About The 1981 Mustang
Although Ford realized the 1980 Mustangs had some engine issues and had dropped in sales since the 1979 Mustang they ran into more problems in 1981. The sales dropped even more and only 182,552 Mustangs were sold in the ’81 model year. Due to the engine problems of the previous year there were some engines that were simply not offered in 1981.One of the biggest changes in 1981 was perhaps the introduction of a new VIN, Vehicle Identification Number, system.
Overall the 1981 Mustang was much like the 1980 model, it just had fewer engines to choose from. Unfortunately, 1981 was not the successful year Ford had been hoping for.
Source: Mustang Forums