from 4.0 to 4.0 H.O Q's

i have a 91 xj 4.0 auto that i need to change out motor. what would i have to change/need in order to do a H.O.? how about computers? do i need to find a motor that was lined up with the same tranny? or do the motors change from years that i don't know about? any and all info welcome.

Year 1991 is a H.O.

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.

5-90

the motor in my 91 is dead so i want to swap motors. the basic swap. just wanting to know if i need to change computers or if there's anything to look for that i need to know.

black&blue said:

the motor in my 91 is dead so i want to swap motors. the basic swap. just wanting to know if i need to change computers or if there's anything to look for that i need to know.

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you already have everything, you have an HO cherokee................. just put a new HO motor in an go....

now let make sure i get this right.
91+xj = drop right in.
now on a 90-xj what would i need to do a h.o. or any + year motors?

my friend has a 90 and wants to swap it too. now can he just drop in a 91+ motor? or what would have to be changed? or does he have only the 90- years option to drop in?

Sure, just use your 90 head on the 91+ block keep all the pre-HO senders and sensors, use your old computer. If you want to swap to a full HO setup goodluck, and have fun. Not worth it IMHO.

You can also bolt the HO head on too but you have to do some modifications and run the renix fuel rail.

Check the search, it has been covered a couple of times and there is lots of information. :repair:

HodgePodge said:

Sure, just use your 90 head on the 91+ block keep all the pre-HO senders and sensors, use your old computer. If you want to swap to a full HO setup goodluck, and have fun. Not worth it IMHO.

You can also bolt the HO head on too but you have to do some modifications and run the renix fuel rail.

Check the search, it has been covered a couple of times and there is lots of information. :repair:

Click to expand...

Have you actually ever done this swap? A Renix fuel rail will not bolt up to an HO intake manifold.

If it's a 91-95 HO you're swapping in then make sure to get the whole engine. Intake and exhaust manifolds changed from Renix to HO and are not compatible. You would need a new transmission kickdown cable and throttle cable, adapt the TPS to the new throttle body. Bolt all of the Renix accessories to the HO block on the passenger side and use the HO brackets for the PS Pump. You will need to rework the fuel lines. Swap all the Renix sensors onto the HO block. It's not that hard of a swap but you should fully research this before delving too deep into it.

i changed the motor in my 89 to a H.O. and all i had to do was change the intake and exaust manifolds and some sensors and the water pump and hardware...