As you know, we here at Jp have a thing for fullsize Jeeps. Everyone on staff owns at least one of them, and all of us have owned more than that over the years. We’ve driven them, wrenched on ’em, wheeled ’em, and broken ’em, repeatedly. The one thing we’ve come to hate in this long-term love affair with FSJs is that none of them ever got good mileage. From a low of 6 mpg to a high of 17 mpg, with an egg on the gas pedal and a tailwind, they just aren’t cheap to drive. We keep dreaming of a smog-legal diesel swap, but none of us can bring ourselves to tear out our running V-8s to do it. That’s why when we saw this Grand Wagoneer a couple of years ago out at Area BFE in Moab, Utah, and heard the clatter from under the hood, we jumped on it. We got a few pictures, but we jumped too late. One of our sister magazines had already nailed down a feature on it, and it actually ended up on their cover. It belongs to Toby Boyer of Camarillo, California, but we didn’t actually meet him until the third time we saw the Jeep. The first two times it was driven by its builder, Todd, from Mercenary Offroad.
But the conversion to the ’98.5 24-valve 5.9L Cummins was so clean and the Jeep itself such a sleeper that we couldn’t get it out of our heads. So, we brainstormed on how to bring this Jeep to you in a fresh way that didn’t just regurgitate all the same information that our sister mag gave you. We ended up inventing a new kind of feature. We decided that you guys would want to see more information about this Jeep than what we normally run, so we went the extra mile. We blended a normal feature with a tech article. So, in addition to the usual feature information, there are build pictures to show some of the more complicated parts of the modifications and more.
We ran the Jeep on the dyno to find out what kind of power it was putting down. We even broke out tape measure to take critical dimensions and weighed it so that you could see how it measures up to your own Jeeps. “Feature Plus” is what we are calling this kind of story, and it’s our first one ever as well as the only one we have in the stable right now, so there is no telling if or when we might go this deep into a build again. By the time you read this, two years will have passed since we first saw the Jeep—and hopefully it was worth the extra work.
We really want to start in on how the Cummins got in there, but without the foundation, what would be the point? The frame was extensively boxed with plate for stiffness. The Jeep rides on about 6 inches of lift thanks to some custom Atlas Springs. Heavy-duty plate steel boxes were fabricated to move the front springs forward about 2 inches and move the rear rearward about 4 inches. That helps to get those 39-inch BFG Krawlers out of the wheelwells, but more on that later. As nice as it was to have leaf springs still under the Jeep (and it still rides nice too), in preparation for the weight of the axles and engine, 3⁄4-inch-diameter U-bolts are threads-up for better clearance to hold the axles to the springs with some 1-inch-thick steel plate.
Custom frame-side engine mounts that ape the factory Dodge mounts hold the Cummins to the frame with the factory engine-side brackets and rubber while three transmission mounts support the transmission and T-case. Energy suspension was used for the soft parts of the transmission mounts but some had to be modified. The top of the frame right behind the grille shell was notched and plated to provide clearance for the radiator that was needed to keep the mill cool. A custom fuel tank was fabbed up from aluminum and hangs between the rear framerails, thanks to a steel skidplate. The tank features internal baffling and all –AN style fittings.
The BBCS shocks are hung off of custom upper mounts. Out back a channel just in front of the fuel tank with multiple holes allows for changes in the shock angle. That helps with both potential shock length issues and allows for effective adjustment of the valving. Up front, much taller towers were fabbed up and welded to the frame to properly locate the shocks with the pushed-forward axle. A custom front bumper was built from many pieces of plate, ties into the new front spring mounts, and holds a 12,000 pound Warn Endurance winch. Underneath is a fabricated skidplate of tube and plate steel is covered with UHMD plastic that guards the belly from just behind the front axle all the way aft of the T-case for engine, transmission, and T-case protection. The rocker guards are impossible to see because they are 3⁄16-inch-thick plate bent to the contours of the factory sheetmetal.
Ah, finally. You might have been wondering about the “’98.5” we have up above. In this case, the 5.9L inline-six came out of a ’98.5 Dodge Ram 2500 and that “.5” is important because for the beginning half of ’98 the Ram got the 12-valve engine. This 24-valve engine didn’t show up until mid-year to meet new emissions requirements. It is also known as an ISB engine and was rated at 215hp and 420 lb-ft at the flywheel. In this Jeep, however, it has a big Mishimoto intercooler with a Diablo Sport Power Puck, so it puts down a bit more. Strong ARP head studs make sure that extra air and consequent power stay in the combustion chambers.
