One of the more popular tech questions we get asked around here is "How do I put a Cummins diesel in my Jeep?" At over 1,000 pounds dry, the huge weight and massive torque output of the 5.9L 6BT engine precludes most CJ and Wrangler owners from realizing such a swap. However, the smaller 3.9L 4BT commonly found in many fleet trucks is a pretty good fit in the engine bay of some Jeep models. So when we heard that distinctive diesel clatter coming from Dave Smith's '80 J-20 while photographing a trail run with the Western Maine Mountain Jeepers back in late 2006, we couldn't say "pop the hood" fast enough.
As a self-professed Jeep junkie, Dave maintains a good supply of vehicles at his residence in Unity, Maine-whether drivers, projects, or donors, he's got it covered. So when he decided to swap the powerful yet thirsty AMC 360 from his '80 J-20 for an efficient and reliable Cummins 4BT, the only thing he really had to scrounge up was the engine itself.
Vehicle:'80 Jeep J-20
Engine:3.9L 4BT turbocharged Cummins
Transmission:Ford ZF five-speed
Transfer Case:Ford BW 1356
Axles:Stock 8-lug Dana 44, 3.73, open diff (front); stock 8-lug Dana 60, 3.73, open diff(rear)
Tires:Light truck 235/85-16
Built For:Daily driving with mileage and reliability in mind
Chassis And Driveline
Aside from the drivetrain, Dave's '80 pickup is pretty much factory. The frame, suspension, and front Dana 44 and Dana 60 8-lug, 3.73-geared axles are all as Jeep made them.
A local bread company in Dave's area used a fleet of P350 dual rear-wheel Ford trucks that had had their 300 inline-sixes converted to 3.9L Cummins engines back in the early 1990s, so finding a donor truck was relatively easy. Dave bought the retired bread truck already sporting the Cummins-to-Ford adapter and wearing a Ford flywheel. Coincidentally, Dave also had a Ford truck in his collection sporting a ZF five-speed tranny and BW 1356 T-case hanging around, so the die was cast.
After mating the tranny to the engine with a new Ford clutch and building up some motor mounts and a crossmember mount, Dave dropped the engine into the chassis. The bread truck donated its radiator, and Dave whipped together a 1-inch body lift using leftover Daystar CJ pieces to gain some clearance between the drivetain and the floor. Even with a large electric puller fan there seems to be room to spare under the hood.
Dave left the stock gas tank and hard lines in place, but replaced any rubber in the fuel system with Viton because he operates the truck on a blend of diesel and biodiesel. He'll eventually run it on straight biodiesel, which isn't friendly to rubber. The engine feeds off of the stock pump mounted on the side of the block. Dave rigged up a fuel heater and filter using tractor parts to keep the fuel clean and prevent gelling in the cold Maine winters.
While it's true the star of this vehicle is its drivetrain, the sheetmetal is exceptionally clean for a 27-year-old New England truck. Likewise, the interior is pretty plush by Jeep truck standards, with new vinyl flooring from BJ's Off Road and transplanted leather power buckets from a Grand Wagoneer.
The front bumper is homemade, and the truck sports front and rear receiver hitches so Dave can plug his winch into either end as needed.
Driving The Diesel
The little Cummins puts out 105 hp and 286 lb-ft in stock trim with no intercooler. Dave has turned his up just a bit but puts his priorities squarely on dependability, not power, so moderation is the name of the game. As such, the 238/85-16 tires and 3.73 gears translate into a comfortable 2,000 rpm at 65 mph on the highway, during which the big J-Truck knocks down 24 mpg. Dave reports figures of around 21 mpg flat towing his flatfender.
Dave admits acceleration just isn't up to par with the 360, but he says the big truck will certainly get out of its own way. Dave also says that on back roads he's able to pull most hills without slowing in Fourth gear as long as the turbo is spooled. On the interstate, he's able to maintain Fifth gear over pretty much any grade at legal freeway speeds.
Why I Featured It
As the technical editor, I'm frequently inundated with questions about swapping these engines into Jeeps. While I still don't think they're the best choice for everybody, Dave's J-20 represents a cleanly executed, owner-built-and-fabricated diesel conversion that looks like the factory could have built it. In my opinion, it's the right engine in the right truck for the right owner.-Christian Hazel