engine differences?

i tried to search but couldn't come up with anything. i know pre-90 it was the renix system, then it was the H.O. till..? are all the rest H.O. or did that stop when the high output badge stopped?

Renix 87-90. After that all 4.0 are considered HOs. There were changes along the way. 91-95 are basically the same. 96+ the block was upgraded, they stiffened the cylinder walls and added a main cap craddle. The heads change also 96-99. Mid year 99 the head changed again. The exhuast ports were smaller in size. Some time after 2000 they did some changes on the block. They water pump has an improved impelor. It is not intercahngeable with older blocks. It has an extra bolt and the bolt locatoin is different for two of the holes. The oil pressure sensor hole is located towards the back of the block instead of the oil filter boss. Two of the engine mounting bosses have different hights. At least on my '03 block. I would guess these changes came in '02.
Tom