Positive pulses

The most significantly new aspect of the latest Audi 100, now the fourth generation of that model, is its V6 engine. This completely new 2771 cc engine is slightly undersquare with a bore of 82.5 mm and a stroke of 86.4 mm, and was developed with an eye to producing an even spread of torque and power throughout the rev range, rather than top revs bhp. To this end Audi has made use of an old, but beautifully simple idea of an inlet pipe of variable length and diameter. When a valve opens it causes a negative pressure wave to be generated, which travels outwards along the inlet pipe towards the open air. When it reaches the open end it causes a partial vacuum in the region of the inlet pipe mouth. Air rushes in to fill this vacuum, forming a positive pressure wave that travels back down the inlet pipe.

Ideally this positive pulse should reach the combustion chamber before the valve closes, providing a second wind so to speak. However the rate at which this pulse travels up and down the inlet pipe is relatively constant; it is the speed of sound (about 1080 feet per second, but variable according to atmospheric temperature and pressure). So with an inlet pipe of fixed length the pulses are only perfectly tuned at one point in the rev range. Varying the diameter of inlet pipes, as a means of achieving optimised air velocity and air/fuel mixture, is not uncommon in modern cars, but combining these two concepts as Audi has done is rare. Each cylinder of the Audi V6 is fed by effectively two pipes: at engine speeds below 4000 rpm the long narrow pipe is used, and above this speed a vacuum-operated valve opens enabling air to by-pass the long pipe, and use a short pipe of larger diameter. This keeps the pulses, which vary in frequency according to engine speed, approximately tuned to inlet length, and provides high air velocity at low rpm.

Thus the Audi V6 is a flexible engine, with generous amounts of low down torque, 186 lb ft (245 NM) at 3000 rpm, and an even spread of torque throughout the range, 164 lb ft (220 NM) or over between 2000 and 5500 rpm. Peak power is not unduly sacrificed at 174 bhp at 5500 rpm.

The compact 90 degree V6, which is fuel injected, with two catalytic convertors, is an undeniably smooth and capable engine, giving the car brisk acceleration in all the gears, a comfortable cruising speed of over 100 mph and a top speed of 135 mph. The Audi 100 will also be available with a 2.3-litre five cylinder engine that despite its 133 bhp and 138 lb ft felt disappointingly gutless on the road, especially in four-wheel drive form.

The design of the front suspension on both front-drive and four-wheel drive models is a developed rather than completely revised version of that on the previous Audi 100, but improvements in handling response have been gained by widening the track. The rear suspension of the quattro model is independent with transverse links and trapezium arms. The suspension is compliant and the handling responsive in both four-wheel drive and front-drive specifications, but both are more suited to fast A roads and motorways than twisty lanes. The suspension and the steering both lean slightly too far to the comfortable rather than the precise, to make really quick driving enjoyable.

Both five-cylinder and V6 models are extremely capable cruising cars. They are quiet, with comfortable seats, and an excellent interior. But after only a brief acquaintance with either model it seems clear that the driver with fast cross-country driving in mind would be well advised to plump for the V6 quattro.