Mercedes-Benz and Volkswagen are soon to bring more Bluetec diesel engines to North America, and BMW has its latest inline-6 diesels set to come over by 2009. So, what about Audi?

Lacking the sheer riches and reach of Mercedes, BMW, or VW, Audi needed to be strategically wise. Hence the announcement at the last Detroit show that it is focusing for now solely on the 3.0-liter V-6 TDI Bluetec diesel beginning with the Q7 in the third quarter of 2008. (Mercedes, VW, and Audi are equal partners on Bluetec diesel, with Mercedes being more equal than the others.)

We’ve driven the current 230-horsepower, 3.0-liter V-6 TDI (non-Bluetec) with 369 pound-feet of torque in Europe and it is a powerful and pleasing engine. But it ain’t no 320-horsepower, 561-pound-foot V-8 (also not yet Bluetec) as driven here. And a larger, very premium SUV in the States still almost requires a V-8, like it or leave it. First-year sales as projected by Audi for the 3.0 TDI Bluetec Q7 in North America through late 2009 come to roughly 2500 units.

The argument from Ingolstadt is a sound one, however, because the V-6 that’s coming over can out-pull all gas V-8 engines on the U.S. market. Plus it will thrash any V-8 gas engine when it comes to fuel economy (22.4 miles per gallon average), meaning considerably more time between fill-ups on those long interstate journeys that make America great. Lastly, the CO2-emissions per mile are also much lower versus gas. And with the advent of Bluetec and its AdBlue water-based urea solution smashing nitrous oxides for an 80 percent drop in NO emissions, X even California and Massachusetts will allow their citizens to buy these Q7s close to home.

However, this 4.2-liter V-8 TDI, used first in the A8, once it gets the Bluetec system and comes to North America in perhaps 2011, will also seriously outdo any V-8 and most V-6 gas engines in all ways similar-displacement gas engines. Cruising an empty autobahn at top speed—147 mph—was a great sensation, with stability and wind noise respectively at the maximum and minimum.

Ideally, therefore, this V-8 diesel as a Bluetec would be best for the Q7 in America. But we understand Audi’s more practical choice of bringing the V-6 to help more North Americans warm up to the concept and then buy it in other models in the future.

But what’s up exactly with that albatross-like 500-horsepower 6.0-liter V-12 TDI Q7 unveiled at Detroit last January? Maybe if Audi ever plans a Q350 Super Duty pickup for corporate farmers and contractors, but we see that Q7 as a vehicle nobody on earth can want. We say bump up the development of Bluetec for this V-8 and bring it into North America about a year after the V-6. just mentioned. It also accelerates the beefy Q7 to 62 miles per hour in just 6.4 seconds. With a diesel particulate filter as standard, this V-8 rates a European equivalent of 21.2 mpg average per U.S. gallon. (N.B.: that’s better than the commonly registered 13 mpg from a bigger SUV with a V-8 gas engine.)

Driving the Q7 4.2 TDI Quattro for around 100 miles near Munich revealed it to be an eminently mature choice for any armchair-loving executive or family with lots to do. The default suspension package here (no adaptive air suspension on our tester) is up to every everyday task, steering is par for this class, the six-speed Tiptronic S automatic with paddles is above average, and the 40/60 split Quattro is excellence as ever. We’re still not convinced by the overall Q7 proposition, but this V-8 TDI version is easily our favorite treatment so far.

And, just as on all other larger-displacement diesels out of Germany, the diesel noise coming through to the cabin or reaching the ears of passersby is roughly equal to the noise of similar-displacement gas engines. Cruising an empty autobahn at top speed--147 mph--was a great sensation, with stability and wind noise respectively at the maximum and minimum.

Ideally, therefore, this V-8 diesel as a Bluetec would be best for the Q7 in America. But we understand Audi's more practical choice of bringing the V-6 to help more North Americans warm up to the concept and then buy it in other models in the future.

But what's up exactly with that albatross-like 500-horsepower 6.0-liter V-12 TDI Q7 unveiled at Detroit last January? Maybe if Audi ever plans a Q350 Super Duty pickup for corporate farmers and contractors, but we see that Q7 as a vehicle nobody on earth can want. We say bump up the development of Bluetec for this V-8 and bring it into North America about a year after the V-6.