91'+ had the 4.0L HO from the factory, 87-90' had the Renix non-HO from the factory. You can tell too from the air box as the 87-90' non-HO had a temp sensor, and silver air duct flex pipe from the box to the motor, also had 2 small hose lines coming from the back of it. The HO had a very simple airbox with just 2 large tubes coming from it and no temp sensor in the box. The HO has 190 hp and the non-HO 177 hp from the factory, both are good motors being fuel injected 4.0L 6 cylinders. The slightly reduced hp in the non-HO isn't that big of a deal, just the HO has more off the shelf modification stuff to get more hp out of them and they are easier to find donor stuff for them being that they were used from 91'+, not just 87-90'
as the Renix non-HO.