up BMW’s highest current compression ratio. The higher the compression ratio, the more power can be extracted from a given engine size, so: another factor in the V-10’s performance.
Ionic-current technology. In a “world first” for such a high-revving engine, and a first for BMW, BMW M’s engineers have applied an exotic technology to the knock control that is crucial with such a high compression ratio. Instead of piezoelectric knock sensors positioned in the cylinder block to detect detonation via sound-pressure level, the sparkplugs themselves do the knock-sensing – and do it incredibly quickly and directly.
The air-fuel mixture in an engine’s cylinders has a certain electrical conductivity, which varies – especially as “knock,” or detonation, occurs in a cylinder. Built into the engine’s electronics is circuitry that applies a small voltage across the sparkplug’s electrodes just after it fires; the resulting ionic current is a function of the combustion process taking place. If knock (detonation) is incipient, the ionic current will so indicate; a signal goes to the engine’s electronic control system and ignition timing is retarded appropriately.
In all BMW knock-control strategies, each cylinder’s combustion process is measured individually; if only one cylinder is tending to knock, only that cylinder will have its ignition timing retarded. The advantages of this technology over conventional block-mounted knock sensors are
- Ignition adjustment occurs more quickly. The whole process – onset of knock, measurement of ionic current, adjustment of timing – takes place in just 0.8 milliseconds at 8000 rpm, rapid enough to adjust the timing before that sparkplug’s next firing.
- Because of its extreme sensitivity and speed, this technology allows the engine to “edge closer” to its full potential in the upper reaches of power and rpm.
- Helps control fuel consumption and exhaust emissions.
Ionic-current technology does not require a different type of sparkplug, nor is the sparkplug’s durability affected by its extra “duty.” The current-measuring voltage is only about 100 volts, vs. more than 25,000 V applied to fire the sparkplug.
Weight-efficient construction. The V-10 engine is of aluminum, with bedplate construction for the lower portion of the cylinder block. This construction is inherently ultra-rigid; for the strength to withstand the massive internal forces of this engine, however, the main-bearing inserts are of cast iron. Sharing a major feature of most current BMW engines – N52 6-cylinder as well as all V-8s and V-12 – the cylinders are silicon-impregnated, with “soft honing” removing just enough of the aluminum to leave the silicon crystals as ultra-hard cylinder surfaces.
The entire engine weighs just 240 kg/529 lb. – almost exactly the same as the prior M5 V-8 engine, yet the V-10 delivers fully 106 hp more output!
G-sensitive lubrication system. Given the 45˚ cant of the cylinder banks and the M6’s cornering ability, special attention has been directed to ensuring natural return of oil to the main sump. There are two sumps, the main and larger one behind the front frame crossmember and a smaller one forward of the member; a baffle separates the two from each other in this “semi-dry-sump” system.
First, the mechanically driven main oil pump is newly of the variable-volume type, an innovation also found on the new 6-cylinder engine. By varying the output of its pump element according to engine oil pressure, the engineers have achieved a pump that always delivers sufficient pressure to lubricate this demanding engine, yet never pumps more oil than is necessary. Thus it –
- Contributes to the high power output, by requiring less power from the engine.
- Doesn’t require a bypass to divert excess flow; this helps avoid excess oil temperatures and oil foaming.
There is also a recirculating pump that picks up oil from the small front oil sump and transfers it back to the main sump. Additionally, there is an electrically driven scavenging pump for each cylinder bank. In straight-ahead driving, these pumps pick up oil from the rear of the engine and return it to the sump. In hard cornering (0.6g or greater), the Dynamic Stability Control system’s lateral-g sensor switches magnetic valves to different pickup points, at the curve-outer side of either head and the oil pan. This system remains active even if the driver switches off DSC.
The oil level and temperature are monitored by a thermal sensor; a warning is displayed if the level falls low, and an oil-temperature gauge is included in the tachometer face. Oil is cooled by a coolant-oil heat exchanger.
Low-back-pressure, tuned exhaust system. Exiting the engine through stainless steel exhaust headers that Germany’s auto motor und sport magazine (November 10, ’04) called “an artwork in stainless steel,” exhaust gas from each cylinder travels an ideal length of 560 mm/22 in. before reaching the engine-close first catalytic converter on each side. A high-pressure forming technique is used to shape the headers, enabling them to achieve the necessary strength and desirable light weight with only 0.8-mm/0.03-in. wall thickness. Two additional catalytic converters, one per side, are located farther back in the system.
Further advanced engine electronics. Once again BMW M has developed a new, ultra-powerful electronic control system; called MS S65, this scheme incorporates much of the experience gained from the V-10 racing engine. All the engine’s electronic functions – throttles, ignition, fuel injection, lubrication, dialog with the Sequential Manual Gearbox, and more – are overseen by a central electronic module employing three 32-bit microprocessors capable of 200 million operations per second.
This system also incorporates the MDrive functions, including the three power levels and throttle calibrations described under MDrive.
Variable tachometer warning zone. Pioneered on the previous M5 engine and now found on 5, 6 and 7 Series as well as M3 and Z4 M Roadster tachometers as well, this feature reminds drivers that a cold engine should be treated with care. Displaying a yellow “caution” segment and a red “limit” segment, this zone moves from a minimum point with a cold engine to the full 8250-rpm redline as the engine oil warms up.
|Performance & efficiency...||7-speed Sequential Manual Gear...|
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| Transmission:||7-Speed manual |
| Horsepower @ RPM:||500@7750 |
| MPG(Cty):||18 |
| MPG(Hwy):||12 |
| Torque @ RPM:||6100 |
| Displacement:||4999 L |
| Curb Weight:||3909 lbs. |
| 0-60 time:||4.5 sec. |
| Top Speed:||155 mph