The DC-9-80 introduced a 30% larger wing,larger horizontal stabilizer,larger engines,different main gear legs,different flight guidance system,autoland capability and autobrakes,brakes being an option.There were also upgrades to the electrical and hydraulic systems.
Launched in the very late 70's as the DC-9-80,or Super-80,initial versions were the DC-9-81 and the DC-9-82 with uprated engines and higher operating weights.DC-9-83 had even more powerful engines,weights and 1 or 2 auxiliary fuel tanks in the cargo hold for long-range operations.Some airlines upgraded their DC-9-80's with an EFIS cockpit (Scandinavian Airlines did this) and started calling them MD
-80's which was the designation McDD was using in their marketing efforts.
-87 was a shorter fuselage version of the MD
-82 with a taller "top hat" fin also used on the MD
-90 and MD
-95.It also had the option of extra fuel tanks.
-88 was the initial factory version with EFIS,later introduced on the production line for the other versions,as well.It also had provisions for mounting propfan engines under development at the time.
-90 did away with the PW
engines and had V2500's instead,new electrics,changes to the flight control systems and many,many more minor modifications.Main failure of the MD
-90 was it's wing.It was really too small for the added weights of the MD
-90,with high altitude performance suffering.
The major difference in the old "classic" DC-9-series,apart from the fuselage lengths,was the introduction of the wider span,slotted wing of the -20/-30/-40/-50 versions.