(November 28, 2008) Over the past year, Audi has completely redesigned its mainstream mid-sized car platform which debuted in 2007 under the new A5 coupe, followed by the A4 sedan and station wagon this year. In 2009, the so-called B8 platform will also provide the basis for the new Q5 mid-size SUV as well. Americans are only able to get gasoline engines in the A4/A5 so far, but European consumers get a wider array of choices, particularly in the sedan.

On the diesel side, the A4 can be purchased with the same 2.0L four cylinder already offered in the US spec VW Jetta TDI, or a choice of 2.7L or 3.0L V6 oil burners. The A5 only offers the two V6 engines for those opting for diesel. Here at Green Fuels Forecast we drove an example of each car powered by the 3.0L V6 TDI and a six-speed manual gearbox. The A4 was provided by fuel system supplier Bosch while the A5 came from turbocharger supplier Honeywell. While the powertrains were nominally equal, the coupe lacked some of the dynamic options that were installed on the sedan.

The A4 had the new Audi Drive Select system that provides driver adjustable damping and an active steering system. The adjustable dampers are combined with a sport suspension system that lowers the ride height by 0.79 inches. The difference between sport and comfort modes for the damping was not dramatically obvious but the lowered suspension of the sedan definitely had crisper responses than the base setup in the coupe.

All Audi B8 platform cars have electric assisted steering but the drive select system supplements that with a superposition system that can vary the steering ratio depending on vehicle speed. An electric motor tied to the steering gear, can either increase or decrease the motion of the gear relative to the driver input. When active the system not only varies the effort to turn the steering wheel but also the actual angle of the front wheels relative to the steering wheel. This can be used in a number of ways.

At low speeds the ratio can be reduced allowing easier maneuvering in parking lots with reduced steering angle. This reduced ratio requires more effort to achieve the same angle at the wheels. The A5 which is missing this variable ratio mechanism feels like it must have the same assist calibrations. As a result at speeds below about 15 mph the steering effort feels far too light. The steering superposition system is also used in conjunction with the stability control system.

When the car is at the limits of stability, the control system normally adjusts the engine torque output and individual wheel brakes in order to modify the attitude of the car and help it follow the track requested by the driver with the steering wheel. The steering system can also adjust the angle of the front wheels to help manage the vehicle's direction. We didn't really have the opportunity to explore the limits of either car during this test. However, when we drove the A4 in Europe last spring, the car proved to be beautifully balanced during hard cornering.

Inside the two and four door cars share basically the same dashboard design which is a good thing. Like virtually all Audi's of recent vintage these have wonderful interiors. The surfaces are covered in high quality soft touch materials and everything is well laid out and easy to use. In spite of the not having the sport suspension, the A5 did have the optional sport seats which the sedan was unfortunately missing. The standard seats in the sedan are comfortable but could use more lateral support. The sport seats offer excellent support and have adjustable thigh supports which will be welcome to anyone with longer legs.

Part of the A5's sportier appearance compared to the A4 comes from having a roof two inches lower and a body one inch wider than its sibling. Inside the car you sit slightly lower and the coupe's roof is closer to your head. Nonetheless both cars have plenty of room in the front seats. The combination of a two inch shorter wheel base and lower, sloping roof-line mean the A5's back seat is noticeably cozier than the A4.

What separates these two test cars from similar looking examples that can be purchased from American dealers is the powertrain. Both cars are powered by the same 3.0L V6 TDI diesel that will power the upcoming US market Q7 TDI. In European specification, the V6 generates 240 hp and 369 lb-ft of torque. The engine uses the latest high pressure common rail injection system that reduces fuel consumption and emissions as well as noise levels. The current European version of this engine is also equipped with a particulate filter to eliminate soot emissions. Unlike the Q7, these cars are not yet equipped with a urea injection system to reduce NOx emissions, so they cannot pass Tier 2 Bin 5 emissions standards.

A driver unfamiliar with diesel engines stepping into one of these cars for the first time will likely never realize what it is. This is one of the smoothest and quietest engines available regardless of what type of fuel it uses. The V6 has a 5,000 rpm red-line and the variable geometry turbocharger provides excellent response to the accelerator at all speeds. Squeezing the pedal makes either car surge forward, making merging onto freeways easy. Acceleration to 60 mph comes in the low 6 second range from a standing start. The exhaust note has a pleasant growl that is perfectly in keeping with the performance capabilities.

In light of the run up in diesel fuel prices over the past year, German automakers like Audi and BMW have been promoting the idea of diesel engines as premium performance power-plants that still provide excellent fuel economy. Equipped with a diesel both of these cars exhibit all the characteristics one would expect of a premium sport sedan or coupe. Even with the performance capabilities of the A4 and A5, the cars still averaged 31 mpg and 32.5 mpg respectively during our test period which consisted mostly of urban driving. During the recent Audi Mileage Marathon, A4 TDIs averaged as much 44 mpg on some stages with the same powertrain. At this point Audi has not made any official announcements about whether it will bring the A4 TDI to the US market. Off the record officials have said that at least the A4 TDI will likely come to the US at some point, perhaps in 2010. If BMW's new 335d proves to be a market success, the A4 TDI will surely follow close behind.