Haha I just mentioned this..I'm on peachparts, dieselbombers ect ect. I started this search about 3 months back, just found this thread by accident.
Saw some of mis-information though, so I'd figured I toss out what I know and learned over the last few months of constant research!
The Om617 aka the million mile motor is a great swap! Its virtually the same demisions as the 4.0. Slight shorter. Its a front sump & the oil ump/pick up are 1 piece, so you can only mod the pan so much, you can notch it slightly, but yes a 3" would be needed to have any kind of flex! This is the only bit worrying me about the swap..if it'll flex decently with my 3-4" lift.
The OM617 will never see 30mpgs. Sorry guys
Highest reported in a hyper mileaging MB was 31mpgs, he blocked his grill, lowered his car ect ect ect..us lifted Xj will probably get 25-27mpg MAX! But alternative fuel & such will save, and diesel is pretty cheap in some states. It would be worth it in my mind! Also think if an appocolyse happened? You can run on oil, veg, ATF, ect. AWSOME!
Anyway the OM617 weighs very very similar to a 4.0..I can get exact specs, I've written them down, the OM617 was something like 60-70lbs lighter fully dressed and has similar output to a jeep 4.0, even though the stated numbers are lower. The stock IP on the OM617 has 5mm elements and the will be maxed out around 210ftlbs. They likes to be ran around 2300-2400rpm with a stock turbo. You can run a W122 N/A serious intake manifold, its got long runners and helps with low end slightly and get a cheap VNT turbo. Otherwise they don't make much for torque below 2k. The VNT setup, will spool sooner, make more power way lower/sooner, and still keep stock or better power up top.
You can run these around 2000-2100rpm but you gonna feel a little slow on the highway if its stock turbo, especially on 33's with extra weight. Similar to the feeling of big tires & stock gears. I have a 5speed and 31's & 3.55's. If I do this swap I'll be downing to 30's. Better for MPG's anyway. Currently looking for 3.73 or 4.10 axles as well, they would be a good upgrade even if I stay with the 4.0.
Cheap upgrades, gauges first! EGT, Boost, and EMP (exhuats mani pressure), run a intercooler, crank IP, stock turbo will run 12psi without issue. 3" downpipe, straight through exhaust. W112 N/a long runner intake.
After that you'll be maxed on fuel, no more to add, no more power. Options are a $1500 Myna pump upgrade, upgrade stock IP with bigger elements, or meth injection.
I actually picked up a W112 N/A manifold at a yard because I saw it and saw people selling them for 120+, so I already have that incase I do the swap if not I'll sell it.
Honestly for me its searching the clearence and the wiring for the swap. If its easy enough then I will be undertaking it!
If not then my 4.0 gets a 4.2L mini stroker/efficient rebuild that I've mentioned a few times...
.038 quench, 9.6:1 on 91 oct...250/310ftlbs..with the higher comp and tighter quench it should get better than a stock 4.0 and it'll have better emmisions to boot! I'd guess my mileage would increase from 22 to maybe 25mpgs. But with gas going up, I want a diesel for bio fuels.
I just think of it this way, find a good donor OM617, with a solid bottom end. Cause a full rebuild is around 3000 in parts. Junkyard charges 150 or so complete motor. Rebuild & port/clean up head, polish intake & exhaust runners glass smooth, no roughness on intake since its diesel, that'll help spool and flow & power & efficiency. Shave head .01 to raise comp to 22.5:1 from 22.2:1, its apart so why not lol, get a little more out of it, 3 angel back cut on the valves since its cheap & its apart, to pick up a little more flow and then rebuild head to stock specs. New timing chain and that'd be it! Cost maybe 750? I can port & rebuild it myself, its one of my favorite hobbies.
Then sell my 4.0 - $350 (and thats being cheap, usually 400-550 here)
Sell Ironman motor mounts - $70
Sell Safc II - $200
Sell LS1 valves, springs, retainers - $120
Sell 61mm renix bored throttle body & TB spacer (biggest bored renix TB to date haven't found an equal size or bigger yet)
That alone would put around 800 back in my pocket..so essentially I could do this for almost free!
The OM617 is not smooth, saw that posted lol. They are mildly loud, clatter when your running it hard, so don't expect a quiet perfect plush ride lol
...I mean its a early 70s tractor motor what do you expect. I'd say its no worse than the 4.0 when running properly though.