Saturday, May 9, 2009

2010 BMW X6 M / X5 M - Auto Shows

Some will consider the announcement of the M versions of the BMW X5 and X6 arriving this fall to be heresy, a crime against the enthusiast faithful not seen since the debut of the Porsche Cayenne. Well suck it up fanboy, because the X6 M will lap the Nürburgring faster than the E46 M3. And if you really want to throw stones, blame the success of the Porsche, which demonstrated to BMW that a market exists for sporty SUVs. Both Bimmers—the powertrains are identical—put out 555 hp from a twin-turbo V-8, which is a 5-hp middle finger to the Cayenne Turbo S. The 500 lb-ft of torque in the BMWs, however, is 53 short of that of the top-level Cayenne.

The Mighty Boost

The heart of the X5/X6 M, as in all M cars, is the engine. And the heart of this engine is a trick exhaust manifold and a pair of twin-scroll turbochargers. Of course this is BMW, so there has to be an acronym: Cylinder-bank Comprehensive Manifold, or CCM. The direct-injection 4.4-liter V-8 is based on BMW’s regular production engine, adding new pistons, a new intake manifold, and revised intake cam timing. Compression has been lowered slightly, to 9.3:1 from 10.0:1. The alloy used for the cylinder heads has been altered for greater strength, the oil pan is now aluminum with integrated cooling fins, and both the intercoolers and radiator have been enlarged to deal with increased engine heat.

The secret to the exhaust manifold, which sits in the valley between the two cylinder banks, is a convoluted plumbing system. Exhaust gas from each cylinder is routed to the dual twin-scroll turbochargers; each scroll is fed by a distinct pair of cylinders. The pairs are matched in such a way that the two cylinders fire a full crank rotation apart from each other. (In a four-stroke engine, every cylinder fires every two rotations.) This facilitates steady pulses of exhaust gases to spin the turbo, whereas a more conventional exhaust manifold will form waves of pressure. The M solution, while expensive, results in better engine response and less turbo lag. The torque peak, which runs flat from 1500 to 5650 rpm, backs up this claim. At max boost, the Garrett turbochargers produce 21 psi of pressurized air.

Keep Reading: 2010 BMW X6 M / X5 M - Auto Shows

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