Diesel luxury vehicles have been on the scene in Europe for a long time now, but they remain a novelty in the United States. With ever increasing fuel prices, diesel offers increased economy figures while at the same time increasing the performance figure most people actually use in everyday driving—the torque.
Horsepower has not always been the clincher it has seemingly become in the past few decades. There once was a time, believe it or not, where Rolls-Royce would only describe the power made by their cars as “adequate,” without a number or measurement to be seen. Luxury cars used to be judged by their ride, their creature comforts, and other subjective factors. As long as the cars weren’t laggards when called upon to scoot faster, the actual performance figures were next to irrelevant.
Germans racing their car platforms around Europe generally, and then specifically flogging all of their production models around the Nürburgring race track changed all of that. Honing these chariots of the upper classes into “driver’s cars” made the performance numbers all important, to the point where BMW could, and did, sell limited edition sedans with carbon fiber roofs, no air conditioning, or sound systems.
Diesel, because it has about 11% more energy content than the equivalent gallon of gasoline, changes the performance equation. An oversimplification of the differences between gasoline and diesel engines boils down to this: That “extra” power in the fuel translates into longer power strokes in the cylinders to deal with it. These longer strokes create more mechanical leverage for the connecting rods to act upon the crankshaft. That force on the crankshaft is called torque. In short, the lower revving diesel engine with its long bore strokes creates more torque rather than higher horsepower. The diesel has greater fuel efficiency than an equivalent displacement gasoline engine can manage. To make the disparity between the torque and horsepower less glaring in a diesel, turbocharging is generally employed.
These engineering facts of life actually make owning and operating a luxury SUV compelling again. For 2014, Audi has revised its Q7 tdi (turbocharged direct injection) model’s 3.0L diesel engine generates an astounding 406 ft.lbs. of torque out of a relatively modest 240 BHP. Mated with an eight speed Tiptronic automatic transmission, when the hammer is dropped by your right foot, the Q7 responds like there is a rocket pack bolted to the back. With Audi’s quattro all wheel drive keeping the torque usefully put to forward acceleration, the smile on your face won’t be about the 28 mpg highway figure that the Q7 is capable of.
With seating for seven and filled to overflowing with standard and optional luxury features, the only fly in the soup is the $5,000 upfront premium for the turbo diesel. In Europe, the common pricing practice finds that there is little to no pricing differential between gasoline and diesel powered versions of the same luxury cars, be they Jaguars, Mercedes Benzes, or Audis.
Demand will reduce that price disparity. If you are looking at a Q7, test drive the 3.0 tdi before deciding.