New Audi: Fast and fun but not for you

NEW YORK (CNNMoney.com) -- I just spent a week driving an SUV with a 4.2-liter V8 engine that produces as much torque as the 6.0-liter V10 engine in a Dodge Viper. It can go from a dead stop to sixty miles per hour in about 6.4 seconds.

But this SUV is estimated to get roughly 21 miles per gallon in combined city and highway driving. Now, a gasoline-powered Audi Q7 with a 4.2-liter V8 doesn't even come close to 21 miles per gallon in highway driving alone, but the version I was driving was powered by a twin-turbocharged diesel engine. (For comparison purposes, the Dodge Viper gets 16 mpg overall, but fuel economy isn't high on the list of priorities for its drivers.)

Audi Q7 4.2 TDI

Step on the gas from any legal speed, and the Q7 4.2 TDI moves out like a beast. From a standing start, there's a brief moment of gentle acceleration while the SUV's two turbochargers - one for each bank of cylinders - get spinning. Then there's a quick rush of boost that pushes you back in your seat.

Before you get too excited, though, I should tell you that Audi is not planning to bring this particular SUV to the United States. It's a diesel-powered vehicle that only recently went on sale in Europe.

For one thing, it would be really expensive. At a rough guess - based on European prices minus high European taxes plus the cost of emissions-cleaning equipment to meet strict U.S. regulations - an American version would cost about $100,000.

Audi will be bringing a diesel Q7 with a 3.0-liter, V6 engine to the United States in early 2009. Besides being less expensive, it will also get somewhat better fuel economy, which is what Americans look for in a diesel. According to Audi, the European diesel V6 diesel gets an estimated 22.7 miles per gallon in combined city and highway driving. (That's a European estimate based on the European version of that SUV. U.S. mileage will be different.).

But Honeywell (Charts, Fortune 500), the company that made the twin turbochargers in the Audi 4.2 TDI, wanted to show off what a diesel could really do. And it was an impressive demonstration. It's worth paying attention because you'll be seeing a lot more diesels coming to the U.S. in the next few years and this proves that, yes, they can be fun.

On a racetrack, this engine would be no match for the Viper's, despite its impressive torque. When revved to its limits, the Viper's V10 engine produces a maximum of 600 horsepower compared to the diesel's 326. That means that, running full out, the Viper is much more powerful.

But to use horsepower, you need torque. Horsepower is the maximum amount of power an engine can belt out when everything's going full blast. An engine always produces its maximum horsepower at nearly its maximum operating speed.

Torque is the hard twisting effort needed to just get things moving. Diesel engines produce a lot of torque at relatively low engine speeds. That's one reason they're popular for trucks that need to move heavy loads. It's also why, even though we always talk about horsepower, torque is what really makes a car fun to drive in the real world where engines rarely touch top speed.

In a vehicle with less low-end torque, a quick start requires high engine RPM's. With an engine like the Audi's diesel V8, though, this SUV takes off like a rocket without the screaming, high rev drama you get with a gasoline engine.

That's one reason that a diesel engine generally uses less fuel than a similar gasoline engine - it just doesn't have to rev as high to get going.

There are other reasons, too. But it all adds up to more oomph per gallon.

The downsides you may associate with diesels are gone from modern engines. Driving the Audi Q7 4.2 TDI is nothing like driving a trash truck. There's no smelly black smoke puffing out of the tailpipe. While the engine sounds a little different from a gasoline-burner, it's not the loud, clatter Americans usually associate with diesels.

Besides the V6 Audi Q7, you'll be seeing other diesels entering the U.S. over the next couple of years. Honda, for instance, plans to introduce a diesel version of its Accord to replace the discontinued hybrid.

Up until now, they've been scarce here because of strict emissions rules. Those standards were all but impossible for diesels to meet until the recent introduction of new technologies and ultra-clean diesel fuel. (The V8 Audi I tested would not pass emissions muster in the U.S. Honeywell got a special exemption from the EPA for "testing purposes.")

Also, car companies were worried about the bad public perception of diesels. Those negative images shouldn't take long to dispel. A simple test drive should do it.

(Thanks to Don Altermatt, director of diesel engineering for Chrysler Corp., for helping me boil down some of the technical details for this report.)