If you have a 1991 XJ, you have the High Output version of the motor. The break came between 1990 and 1991 - the 1987-1990 engines are called RENIX, and are pre-OBD-spec, while the 1991 to current powerplants are "High Output." Of course, there are some changes to the HO line (shift to OBD-II and addition of mains brace for 1996, deletion of distributor for "coil rail" ignition in 1999, and a couple changes to intake and cylinder head design,) but they are otherwise basically the same.
If you are looking for more power, you options depend upon how subject to the Air POlice you are. If you're in CA, they are better for 1991-up than they are for 1987-1990 (no, I don't know why - and CARB hasn't given me any useful answers yet. I've been trying for years.) Supercharger and nitrous kits happen (and the supercharger kit is much better for off-road XJ's - nitrous is pretty much a "street only" mod) and turbochargers are beginning to appear on the scene - although I am not yet familiar with any that have gained CARB EO approval. It is also possible to do a "home-ported" cylinder head - worth a few ponies - and to change the intake manifold for a newer design. Also, there are a few more header and electronic options for the OBD 4.0.
Let us know what you are wanting to do, what you are willing to spend, and (IMPORTANT!) where you are, and we can work forward from there.
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