Let me start this off by saying that I absolutely adore Alfas and that the Alfa 6 was an amazing car. The equipment level offered was phenomenal, driving it was splendid and the built quality was great. So what went wrong?
In 1960s Alfa was looking to replace its flagship 2600 series and borrowed heavily from the upcoming Alfetta's part bin to design the replacement. The Alfa 6, known internally as project 119, was the replacement and was originally designed to be launched in 1973. However, strikes within the company delayed production of all Alfas. Furthermore, the oil crisis of that year put a big dent in the demand of large, gas guzzling cars (look at what happened to the mythical Citroen SM) and introducing a new big car at that time made no sense. The launch was postponed until 1979, just in time for the second oil crisis. That created two big problems: first off, launching a big car in any oil crisis was bad news. Second off, precious little had changed since 1973 design and by the time it hit showroom floors the new car looked very dated.
Available with either a 5-speed manual or a 3-speed ZF automatic, the 2.5 V6 (with six carburetors!) that gave the car its name often cost a lot of money to register in countries like France and Italy where strict tax systems on large-displacement engines were in place. To salt the wound, its often-fuel-injected German rivals benefitted from better gas mileage.
The interior was more luxurious than any Alfa before it. This was to represent Alfa Romeo on the high-end market and the factory paid special attention to the materials used. This resulted in an interior that benefitted from a quality level that was maybe a tad subpar compared to its German rivals but that was way above anything that had previously come out of Alfa Romeo's gates.
To address the aging looks the 6 was facelifted in 1983 by Bertone - the most notable exterior change was swapping out the four square lights for two more modern-looking units. The interior was barely touched. New engines were added to the lineup including two new V6s; one was a smaller 2.0, designed with fuel economy and federal taxes in mind. This engine was mostly for the Italian market (where cars with a displacement of over 2.0 were heavily taxed) and was not available in all countries. The other was the same injected 2.5 found in the GTV6 and later the 75. A new 5-cylinder diesel engine was also added - a 2.5 turbodiesel that only produced 105hp, a small amount of power for a car that heavy. The engine was praised at the time as being one of the quietest diesels on the market.
The two new V6s were eventually dropped from the lineup and the Alfa 6 soldiered on largely unnoticed with either the carbureted V6 or the 5-cylinder diesel until its demise in 1987. This brilliant sedan suffered from a dicey introduction and despite its numerous strengths was often overshadowed by its German rivals.
While the Alfa 164 (and the 166 after it) filled the hole the 6 left in Alfa's lineup, the Alfa 6 has yet to be truly replaced; there hasn't been a true, top of the line luxury Alfa since.