Some would argue that the Audi A4 2.5-liter V6 TDI quattro is the best all-rounder in the range, and I would not disagree. But if ultimate performance is what you are after, then you have to bypass the silky smooth V6 cars, buy a 1.8-liter turbo and take it to a reputable tuner.
No Audi tuner is more reputable than Abt. Now 102 years old, this thriving family business is at the cutting edge of the tuner's art. Abt exemplifies the old saying, "Racing improves the breed." Abt is a master of that game, too-it has five Audi TTs in the German DTM Championship, which it is currently leading.
Drive the basic 193-bhp Abt AS4 1.8T conversion, and you find that the expected mild power and torque lift obtained through an ECU reprogramming also improves throttle response and livens up the car considerably. Stage 2 brings 220 bhp, but this time I had come to test a much more thorough Stage 3 conversion with 260 bhp.
The 20-valve Audi 1.8t motor is capable of producing well over 300 bhp with the right modifications, but for everyday use in a four-door sedan, Abt considers the 260-bhp conversion to be optimum from a performance/price point of view.
It is well known that the base 150-bhp engine is literally strangled at birth, with a small turbo and restrictive inlet and exhaust manifolds. Thus, the first step is to improve breathing. The first job is to get more air into the engine, so the inlet tract is modified and a larger carbon-fiber airbox fitted. A K04 unit modified to Abt's specification replaces the small KKK K03 turbo, and this sits on a new cast-iron, high-nickel-content manifold. The exhaust gases exit through a new stainless-steel sports exhaust, which eases the backpressure that throttles a turbocharged motor. This retains the stock catalytic converters and is a twin-pipe system all the way to the back boxes, which have a pair of pipes per side.
Larger fuel injectors are used to fuel the uprated engine, now running 1.3-bar maximum boost with 1.5 bar on overboost. It is vital to keep charge-air temperatures down, so a larger intercooler is placed low in the airstream up front to provide 30% more cooling than standard. Finally, a remapped ECU provides the right fuel and spark combination.
The end result is 260 bhp at 5000 rpm with 345Nm (254.6 lb-ft) of torque at 3500 rpm. With so much extra power and torque on tap, the car is under-geared, so Abt fit a taller fifth gear along with a dual-mass flywheel to damp drivetrain vibrations at the source.
With just 150 bhp in standard form, the heavy A4 quattro struggles to be called quick with a 0-to-100 km/h (62 mph) time of 8.9 sec. and a 220-km/h (137-mph) top speed. But with its new infusion of power, the AS4 really scorches off the line, shaving 2 full seconds from the 0-to-100-km/h time and charging on to 243 km/h (152 mph).
Numbers are one thing, but it is how the car delivers its power and feels like to drive that are important to the enthusiast. In this respect, compared to the standard A4 1.8T the AS4 experience is like an awakening. Where the factory car feels burdened by its mass, and lethargic in its responses, the Abt car shoots forwards like a hungry animal leaping on its prey. With an urgency about its power delivery, which sets the driver's adrenaline flowing as well, it completes the transformation from the mundane to the exciting.
Even at high speeds on the autobahn, acceleration is impressive, and the tuned motor, freed of the standard constraints, revs happily to redline with both gusto and really strong mid-range punch. In fact, there is so much torque on tap, you will generally end up driving one gear higher than in the factory car, unless you are in a desperate hurry.
The AS4 conversion is about more than just an engine, though. It is a complete package that includes suspension, wheel, tires and body styling. The latter comprises new front and rear lower bumper sections, side skirts and a unique color-coded front grille.