Like Joel, I am amazed at the number of IO-360 engine variants that Lycoming has produced. I can see why, over the history of aviation, there are many obscure engine builders that have faded into the past. Look into automobiles and you will see a similar history. But why on earth, do Lycoming and Continental persist in making a zillion different versions of the same thing? If nothing else, the paperwork burden needed to keep the FAA happy must be horrendous. It does seem that they are willing to let the airframe builders dictate to them how an engine must be modified to fit their designs, rather than the other way around. Add to the Lycoming misery the sad tale of the -D mag engines and it would be comical if it weren’t so frustrating - and costly to owners.
Long ago, auto manufacturers learned to make part commonality a key to cost containment. Remember the Chevy 350 engines that turned up in Oldsmobile cars? That one prompted a lawsuit, in spite of the fact that the Chevy engine was better than the Olds version. Part of aviation’s willingness to perpetuate this mess may have come from the fact that the engine builders were the sole source of replacement parts. Why worry about the wide variety of engines if you can charge what you want for new parts. At least that has changed a little with regard to replacement cylinders. When Continental began producing cylinders for many Lycoming engines, the Lyc versions magically got cheaper. Unfortunately, Lycoming still has the lock on their angle-valve cylinders for the 200 hp IO-360 in my Cardinal. So those precious items are still (almost) worth their weight in gold.