The Audi S4 Avant An InDepth Look at the Fuel Efficient Diesel Engine

The Audi S4 is the most powerful diesel of all the Audi currently selling and the only 'S' model that does not have a gasoline engine. Actually, it is an even greater rarity, since while in the rest of the world, the Audi S4 is marketed with a V6 3.0 TFSI engine with 354 hp and 500 Nm, under the hood of the European Audi S4 we find what will probably be the swan song of the V6 3.0 TDI engine. And although most purists may think otherwise, the truth is that we Europeans came out ahead with the change.

Available with Sedan (sedan) and Avant (family car) bodies, the Audi S4 is actually a specially tuned version of the A4, with specific adjustments to its frame that, if you really enjoy driving, you perceive this vehicle as something completely different from the conventional A4.

It's not that the A4 is bad. The surprising thing is that the Audi S4 offers a clearly sportier touch without giving up a level of comfort comparable to that of any of its smaller brothers. The only consideration of choosing it over them is its higher price. Everything else is advantageous.

The forgotten V6 3.0 TDI engine of the Audi S4

Starting with the engine, we are facing a mechanical jewel that develops 700 Nm at 1,750 r.p.m. and it maintains them until 3,250 laps, a very high regime for diesel to be in its zone of maximum efficiency. Obviously, this wide range is due to the fact that the maximum torque is limited to protect the transmission and contribute to a greater sensation of acceleration since if the engine gave its maximum performance at medium speed, it would seem to us that it turns worse at high speed.

In any case, with this torrent of torque, 341 hp is achieved in a rev range of between 3,800 and 3,950 rpm. The engine offers a nice feeling of power and progressiveness when it revs up, but the best thing is its low latency when we press the accelerator pedal in low load and low rev situations. It is not that there is no lag, but it is much lower (in time) than usual in any other high-performance turbodiesel.

The main culprit is a small electric compressor built into the engine. And it is that in the Audi S4, the variable geometry turbocharger receives the invaluable help of a revolutionary compressor driven by a 7 kW (9.5 CV) electric motor instead of using exhaust gases.

Thus, when these are scarce (in the aforementioned situations of low revs and low load), the electric compressor connects, allowing the engine to be supercharged immediately by spending part of the accumulated electricity in a small lithium-ion battery of 0 .5 kWh located under the boot floor. According to Audi, in 0.3 seconds it is capable of rolling to 65,000 rpm, generating the necessary supercharge to reduce lag significantly.

As the problem of lack of response in low gear is solved, it is possible to mount a "fatter" turbo and obtain the extra horsepower that this TDI gives, and since we have a battery in the trunk, we can also feed an 8 electric machine kW (almost 11 CV) water-cooled and connected to the crankshaft by a belt that acts as a generator and starter motor (ISG), which allows the Audi S4 to become a light hybrid and enjoy the DGT Eco label.

There's also a 2,500-bar common-rail system and a 2.4-bar maximum boost pressure, and many components have been specifically strengthened to handle the increased performance of this engine; including the crankshaft, pistons, and connecting rods.

Also specific are the lubrication system, with a larger capacity, the cooling system (in which the new components are integrated) and there is complex "plumbing" to make the most of exhaust gases and be able to efficiently "fill" the new oversized turbocharger. which, of course, has variable geometry blades.

With all this, the umpteenth evolution of that V6 3.0 TDI engine released in 2004 by the Audi A8 (D3) is a true performance factory as well as a tremendously efficient engine. On the highway, at legal cruising speeds, we can drop below six liters per 100 km, while going agile in a mix of scenarios we will settle for just over eight liters. And, as if that were not enough, its exhaust sound achieves an attractive, very sporty timbre, without showing an intensity that makes it annoying. You won't stop buying this Audi S4 because it sounds diesel

The definitive differences between the Audi S4 with respect to the other A4

The V6 TDI engine, mounted longitudinally at the front, is coupled via a torque converter to a conventional eight-speed planetary automatic gearbox. At its output, the torque is distributed permanently between the two axles, thanks to a conventional transmission shaft that carries the torque to the rear axle and a semi-shaft integrated into the gearbox itself that connects with the front differential.

In the rest of the A4, the change is conventional gears aligned on two axes coupled by two disc clutches, and the drive to the rear axle is connected by means of a dog clutch, like a switch, when necessary.

The Audi S4's gearbox contributes to the vehicle's silky smooth running; it works flawlessly both in D mode and in the sports program and, of course, if we decide to drive it sequentially.

The “sail” operation is not so well-tuned. If we select the Efficiency mode and we are driving at a constant speed, the system will choose between having the gear engaged in the highest possible gear, putting it in neutral and even turning off the engine to take advantage of the inertia (for example, when we are driving on a highway with a favorable slope).

This means that, if we select the cruise control, the aforementioned Efficiency mode, and set the speed at 120 km/h, we will repeatedly see how the change goes from D8 to D (that's when it goes to neutral), the speed it quickly drops to 114 km/h, the gear switches back to D8, the vehicle accelerates to 121 or 122 km/h and, after a short time, the operation repeats itself, forcing us to abandon the Efficiency mode to select the Comfort program and avoid this roller coaster of speeds.

And it is that driving with cruise control is the only way to avoid being surprised traveling at 150 km/h or more completely inadvertently in a climate of absolute tranquility and comfort.

The Audi S4 is a fantastic machine for traveling on highways and all types of roads. Its compromise between stability and comfort is outstanding. The feel of the steering, the biting of the brakes, and the quick response of the suspensions, which are firm but never uncomfortable, constitute a difficult cocktail to improve.

Perhaps drivers looking for that "beyond" the limit of adhesion prefer the more oversteering behavior of a BMW 3 Series, but to this day there is no other sedan or diesel family car that offers the response and dynamism of this Audi S4.

Is it worth spending the 82,710 euros that this Audi S4 Avant costs? If we take into account that we are dealing with a representative vehicle, well equipped, that drops from five seconds in the "zero to 100" and that allows us to travel as a family with maximum comfort, the answer is a resounding "yes".

If we simply think that we are paying a supplement of 25,000 euros compared to a 204 hp A4 40 TDI Avant quattro for a more powerful engine, six-piston brake calipers, and 23-millimeter shorter springs, we surely think that it is not worth it. But if you get the chance to get behind the wheel of both models and really enjoy driving, you'll quickly understand that the price premium is more than justified.