Audi R8 diesel?

If you were out to change the notions that diesel engines are smelly, noisy and slow — when they’re anything but — what would you do?

If you were Audi, you would wedge a virtually smog-free 500-horsepower V12 turbo-diesel into one of the hottest sports cars on the road.

A decade ago, the idea of splicing the DNA of a sports cars such as the R8 with a diesel engine (you might be thinking of a tug boat or a tour bus, here) would have flopped. Remember the smelly, noisy and slow part?

Today, however, the turbo-diesel has been redesigned to meet stringent new emissions regulations and is seen as a viable powerplant that rivals a gas engine for quietness and, in the case of sports car, power.

The 6.0-liter V12 in the R8 TDI, which stands for turbo direct injection, also pumps out close to 750 pound-feet of torque. Audi claims the car will hit 60 mph from a standing start in about four seconds flat. The standard R8 with its 400-horsepower 4.2-liter gas V8 and 317 pound-feet of torque — less than half the V12’s twisting force — is only about a four-tenths of second slower. The top speed, as reported by Audi, is nearly identical between the two vehicles.

While you could go back and forth all day long on which will make more power, gas or diesel, there is one significant advantage to the diesel: an abundance of torque at low engine speeds. Diesels are low-revving engines, which means that all that wonderful seat-in-pants torque is available at just 1,750 revs per minute. Talk about snappy acceleration. There’s no dumping the clutch at 4,000 rpm looking for a good launch as is the case with many gasoline-powered sports cars.

In terms of fuel economy, the generalization is that diesels are better. Given the output of the R8 TDI, Audi is happy to report 25 mpg. There’s not much to compare that to, although the base 430-horsepower Corvette, which is about 800 pounds lighter than the Audi — with about the same acceleration and top speed — is good for about 25 mpg highway, at best. It’s by no means an apples-to-apples comparison, but you get the idea.

Audi prefers to think of the big picture and is quick to point out that the Environmental Protection Agency predicts that if just one-third of the vehicles on the road today were diesel that we would conserve about 500 million barrels crude oil every year.

So, let’s hurry up with that R8. Getting the V12 to fit into the spot occupied by the V8 wasn’t easy. In fact, it was impossible. With two extra pistons tacked to the end of each cylinder bank, the diesel is about six inches longer than the V8. Since the engine sits behind the passenger compartment, the “firewall” (between the engine and the passenger compartment) was moved ahead several inches. The V12 is also taller since instead of the traditional 90-degree “V” arrangement with the V8, the diesel has an angle of just 60 degrees: it’s more upright. To feed cool air into a hotter engine bay, the R8’s “side blades” are now more like “side scoops” while the roof has a large duct built into the top.

Just how good is the R8’s V12? Winding Road magazine (www.windingroad.com) quotes Thomas Kräuter of Audi in a video as saying, “If every possible customer had the chance to take a ride with this car, he would never ride a gas engine in a sports car.”

That might be true, but without knowing the cost upgrade to the V12, which should be available later this year, from a base price of $119,000, buyers will ultimately be the ones to decide if the extra money is worth it.

Still, that’s not exactly the point of the R8 TDI. As the “halo” vehicle for what the company calls a diesel “offensive,” the car should set the tone for future models. Audi has plans to offer a diesel V6, V8 and the R8’s V12 TDI under the hood of the Q7 sport-utility vehicle, which probably makes the most sense given fuel-economy concerns and the weight of this big rig: a torquey diesel would make the Q7 feel lighter on its feet without breaking the fuel budget. The V6 TDI, displacing 3.0 liters and having an output of 221 horsepower and 406 pound-feet of torque, will also find its way into the new A5 coupe.

And then there’s the R8 TDI, the leader of a diesel offensive, without the offensive diesel.

Jeff Melnychuk is Wheelbase Communications’ managing editor. You can drop him a note on the Web at www.wheelbase.ws/mailbag.html. Wheelbase Communications supplies automotive news and features to newspapers across North America.