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Ken Gratton19 Apr 2015
NEWS

X5 M's high-end innovation

BMW's performance flagship SUV takes engineering to a whole new level

The advanced engineering and mechanical features of the BMW X5 M and its sibling, the X6M, would do any range-topping prestige SUV proud.

But there's simply too much to describe or explain in one simple review and a news story.

So, here's the rest of the story for the M-powered X5 and X6.

At the core of the new variants is the 4.4-litre TwinPower turbocharged V8, which has been revised for optimal rigidity of the block. Developing 423kW (15kW more than the previous generation), the engine produces 750Nm – a 70Nm hike. A forged crankshaft with the capacity to handle such high torque is integral to the engine's barely oversquare design, with a bore of 89mm and stroke of 88.3mm. This helps it build and maintain peak torque across a range of engine speeds from 2200 to 5000rpm.

Designed to meet the Euro 6 emissions standard, the V8 features a fuel delivery system combining high-pressure direct injection with BMW's Valvetronic and Double VANOS systems. Air is drawn in through a twin-turbocharged set-up with each turbocharger a twin-scroll unit fed by a cross-bank exhaust manifold. Each time exhaust gas acts on the turbine it's drawn from two separate cylinders, each firing at a 360° interval from the other. This ensures "mutual interference" in the gas streams is eliminated, improving efficiency and reducing lag to the minimum.

An advanced lubrication system ensures the engine receives the proper supply of oil while lapping the Nurburgring – generating roll and pitch of up to 1.2g. Employing a snorkel that sits very low in the sump, the lubrication system will always be primed to deliver oil even in the most demanding situations.

Cooling has been a major priority for the engineers working on the X5 M and X6 M. Four distinct cooling systems ensure engine temperature, engine lubricant, transmission oil and intercooled air all stay at the right temperature.

An eight-speed ZF automatic transmission was chosen for the performance SUV application, since it is durable enough for the torque produced by the engine – in excess of the 700Nm ceiling for BMW's dual-clutch transmission used in the M5. BMW calibrated the ZF unit to operate much like the DCT however. In Drive, the car will not crawl forward when the driver releases the brake pedal. Instead, the vehicle needs even a light prod of the accelerator to move forward. This provides the ZF auto with the same sort of operating character as the DCT.

For the same reason, the transmission does not have a detente for 'Park'. The transmission will indicate it's in Park when the driver turns off the engine or a passenger opens a door. A sequential shift facility can be accessed by tapping the lever across to the right. To return to conventional Drive mode, the driver taps the lever across to the right a second time.

In sequential-shift mode, gear changes can be effected from the lever – back for upshifts, forward for downshifts – or from the shift paddles. The transmission will not shift up a gear at the engine's maximum revs when in manual operating mode. A button just aft of the lever changes the shift mapping from smooth and gentle to aggressive and fast – with an intermediate setting as well.

The transmission is coupled to the engine in a longitudinal (north/south) configuration, with torque split 60:40 (rear to front) for the M models. For standard X5 and X6 models, the torque split is 70:30. BMW's xDrive system for the M variants incorporates torque vectoring, delivering more torque to the outside wheels in a corner, and braking the inside wheels at the same time. BMW has specified the heaviest ever tailshaft for drive to the rear wheels of the X5 M and X6 M.

The performance is complemented by sophisticated suspension, steering and brakes. Hydraulically controlled anti-roll bars and air suspension with adaptable dampers and self-levelling rear allow the driver the flexibility of choosing comfort or sport settings on the fly to suit the driving environment. The M Servotronic steering is also manually programmable for comfort (lighter weight) or sports settings. Brakes comprise compound material for the cross-drilled and ventilated disc rotors, which are gripped by six-piston fixed callipers finished in blue and bearing the M logo at the front, plus floating piston callipers at the rear.

BMW and Michelin have joined forces for the development of the mixed tyres fitted to the X5 M and X6 M. After two years of development time, and the testing of 1400 prototypes and 500 pre-production tyres, the companies decided against run-flat tyres for the high-performance application. Instead, they settled on Pilot Super Sport tyres that provide enhanced steering feel and precision at the front, and uprated traction and lateral/directional stability at the rear.

Measurements are 285/35ZR21 105Y for the front, and 325/30ZR21 108Y for the rear. Standard wheels for the X5 M and X6 M in Australia measure 21-inch diameter, rather than the 20-inch wheels specified in overseas markets.

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Written byKen Gratton
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