In the Ram, the engine was originally bolted to a 47RE automatic transmission, and that hasn’t changed. The factory computer is still in control of things and the transmission was left basically stock. A 10 x 19-inch B&M cooler helps it deal with all the extra power. After that a 3.8:1 Atlas II gets abused and sends it on down the line through some 1410-jointed driveshafts held in by U-bolts rather than straps. The front axle is a high-pinion Ford Dana 60 packed with a Detroit Locker, while out back is a 14-bolt stuffed with an ARB Air Locker. Front brakes are the factory dual-piston Ford brakes while the discs from the Ram 2500 were adapted to work on the 14-bolt by Mercenary Offroad. Both axles have 4.56 gears and thick steel fabricated covers keep those gears safe. The big 39-inch Krawlers are wrapped around eight-lug Huchinson beadlocks.
Power to push those discs comes from the Dodge master cylinder and the Saginaw steering box is helped along by a hydraulic-assist ram from Lee Power Steering. The monster single exhaust runs inside the passenger side of the frame to just past the back seat, where it goes under the frame in front of the rear axle and above the leaf spring. From there it is tucked tight to the frame and dumps through the rear lower quarter-panel.
Body and Interior
The blue paint and faux wood grain are all factory original as is most of the interior, including the seats that have some custom 3M patches. The rear of the rear inner fenderwells were cut and moved to provide clearance for the tires but the factory fuel filler neck was reused and adapted to the new tank. Also, the outside of the fenders were trimmed almost up to the wood paneling. Up front the fenders were similarly trimmed right up to that “wood” and a lip welded back on to keep from cutting the tires.
The grille shell was modified to accept the bigger radiator and the intercooler sits between the plastic chrome grille cover and the grill shell itself—so if you had a FSJ with an older nose and wanted to do this, you’d have to get more creative with the intercooler, or just not run one. Both the radiator and the intercooler are hung from custom steel dimple-die brackets that add a finished look when you first open the hood. The tranny cooler is mounted out in front of the intercooler, and it is all well hidden once that plastic grille is on.
Under the load bed near the driver-side framerail you can find an ARB air compressor and a water separator/filter for the diesel fuel to keep the injectors happy. The factory heater assembly was tossed for turbo clearance and a custom aluminum plate fabricated to block off the holes. The factory column shift automatic transmission shifter was adapted to the Dodge transmission with an adjustable heim-jointed linkage and custom body-side brackets.
Good, Bad, and What It’s For
In person, sure you notice the tires, but this thing just isn’t that tall, and it is the furthest from outlandish you can find. But, it still works great off-road. One of the best diesel engines ever was crammed under the hood, but until it fires up you’d have no clue. There are no stacks, no huge RBP stickers, nor anything similar on the Jeep. Many won’t agree with us, but the more you modify stuff from its factory form, the more likely you are to have problems with it. Aside from minor mods, the engine and transmission are stock, as is the aftermarket T-case. This should make for an awesome and reliable wheeler that gets mid-to-high 20 mpg numbers on the road, if you could ever stay out of the fun pedal.
Vehicle: 1984 Jeep Grand Wagoneer
Engine: ’98.5 24-valve Cummins inline-six
Transfer Case: 3.8:1 Atlas II
Suspension: Custom Atlas springs in spring-under configuration (front and rear)
Axles: Dana 60 (front); 14-bolt (rear)
Wheels: 17 x 9 Hutchinson beadlocks
Tires: 39x13.5/17 BFG Krawler
Curb Weight: 6,717 lbs
Track Width: 831⁄2 inches (front); 82 inches (rear)
Overall Height: 76 inches to roof; 78 1⁄2 inches with roof rack
Wheelbase: 112 1⁄2 inches
Axle Clearance: 11 3⁄4 inches (front); 10 3⁄4 inches (rear)
Belly Pan Clearance: 18 1⁄4 inches
Rocker Panel Clearance: 221⁄4 inches
Built For: Wheeling, squeezing a big engine in a smaller rig.
Why I Wrote This Feature
It is the Jeep’s equivalent of a wolf in sheep’s clothing. From the largely stock interior, to the stock paint, and even the faded fake wood trim, it doesn’t look like all that much. All the cool stuff is done under the skin where only true gearheads would be able to appreciate it. Unlike some of the rigs we feature that draw freaks likes moths to the flame, this will only draw those in the know making it possible to drive this Jeep every day, if one was so inclined, and I know I sure would be.