Tm Chou of Thrasher fame posted many years ago on the gnttype list that the 84-86 esc module contained a more selective 4 pole filter while the 87 unit had a less selective 2 pole filter (that passes engine noise from a slightly wider frequency range to the level detect circuit). According to him the 2 pole filter was a little cheaper to make and worked just as well, and they seem to interchange with no difference that I've seen. Oh, the esc module detects knock but it's the ecm that decides what to do about it (how much spark to pull). I know the same esc module was used on lots of other gm engines, like the 5.0 V8 in my 88 Chevy truck, so they are plentiful in the junkyards but I've never heard of one going bad.
