This is where the 750d and 750Ld really stand out from the rest of the 7 Series lineup. That new quad-turbo diesel engine hiding under the hood of the two 750s is pretty amazing. That 3.0-liter, six-cylinder engine manages to deliver a total output of 400 horsepower and 560 pound-feet of torque, which is enough to take the 750d to 62 mph in 4.6 seconds and the 750Ld to the same benchmark in 4.7 seconds. Furthermore, it’s able to achieve a combined fuel economy of 5.7- to 5.9-liter per 100 km, or between 47.9 and 49.6 mpg on the imperial scale.
The 3.0-liter, six-cylinder engine manages to deliver a total output of 400 horsepower and 560 pound-feet of torque
This kind of fuel economy and performance is made possible by multi-stage turbocharging and an upgraded common-rail direct injection system. As always, the cylinder heads and engine block are manufactured using a high-pressure compression process, while a hot isostatic pressing process is used to ensure the aluminum castings can hold up to the extreme combustion pressures of the engine. The head gasket is now five layers think and the cylinder bores have a twin-wire, arc-sprayed coating. The pistons are made from aluminum and silicon alloy with remelted bowl rims and bronze lines inside the pin eyes. The fuel injection system has been upgraded to withstand fuel pressure higher than 2,500 bar – a way to increase the precision and automization of fuel, which essentially means less fuel is wasted.
The most important part is how the turbocharger system works. To make this next-gen engine, BMW had to add a fourth turbocharger to the mix. The high-pressure turbochargers are built into the same housing with variable turbine geometry. The larger, low-pressure turbocharger that was used in the past has been replaced by two smaller units that are able to respond faster. The result is almost zero turbo lag on takeoff and excessively quick response when the accelerator is mashed at higher speeds.
The way it works is, the two low-pressure turbos and one high-pressure turbo are in use at all times. Under hard acceleration from idle, the low-pressure turbos are bypassed. This allows turbo pressure to build up faster than ever, with the second high-pressure turbo kicking in at 2,500 rpm. Both low-pressure and high-pressure turbos now feature exhaust gas recirculation, which increases not only turbo performance but the performance of the engine itself. All told, this is the next evolution of the diesel engine, but Bimmer is keeping it on that side of the world for now, so don’t expect to see it in the U.S. anytime soon.
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