The original high revving four cylinder is replaced by the splendid big straight six which many argue is the best in the world both in terms of performance and smoothness. The standard 3 was available with a 2.5 litre version, so replacing it with the big block 3.5 litre was not too difficult. The bigger engine adds about 118kgs to the overall weight of the M3 and most of that over the front wheels but stiffer front springs and another 60bhp more than compensate.
The result according to official figures when it was new was a top speed up from 146mph to almost 160mph and a vastly reduced 0 - 100mph time, down from circa 20 seconds to just 14.5 seconds.
On the road this translates into what you would expect - a performance better than the standard M3 without the revs required of a 16 valve four. In fact the acceleration becomes deceptive because of the greater torque and low down power delivery. For those who want the excitement a semi-racing power unit will be unenthusiastic about the changes; those who love the sheer urge but find all those revs and gear swapping tiresome will be taken by the Alpina solution.
The Alpina B6S was one of the most relaxing high-speed devices you could find in its day (and possibly still is) with the extra low down torque making progress in heavy traffic a lot easier. There's the added bonus of a much more mellifluous exhaust note; music to the ears when compared with the four cylinder's rather common rasp.