2009 Infiniti FX50 vs 2009 BMW X6 xDrive50i

John Roe

Techtonics : New Wave V-8s (and one trick diff)
By Don Sherman

When General Motors suspended development of its UV-8 (the U stood for ultra), several pundits prematurely wrote obituaries for all V-8s. Confirming that there's plenty of life left in this popular configuration, BMW and Infiniti have both tooled up fresh V-8s for their newest and most sporting SUVs.

BMW's 4.4-liter N63 V-8 is the more clever of the two. This is not only a clean-sheet design, it also employs a revolutionary layout: cylinder heads rotated 180 degrees to relocate the exhaust ports to the center of the V and the intake ports to the heads' outboard flanks.

This shuffle concentrates exhaust heat in one location to maximize the energy available to spin two turbochargers. Shorter exhaust passages also diminish the heat lost to radiation, thereby reducing the time required to warm the catalytic converters - also located in the engine's valley - to proper operating temperature. The third benefit is a taller, narrower package that fits not only the high-riding X6 but also BMW's larger cars.

To avoid singeing the paint, there is ample insulation between the hot engine parts and the hood. Also, the X6's cooling fans run if necessary when the vehicle is parked to flush residual heat from the engine compartment.

Direct fuel injection, nearly equal bore and stroke dimensions, a 10.0:1 compression ratio, variable valve timing (intake and exhaust), and a peak turbo boost of 11.6 psi yield impressive output figures: 450 lb-ft of torque at only 1750 rpm and 400 hp at a hearty 6400 rpm. Watch for the new N63 V-8 to systematically replace the 4.8-liter V-8 BMW fits elsewhere.

Infiniti's new VK50VE V-8 is not an all-new design, but it does bring an impressive résumé to this sport-SUV fracas. In addition to an aluminum block and heads, four camshafts, and four valves per cylinder, this engine uses variable intake valve lift instead of a throttle plate to regulate load. Doing so minimizes throttling losses and improves cruising fuel efficiency. The VK50VE's Variable Valve Event and Lift System, first employed on the Infiniti G37's V-6, has a computer-controlled electric motor driving an intricate adjustable-lift mechanism. Packing a full 5.0 liters (but no turbos or direct fuel injection), the new-wave Infiniti V-8 has a torque peak of 369 lb-ft at 4400 rpm and a competitive 390 hp at 6500 rpm.

One last technical feature worth mentioning is BMW's new Dynamic Performance Control (DPC), a fancy name for the X6's active rear differential. Two planetary gear sets operated by two computer-controlled clutches help steer the vehicle by increasing the amount of torque delivered to one rear wheel. When the issue is excessive understeer, more torque goes to the outboard rear wheel. In the event of oversteer, the inside wheel receives the boost. According to BMW, DPC also improves steering precision and stability when the driver abruptly lifts off the accelerator.

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