CREWE, U.K. – Bentley Motor Cars Ltd. redesigns its 6.75L twin-turbo V-8 engine from stem to stern for the all-new Mulsanne flagship sedan coming later this year, focusing on retaining the motor’s trademark personality while improving its efficiency.
Simon Atkinson, module leader-Chassis Powertrain at Bentley’s famous assembly plant and engineering house here, says early consideration was given to designing an all-new V-8 for the Mulsanne.
But engineers quickly discovered they could achieve their goals by extensively revising the existing pushrod V-8, an engine Arnage, Brooklands andowners covet for its big torque at low revs.
“We knew we wanted to keep that basic trait, the effortless torque to pull away,” Atkinson tells Ward’s.
Mission accomplished. The redesigned 6.75L V-8 pours out 752 lb.-ft. (1,020 Nm) of torque, or 14 lb.-ft. (19 Nm) more than the engine it replaces, and the thump arrives just off idle at 1,750 rpm vs. 2,500 rpm previously.
The engine motivates the 5,700-lb. (2,585-kg) Mulsanne from 0-60 mph (97 km/h) in 5.1 seconds and 0-100 mph (161 km/h) in 11.6 seconds. Top speed is 184 mph (296 km/h), Bentley says.
In total, the 40 engineers assigned to the new engine worked to replace or significantly redesign 300 of the V-8’s 700 individual parts – resulting in a new block, lightweight forged crankshaft and lighter-weight pistons, connecting rods and cylinder heads.
Reciprocating mass and internal friction were reduced, improving responsiveness, Atkinson says. The new engine weighs 10%, or 42 lbs. (19 kg), less than its predecessor.
However, Bentley sticks with the overhead-valve design instead of switching to the more popular overhead-cam.
“It came down to fundamental packaging issues,” Atkinson admits. “We knew we needed to be tall; we had height, we did not have width.”
But looking at stricter fuel-economy and carbon-dioxide emissions rules in Europe and the U.S., Bentley adds cylinder activation and cam phasing to the new engine.
The cam phasing helps improve torque delivery, but also benefits the engine’s breathing at idle, while cutting off four cylinders at cruising speeds boosts fuel economy. Overall, fuel consumption and CO2 emissions are trimmed 15% to 17 mpg (16.9 L/100 km) and 396 g/km in the combined cycle.
“The expectations for refinement were very high, but you can still pick up just slightly the deactivation,” Atkinson says.
A new 8-speed transmission fromFriedrichshafen AG further optimizes engine efficiency.
Bentley also makes the engine flex-fuel capable. Ironically, the premium manufacturer looked down market for part of the design.
“We actually tore down aHemi and got a close look at how they do it,” Atkinson says. “We actually went with the same tappet design.”
Each one of the V-8 engines is assembled by hand, checked for refinement on a state-of-the-art balancing machine and then hot-tested on a dynamometer for 80 minutes to simulate real-world driving conditions before being fitted into a Mulsanne.