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Driver's Logbook: Diesel takes a good SUV and makes it great

Peter Cheney/The Globe and Mail

I've never surfed a thirty-foot wave, but cruising the fast lane of I-75 in a BMW X5 diesel made me think about what it might be like. I was riding a Niagara of torque - the little-understood force that makes the difference between an okay car and a great one.

Like a giant wave on Hawaii's North Shore, torque is something you understand instantly when you encounter it first-hand. It's impressive. It's real - and it's not what bond traders talk about at cocktail parties. Instead, they focus on horsepower, a number that serves as a thinly veiled allegory for penis size, as in: "My Turbo S has 530 horsepower. What have you got?" (In my case, 116 horsepower.)

But never mind the foolish compensation talk. Torque is what pulls trailers, pushes you up a steep mountain pass, and lets you pass a line of motorhomes without downshifting. Torque is the connoisseur's power measurement, and it is the diesel engine's specialty. I've driven several BMW SUVs, but the diesel transformed the car. The X5's six-cylinder turbo diesel burbled away beneath the hood, barely exerting itself as we zoomed from Toronto to Georgia and back, loaded to the gunwales with clothes, camera bags, a crate of aircraft tools and a 300-pound air compressor.

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Although I'm not really an SUV fan, the diesel engine went a long way toward converting me. The motor's lugging power and fuel efficiency made the X5 an extremely useable vehicle. Even with a heavy load and a high cruising speed (106 km/hr. including restaurant and gas stops), the X5 got an impressive 9.4 litres per 100 kilometres (about 25 per cent better than its gasoline-powered siblings.)

Although many drivers dismiss the diesel engine as a noisy, smoke-spewing contrivance best suited to a locomotive, they should think again. The diesel has some inherent advantages that make more sense than ever in the age of high-priced oil.

For starters, a gallon of diesel fuel contains about 14 per cent more energy than a gallon of gasoline, so you go further on the same fuel volume. A diesel engine produces its power at lower rpm. And its forte is torque - the X5 I drove had only 245 horsepower, but its peak torque was a massive 425 foot-pounds.

So what is torque? Think of it as twisting force and you'll be on the right track. A vehicle can have high horsepower, but still feel slow unless you know how to extract the power. A classic example is Honda's S-2000 sports car, which developed power only at high rpm, forcing drivers to shift constantly to keep the high-tuned engine spinning in its lofty power band.

Torque is different. Touch the gas pedal and it's there - it's a Clydesdale that can move a fully laden beer wagon merely by leaning against the reins. Horsepower, on the other hand, is a high-tuned thoroughbred that hits peak speed somewhere on the backstretch. I drive a lot of high-horsepower cars, and they're great. But on the street, torque is what really counts.

The X5 diesel has more going for it than torque. I particularly liked the sound: a gentle industrial thrum that reminded me of an overnight ship crossing. The X5's cockpit has just the right layout for a long trip, with comfortable seats, a Bluetooth hands-free system and satellite radio that let my wife and I listen to the BBC as we headed into the Deep South.

The brakes are powerful, and the suspension did a good job of masking the X5's SUV bulk - it went around corners like a 275-pound NFL receiver running a pattern. There was even a GPS-based concierge system that tracked down gas stations with diesel. Nice. This car costs $77,800, which raises the thorny matter of value. Is a BMW X5 diesel worth it? That's up to you to decide, but the diesel engine really is a winner.

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