Manifold placement matters in BMW M's

  • 04-May-2009 09:57 EDT

The most significant challenge for BMW engineers in developing an M version of the company's X6 crossover vehicle was making a 5324-lb (2415-kg) car handle like an M vehicle should, said M division President Ludwig Willisch.

BMW’s reveal of the high-performance M version of its X6 crossover at the recent auto show in New York and the X5 at the recent auto show in Shanghai marked the debut of the company’s latest engine technology: a reverse-flow V8 with the intake manifolds on the outside and the twin dual-scroll turbos nestled in the valley of the engine’s vee.

The 4.4-L DOHC V8 yields a prodigious 555 hp (414 kW) and 500 lb·ft (678 N·m), with optimized power delivery thanks to a CCM (cylinder bank comprehensive manifold) cross-bank exhaust manifold that matches cylinder pairs that are 180º apart. With two such pairs feeding each turbo, and each of the pairs 90º out of phase with one another, the turbos receive a steady flow of exhaust gas at a maximum pressure of 1.5 bar (21.8 psi). In the presence of this innovation, the direct fuel injection seems almost prosaic.

Having the exhaust heat trapped between the cylinder banks made thermal management a particular challenge in developing the X6 M, reported Ludwig Willisch, President of BMW’s M division. “We had to put some thought into the solution,” he said with understatement.

A pair of air-to-water intercoolers and a finned aluminum oil sump each contribute to the solution. The engine, using a ZF-supplied six-speed automatic gearbox, accelerates the 5324-lb (2415-kg) X6 M from 0-60 mph (0-97 km/h) in just 4.5 s, according to the company. We can expect to see the CCM technology applied to other models in the future, Willisch said.

The xDrive all-wheel-drive is tuned to send most of the power to the rear wheels, giving the X6 and X5 M the feel of rear-wheel-drive cars, but with the security of all-wheel-drive.

But even tougher than accelerating that much mass is making it change direction and stop in a manner suitable for sporting driving, he added. “Agility and dynamism are tough to bring into a car that weighs 2.3 tons,” he said. The tuning challenge was so significant that Willisch himself drove X6 M test vehicles every few weeks throughout its 15-month development cycle to check on the team’s progress, he said.

In addition to retuning the suspension and stability control programming, engineers specified also received larger brakes, wheels, and tires for the vehicles. They have 20-in wheels, with 275/40 tires at the front and larger 315/35 tires at the rear. The large wheels create space to accommodate enormous 15.6-in (396-mm) front brake rotors and 15.2-in (386-mm) rears, each clamped by a four-piston caliper.

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