JK 3.8 swap to a 4.0 how to?

[LTDanslegs]

Hi!

I’m posting this because of a close friend of mine.

My friend who can no longer work on cars because of a prior accident. His only vehicle (a 2008 Wrangler) threw a rod. I was planning on putting another 3.8
in for him, but he told me he would like to do a 4.0 swap (presumably out of a cherokee). I’ve been trying to research if this is possible, I’m assuming I would need different motor mounts and probably would need to relocate them, ecu, trans etc.

While I do work on cars, and have for years. I’ve never completed a swap. Additionally I’m not as familiar with jeeps. I work with mostly Toyotas, or German diesels.

Does anyone have any insight they would be willing to share? Maybe I am searching incorrectly on google? Any advice will be greatly appreciated! I do apologize for my complete lack of knowledge, I am mostly self taught, and didn’t grow up around car enthusiasts.

Thank you in advance!

 

[Speartip]

I've researched many diff topics & mods for my Jeep.
TBH I haven't come across anything written about putting
a 4 Litre in a JK.
I do understand many Jeepers cherish the older 4 Litre .
There are some knowledgeable people on this board so
if it's been done I'm sure someone will chime in .
I do honestly think it would be a lot easier to just replace
the 3.8 with another 3.8 crate engine . Good to go .
As the 3.8 is a bit underpowered compared to the newer
3.6 . The 3.8 can be very reliable & last a very long time .

 

[macleanflood]

If you took a complete 4.0, transmission, transfer case and computer sure with fabbing mounts and custom drive shafts you could get it running. No idea how you'd do the dash...but with some creativity anything is possible.

I'd think a hemi or LS swap would be far easier...many have gone before you...or as mentioned a 3.8 or 3.6. Hell these days you might want to consider electric...a pair of 120 HP netgain motors hooked to each driveshaft would be awesome.

-Mac

 

[Speartip]

If you took a complete 4.0, transmission, transfer case and computer sure with fabbing mounts and custom drive shafts you could get it running. No idea how you'd do the dash...but with some creativity anything is possible.

I'd think a hemi or LS swap would be far easier...many have gone before you...or as mentioned a 3.8 or 3.6. Hell these days you might want to consider electric...a pair of 120 HP netgain motors hooked to each driveshaft would be awesome.

-Mac

Than there’s that , the computer , dash , everything electrical .
It could be the case where the entire Jeep would have to be rewired . Thinking it would be a total nightmare .
It’s probably why I’ve never came across it before .

 

[JerryD]

The most economical/least frustrating/least time fix would be another 3.8. I agree that the reason most(any?) don't do a 4.0 swap is because of mating the canbus controlled everything to the JK. I suppose you could replace gauges with mechanical and get a "Universal" harness from some place like Painless Performance or something. The downside of doing that, depending on where you live, is putting an older engine in a newer vehicle could cause problems getting it registered/inspected (there are often legal ways around this, though). And the cost to go to a Hemi or LS can get very pricey, very quick.

If he's looking for better performance, a fresh 3.8 and a re-gear could be a good route. Shoot, you could even throw a RIPP supercharger on it if you wanted more power. IMO, getting another 3.8 is the fastest, cheapest, easiest way to get it back on the road.

I really miss my old CJ & it's 4.2. Loved the exhaust note of the straight six...

 

[Dirty Dog]

I do not recall ever hearing about anybody doing this particular swap. Probably because there really isn't a good reason to do it.

You would need to fabricate your own mounts, which admittedly isn't super difficult. There is a good chance that fitment issues will mean fabricating your own oil pan, steering components, etc. You're going to need the ECU from a TJ as well as the engine. You will quite possibly need other modules as well. The transmission should bolt up, but the 42RLE is controlled by the ECU. The one you just took out so you could use the TJ ECU...

I'm sure there's more. The point is, you've never done an engine swap, and this will be a very complicated engine swap. And for what?


I stole the 4.0L dyno numbers from Motortrend. It had a CAI, header, and exhaust, so it's a tiny bit higher than stock.

So you're considering a nightmare swap for a net loss in usable power.

It would be easier and cheaper to do a Hemi swap. And even easier and cheaper to do an LS swap.

This is the dyno sheet from my Vette. The lower numbers are a basically stock LS1 with a CAI, long tube headers, high flow cats, and a Magnaflow exhaust.
The top numbers are from the Heads/Cam/Blower LS3 that was swapped in. That engine is currently out. We're install a forged crank, billet rods, and forged pistons. Lowering the static CR from 10.7:1 to 10:1 so we can run a bit more boost and timing.

A stock LS1 would give you a torque curve that is nearly as flat as a table, with over 300lb/ft @ 2400 RPM.

If your buddy wants a swap, it just makes sense to either do the simple swap (3.8 to 3.8) or a swap that will provide an improvement in performance (Hemi or LS1). You're still looking at swapping things like the ECU and transmission, but aftermarket support for these swaps is huge, which means someone else already figured out the HOW.

 

[Danarch]

Like the the old inline 6’s.

 

[Dirty Dog]

Like the the old inline 6’s.

They're great engines. But the proposed swap just doesn't make any sense.

 

[elgordo371]

If you stroke the 4.0, put a performance camshaft in it, and use newer fuel injectors you will be running about 230-240HP and over 300 ft-lbs torque. Your horsepower loss relative to a 3.6 or 3.8 will be theoretical as it will be in the 4000-5500 rpm range where you almost never go in most driving, and even wheeling scenarios. Meanwhile you gain some torque from 1500-3000 RPM. You will need to use the 4.0 computer and instrument cluster appropriate to your engine/transmission pairing.

Or you could maybe let a holley sniper control the EFI and cluster...

If you have a manual tranny, then your engineering problem is to mate the 4.0 to the transmission. If its the NSG 6-speed it can be done. Just use the computer from a 4.0 with a manual.
If you want an auto transmission then you'll have to find/get a discrete trans computer for it, and integrate it with the ECU yourself.

I don't think this would be worth doing with a 2000-2006 4.0. The ignition is too specialized, and the rest of the engine too prone to bad castings and other defects.

But if you use a good '92-'98 4.0 then parts are generic, plentiful, and inexpensive. If you are a "keep my Jeep FOREVER kinda guy this is a viable route.
The 4.0 shines in longevity and rebuildability. If you stroke it, the compression will rise. You will need 89 octane gas. And you will lose some of the longevity...but not the repairability.
With that setup you could get 200,000-300,000 on the first motor, and then just pop another identical motor in, and get another 200-300k miles.