Specific output. That single measure is the small-car enthusiast's last stand when the tides of displacement turn against us and the eyes of the opposite sex turn to that under-endowed Viper driver. "If that inefficient hunk of iron had the same specific output as my Civic Si," you stammer, "it would make 800 hp!" Yeah, that'll help. The high output of our favorite small engines does make you wonder what would happen if the same high-revving, high-efficiency design strategy were taken with something bigger. Wonder no more.
BMW's long-awaited M3 just had its debut and under the hood is the long-awaited answer. Three-thousand, two-hundred-forty-six cubic centemeters of BMW's most exotic reciprocating metallurgy pumps out an amazing 333 hp at 7900 rpm. That adds up to a specific output of 103 hp-per-liter, landing it handily between the Civic Si's 100 hp-per-liter and the Integra Type R's 109.
For those of you who may scoff at my comparing an M3 to a Civic, or at even mentioning a $45,000 BMW in these pages, I have but one thing to say: Look at all these pretty pictures! If you have even the slightest concentration of petroleum in your circulatory system, one look at the inside of the new M3 engine will stop you dead in your tracks, make you forget your complaints, and get all jiggly with excitement.
Let's start at the bottom, where the very fact that an in-line six cylinder can spin to 8000 rpm should come as a shock. The long crankshaft of an in-line six is especially prone to torsional vibration, and very few have cranks stiff enough to withstand such revs (the Nissan Skyline's much smaller RB26DETT being a very notable exception.) The M3's bore and stroke, at 87 mm and 91 mm, respectively, are almost a dead match for the Honda Prelude's. In other words, BMW is also taking advantage of the high efficiency of a small-combustion chamber while tolerating piston speeds that rival those in an F1 engine.
Below that stiff crank, forged connecting rods, and graphite-coated pistons sits a semi-dry sump lubrication system. The cast aluminum oil pan places the engine's oil low, behind the front crossmember where it is shielded from the crankshaft to prevent windage losses. Because the M3's prodigious cornering forces and the engine's 30-degree slant as it is mounted in the engine compartment conspire to prevent oil from draining back into the pan during hard left turns, a scavenging pump has to be used to pump oil back into the pan. Also below those 11.5:1 pistons are oil squirters to carry heat away from the bottom of the pistons.
At the top of the engine is all the magic, however. Horsepower is made in the head; only reliability can be found in the block. (And remember, nobody is looking at that Viper driver to admire the car's reliability.) A shallow-angle pent-roof combustion chamber is status quo for modern, high-output engines, so it's no surprise to see a textbook combustion chamber in the new M3. Upstream are the surprises.
It starts with an intake manifold that consists of a big, plastic box concealing six composite air trumpets. The shape of these trumpets was developed using computer models from the BMW F1 engine development program. Each trumpet feeds its own electronically controlled throttle body that is bolted directly to the intake port. That port is fully CNC machined from its perfectly matchported, O-ring sealed flange, to its three-angle valve seat. Along the way, the divider that splits each intake port in two to feed the two intake valves is cut knife-edge sharp, unlike the blunt wall cast into most heads. Downstream a little further, notice how the valve guide doesn't protrude into the port at all, and how the valve stem has been cut extra-narrow below the valve guide to minimize the effect on airflow. The valve seat, in addition to using a three-angle grind to maximize low-lift airflow is machined flush with the port wall. BMW has clearly one-upped Honda's hand-ported Type R head treatments with this port work. The exhaust ports, though only half machined, dump into a beautiful, equal length, 6-2-1 tubular header.
Above the port, the valvetrain is interesting as well. The cam lobes operate almost directly on the valves, but a short, narrow rocker arm separates them. This rocker arm is 30-percent lighter than a conventional bucket from a direct-acting valvetrain, which allows lighter valve springs and consequently less friction from the valvetrain. Despite the lightweight arrangement, dual valve springs are still used to help prevent high-rpm harmonics.
The cam timing, no surprise, is variable. BMW's double-VANOS system simply varies the timing of both the intake and exhaust cam. Physically moving the camshaft relative to the cam sprocket is managed by oil pressure, and this new, high-pressure VANOS uses a secondary pump to push that pressure to an astonishing 1660 psi to improve response time. BMW has not resorted to a two-stage valve timing system like VTEC, VVTL-i, VVL or any of the many latecomers. There may be less power to be gained by simply varying the timing of one set of cams, but the payoff is a smooth power delivery that is essential to a well-balanced car.
Controlling all these throttle bodies and camshafts is a ridiculously complex engine management system that tailors fuel mixture and ignition timing on a cylinder-by-cylinder basis. Three knock sensors are used, and knock-sensor-based timing adjustments are made independently for each cylinder.
Most complex, however, is the drive-by-wire system. BMW's press materials describe it as such: "The driver's wish for power and performance is measured on the gas pedal by means of a potentiometer and translated into a specific request function. This request function is then adjusted by the power and torque manager by adding the power signals from the auxiliary engine units and is then aligned to the maximum and minimum output and torque curves allowed by the Dynamic Stability Control (DSC) and Engine Drag Force Control (EDFC). The target output and torque calculated in this way is then maintained at the desired level, taking the current ignition angle into account."
Now, that's technobabble!