Actually, mid-1998 was the dividing point between the fully mechanical P7100-pumped engines and the electronic VP44-pumped 24-valve engines.
I used to know how to decipher which was which by the 8th digit/letter of the VIN-number; it seems like one was a "C" and the other a "D", but I don't remember which was which.
My preferences are :
First = 1989 or 1990 Bosch rotary VE-pumped non-Inter-Cooled; these engines have the biggest injector bores and achieve the highest fuel-mileage of them all
Second = 1991.5 -1993; same VE pump; Inter-Cooled; but, with smaller injector bores, a larger exhaust housing, and not quite the power or mileage of the earlier ones.
Third = 1994-1998.5; these engines are the real "12-valves"; Bosch P7100 six-piston-operated inline injector-pump that is capable of insane horse-power; a bigger, quicker-spooling turbo; factory-original piston lift-pump (which many variants have been swapped onto the earlier 1989-1993 diaphragm-pumped engines); as a general rule, these engines lack the high fuel-mileage capabilities of the earlier engines.
Anything after 1998.5, while they are a Cummins engine,