Rebuilding 4.0 Renix vs 4.0 HO

I think I screwed up and bought a 89 Renix. Well I know its a Renix. The only thing that I will use from the 89 4.0 is the block(it came out of a 2wd MJ and is going into a 4wd XJ). I am using a 92 HO head. Should I dump the Renix block and get a true HO block or is there going to be no difference/little difference? Is the block casting the same from the 89 and 92? I have heard a lot of bad things about the Renix 4.0 but is it just because of the head/intake/header airflow? Or is the block weaker / crappy?

Actually, from what I've read/researched, the older 4.0's have slightly stronger blocks due to a higher nickel content.

AFAIK the Renix is a good system, with some minor faults. They have a problem with the way things are grounded, but this can be corrected with cleaning the grounds and/or augmenting them with heavier duty ground wires. The cylinder head and the intake manifold aren't as efficient as the later (1991+) models, and the amount of vacuum lines is another potential trouble area. There isn't as much aftermarket support for the older engines either, so it can be tricky to upgrade them.

On the plus side, they are pretty rugged. A lot of folks claim to have limped their Renix-based Jeeps along with sensor faults that would have stopped a later, more complicated version in it's tracks. I know that the OBD I and II versions can be tricky to mess with because the computers don't like the changes; but they can be reprogrammed, unlike a Renix model.

One of the better things I've heard, is that you can diagnose things on the Renix system with a good multi-meter. No confusing, or misleading error messages like the OBD units can spit out.

I honestly think if you have some electrical know-how, and are willing to keep it tuned, adjusted, and maintained; the Renix should be just fine.

Plus, if you hate it, you can buy a rusted-out POS XJ with a good drivetrain, and swap it ALL over.

Mechanically, RENIX and HO are virtually identical.

ANY 242ci block will accept ANY 242ci head. Manifolds to heads are a bit difference - RENIX will only accept RENIX manifolds easily. HO allows for a bit more swapping, but the later heads (#0630 and #0331) can be difficult, since the #0331 exhaust ports are smaller (if you swap away from an #0331 head, you may want to modify your exhaust manifold to allow the pre-cats and HEGO sensors, have one made, or you'll need an adapter plate so the ports aren't open to air.) Intakes swap freely back and forth between #7120, #0630, and #0331 heads.

The principal changes made to make the 242ci engine an "HO" variant are these:
1) Intake ports raised 5/8", reducing the angle that air has to go through to enter the chamber. This makes the head breathe more efficiently.
2) Engine management is OBD-compliant, which makes it a bit easier to tune (RENIX is "pre-OBD.") However, as mentioned, nearly any diagnosis on RENIX can be done with a DMM - there are only a couple of tests that require an analogue. And, I've known OBD to be wrong. I prefer to do my own thinking.
3) The intake manifolds were gradually improved 1991-2007, eliminating right-angle bends and sharp corners (both of which improve airflow.)
4) The knock sensor (found on RENIX) was deleted for 1991.
5) The EGR valve (never really needed on these engines anyhow) was deleted for 1991.

There's nothing really "bad" about RENIX - I prefer RENIX over OBD-I, and OBD-II runs a close second to it (OBD-I was specified by California, and OBD-II by SAE, so at least it's standardised.) Yes, RENIX could have had more potential than it did, but for the time it came out, AMC actually did fairly well (I'm given to understand that much of the RENIX head was modeled off of Jag heads of the time.) There is also no mechanical difference between the 2WD and 4WD blocks, so that's not going to be a problem for you - it's when you have FWD/RWD versions of an engine that you start to have trouble (mainly, the starter motor ends up on the wrong side of the engine.)

I have seriously flogged Hell out of some RENIX rigs - and swbooking is right, it would have stopped OBD rigs in their tracks and made me tow them home. Thanks - I'll take RENIX.