The 90 Model year was introduced very late in 1989 ( November).
As a result "first 18 months of production" should actually cover most 91's as well, except maybe the last quarter of production, (maybe early spring 91 production being a good guide).
With 92 production starting approx early summer 91 (and in the showrooms mid september).
But the 92 Model year engine is not a bad one still ahs those sodium filled exhaust valves.
Some have stated that one way to recognize the earliest production engines is that they seem to emit a heartier growl on vigourous acceleration.
But later engine's growl pretty good too.:oface
Fred..
