4.0 swap in J20

[AMC-COMMANDO]

Considering a 4.0 swap in my 75 J20. Has anyone done this? How did you like the results?

Currently the truck has a 401. Love the 401 and the power, but hate the gas mileage! I would like to drive the truck a lot more so the 4.0 may be the way to go. I did this swap in my 72 Commando and love it. But it is not a street cruiser, mainly just my rock crawler. And I did notice a difference between the 360 that was in my Commando and the 4.0.

[Heavy_Metal_Thunder_81]

IMO 6cyls in these rigs are a little underpowered for how heavy they are. Just my .02, though.

[addicted]

It's a swap i've always wanted to do. 4.0/NV4500/NP241 in a J20. The 4.0 will do the job and do it much more efficiently. Just slower. I don't drive fast anymore so that isn't a problem.

I put a 4.0/AW4/NP208 into an '89 GW. With the 3.73 gears it moved along nicely and only downshifted on bigger hills at 65ish mph with my 3000lb pop up camper in tow and a long weekends worth of supplies and clothes. Otherwise it cruised along nicely.

[AMC-COMMANDO]

I think I am going to do this in the near future. I have a freshly rebuilt 4.0 on the stand in the shop. And I recently got a part time kit with the overdrive to go in the Quadratrac. I will stick with the Turbo 400. The swap should be an exact duplicate of what I did in my 72 Commando. I know it won't be a power house but sure will make the truck more affordable to drive.

[Mikel]

What would be the MPG gain? Going from low teens to mid teens?

[Carnuck]

Auto or stick?

[AMC-COMMANDO]

It would be going from about 8 mpg to hopefully 12-15? Not exactly sure but has to be better. My Commando which had a 360, I never really checked mpg. It was built for mainly off road use but I do drive it to town once in a while. It went from using 5+ gallons to go up to the mountains to using about 3 gallons. Which is about a 30 min drive from my house. It also has 4.88 gears.

The truck is a 75 with a turbo 400 and 3.73 gears. I know the auto is going to rob some power.

[brielly]

I'd look to swap in an aw4 or ax15 along with the 4.0. The th400 is a bit of a resource hog that will hurt both your speed and mileage. You are going to use the efi with that engine right? If you are going efi you'll need to switch to hydroboost brakes to fit the 4.0 intake.

You have the potential to get 17 mpg or maybe at bit more. Definitely worth the effort!

[DieselSJ]

I'd look to swap in an aw4 or ax15 along with the 4.0. The th400 is a bit of a resource hog that will hurt both your speed and mileage. You are going to use the efi with that engine right? If you are going efi you'll need to switch to hydroboost brakes to fit the 4.0 intake.

You have the potential to get 17 mpg or maybe at bit more. Definitely worth the effort!

Or a NV3500. Definitely swap in some type of overdrive trans.

[AMC-COMMANDO]

I am not wanting to get to involved in swapping out engines. Just the engine swap will help a bunch I think. Switching out trans will take more time and $$$. Especially to switch to a stick.

I had not thought about the booster issue. Might be easier to adapt a Bronco booster instead of a hydro boost. I guess I will cross that bridge when I get there.

[sxp]

I hope to drop a 4.0L into my M715 someday. My plan would be to stroke it to 4.6L or so and mate it to a NV4500. This way I can stay true to it's inline 6 routes, but with a more solid engine when the time comes to replace the Tornado.

The idea of a 4.0L in my wagoneer is interesting, but a 4.8L Vortec with a 4 speed trans is a better/cheaper route for me.

Best of luck with the swap, and keep us updated.

[babywag]

Honestly a stocker 401 while it has the cool factor wouldn't be my first choice in a long shot.
One of my first vehicles was a 401 powered Cherokee, and while it was fun to drive...paying for it sure did suck!

I say go for it!
A nice 4.0 isn't much less hp, it'll rev higher, and obviously get better mileage. Added bonus of multiport FI, etc.

[AMC-COMMANDO]

I love the sound, and the pep of the 401 but not the mpg! I really enjoy driving the truck so I need to do something. I let my daughter use my Grand Cherokee for a few days so I have been driving the J20. Filled it 3 times in 4 days! Kills my wallet! I can't drive it long distances which sucks.

I debated a 4.6 but I have a fresh rebuilt 93 4.0 sitting on the stand. My Dad rebuilt it for his 83 CJ7 and then sold the Jeep to buy a 2003 Rubicon. So I hate to tear it down to stroke it. And best of all the engine was free! Just need to find the time to have the truck down for a while. And still need a few more parts for the swap.

[DieselSJ]

I am not wanting to get to involved in swapping out engines. Just the engine swap will help a bunch I think. Switching out trans will take more time and $$$. Especially to switch to a stick.

I had not thought about the booster issue. Might be easier to adapt a Bronco booster instead of a hydro boost. I guess I will cross that bridge when I get there.

Get the whole driveline - engine, auto trans, xfercase and go with that. The wiring for the automatic will all be there - plug it in and go.