planning ahead (engine rebuild)

ok so my 1990 xj has 300,000ish miles and still runs good. I just had a newer transmission put in along with a rear main seal. The engine is still good to go right now but it is enevitable that this thing will quit and that will probably be pretty soon. I dont expect more than 50,000 more miles out of her. So I have done some searching on this forums and am pretty sure I am going to go with a late model HO 4.0. So here is my question. I keep seeing that you cant just bolt in the HO4.0 and I will have to use my renix head in order to keep the headers in line? What if I get cutom headers built? If I were to say find a late model XJ and yank the whole motor out including the TB and all of the electronics and everything will that bolt in with custom headers? I am kinda new to this so what is a short block and what is a long block? I dont think I want to do a stroker as that is even more money and even more work. I will not be doing this myself as i dont have the room or the tools or really the ability to do so. My goal is to get my jeep running so great that I would not think twice about doing long distance cross country road trips and not worry about breaking down in po-bunk Alabama with a busted up jeep. Another reason for not strokeing it is I want to keep the good gas mileage and if I have a stroker and the power I know I would not be able to keep my foot light on the pedal.

So some advice? thanks a bunch. This forum has taught me more about my jeep and basic mechanical workings that I thought it would.



-Dillon

Custom headers? If you have to customize anything make it the tube that goes between headers & muffler, much less complicated, much less expensive. You could get aftermarket or stock header for the same (later) model motor you want to get. Should fit in earlier xj. Include the intake manifold, headcomputer, gauges, etc. with that. The complete package so everything works. You could also look into a full rebuild kit for your motor (sealed power) that and DIY rebuild, working with your local machine shop if you choose.

As for buying parts, a short block it buying the lower block only. A long block typically includes the lower block and the heads. A crate motor basically is a built motor that has about everything attached needing to make it run.

Start at the beginning: your 1990 with Renix (non-HO) computer needs Renix-era sensors. You can take a whole complete HO motor, swap in the non-HO sensors, and plug everything in- it will run fine. Most swappers have done it this way, since swapping in the HO computer involves a lot of rewiring. The biggest issue is adapting the non-HO Throttle Position Sensor to the HO throttle body. It takes some rigging to get the arm (as found on the non-HO throttle plate shaft) mounted onto the HO throttle plate shaft, so that the TPS can work. There have been some articles on the web about this adptation (madxj maybe?). The article also talks about throttle cable mounting. The fuel rail on late-model HO engines has no return line- easy, just use your non-HO fuel rail/regulator/return line. Some of the temperature sensors on the HO work electrically the reverse of the non-HO sensors- don't mix. If you get a recent HO, there will be no mounting boss for the Knock Sensor. Earlier HO engines have this boss, with a plug in it. I like my knock sensor!!
If you want to put in an HO computer, you are on your own. Everything is so interconnected I have nightmares even thinking about it. There may be a Painless Wiring harness to make it easier- not sure on this one.
For the head question, see the information on Dino Savva's site: he rates them for flow (search for his name, to find a link to his site). You definately want a later model head (but not the last few years, for best flow). The non-HO heads are somewhat restructive at higher RPMs. You may be able to just bend your down pipe up to mett your header, the difference in elevation of the header isn't much. If not, a muffler shop will add a section to extend it easily.
My ideal for my 1990 would use the original Renix computer, late model head, intake and throttle body, and a block with knock sensor capability (either earlier HO or non-HO). This is with stroker in mind, though.

Good point about the HO motor with Renix comp. Got carried away and forgot that the HO & early 4.0 they aren't really all that different. The HO should fit most renix sensors. Could the location for the knock sensor if your new (HO block doesn't have one) be drilled and tapped or a bung (I think this is the correct word) of the right size be attached by welding? I have seen someone, very unprofessionally, attach the knock sensor w/ zipties when doing a similar transplant. It rattled around of course and confused the comp.

You can use a aftermarket header on either head. I have a 90 with a borla header that works great. My mechanic friend has used the stock intake manifold (90) on a HO head with enough overlap to seal. You can check for leaks by spraying brake cleaner around the mating parts while the engine is running, rpm change will indicate a leak.

Welding a bung (right term) into the block is problematic. It usually doesn't work well for castings as the crystaline structure changes with the heat inducing stressors in the material that tend to cause failure (cracks). You might be better off JB welding it in place. All it does is sense vibration in a certain frequency range that is suppose to be the knock frequency. BTW this is a good sensor to have.

I can get an equivalent to a crate motor with less than 10K on the clock for under $900, with electronics @ under $1200, so you may want to do some investigation in your area and talk to people who do engine swaps/ new installations. My source is heavily discounted but even with a 40% markup, it is a good deal.