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4BT Cummins 1984 GMC Jimmy: Mechanical Engineering

A fullsize Jimmy that’s redesigned for the trail

Ken BrubakerPhotographer, Writer

Brian Hays of Ridgecrest, California, purchased this ’84 GMC Jimmy from a used car dealership in southeastern Washington in 2008. And it was very used. The rig had 300,000 miles on it and the 6.2L diesel engine wasn’t running. This wasn’t a problem for Hays, however. “I got it up and running with new batteries, starter, and fuel filter,” he says. Hays then wheeled the Jimmy for a while in mostly stock form until the rear limited-slip differential broke. It was this breakage that launched a major buildup. “I wheeled it around the upper high desert and Panamint Mountains until the rear 10-bolt Gov-Loc broke and stranded me 15 miles from home. I decided to replace the rear axle with a 14-bolt that I had laying around and then it was non-stop for 2½ years,” Hays says. Along the way he tore out the entire drivetrain and sold all the parts to purchase stuff like an Off Road Design Doubler and rear 64-inch Superlift leaf springs.

Hays is a mechanical engineer and he used his knowledge in the buildup of the Jimmy. An example is the front suspension, which he designed and built. Hays says, “I started with 64-inch front springs, but after I was done I did not like it and ripped it out without even trying it. The approach angle was not good. I still wanted great ride quality and have tried a lot of the other leaf spring combinations on other past trucks. At the time there was not a coilover suspension kit on the market so I decided to take the time to design it myself using the tools I use every day (CAD, kinetic analysis tools, structural analysis, and CAE analysis tools). It took about six months from start to finish after long days at work. Designing the suspension on a computer allowed me to optimize the suspension without cutting metal. Many design iterations were gone through and verified with paper templates. The final design was mocked up using poster board. The design was to be simple and self-jigging (for welding).”

But this was only the tip of the iceberg. Hays left no bolt unturned, or unanalyzed, in the buildup of the Jimmy. Among other things, he bolted in a Cummins 4BT turbodiesel engine, Doubler transfer case, Dana 60 front axle, and lots more. The result is a unique, flexy rig that earns an 801 on a 30-degree RTI ramp, has a 50-degree approach angle, a 43-degree departure angle, and gets 16-20 mpg.

Hays’ Jimmy is full of surprises, including the one under the hood. It’s an early ’90s Cummins 4BT turbodiesel engine from a Frito-Lay delivery truck and just like the rest of the Jimmy it has been customized to the owner’s preference. Its mods include a modified Bosch VE pump with 3,200 RPM governor spring; high carbon chromium stainless Denny T fuel pin; Industrial Injection Prince of Darkness injectors; larger Holset HY35 turbocharger; grid heater (from a 5.9L Cummins), and Silicon Intake Systems hoses, clamps, and intercooler. Hays TIG-welded the 3.5-inch-diameter exhaust together using pieces of old exhaust systems from late model diesel trucks. Hays estimates that the Cummins produces 200hp at 3,200 rpm and 380 lb-ft of torque at 1,850 rpm. The ‘plant is kept cool via a 6.2L turbodiesel-application aftermarket aluminum radiator with salvage yard-sourced 15-inch electric fans and an owner-built aluminum shroud. The fans are controlled by a Dakota Digital programmable controller. Other underhood mods include a modified factory wiring harness and Dual Optima YellowTop batteries.

The Details
Owner/Hometown: Brian Hays/Ridgecrest, California
Vehicle/Model: 1984 GMC Jimmy
Estimated value: $11,000
Type: Cummins 3.9L 4BT I-4
Aspiration: Direct injection, turbocharged, intercooled
Output, hp/torque @ rpm (estimated): 200@3,200/380@1,850
Transmission: TH475
Transfer case: NP203/205 Doubler
Front: Custom four-link, 2.5-in F-O-A 16-in-travel remote-reservoir coilovers, Currie Antirock sway bar, Poly Performance limiting straps
Rear: 64-in Superlift leaf springs, 2.5-in F-O-A 16-in-travel remote-reservoir shocks
Front: Dana 60, USA Alloy 35-spline axleshafts, Superior 35-spline stub shafts, Longfield 300M U-joints, Ruff Stuff diff cover, Warn Premium lockouts, ported steering box, PSC hydraulic ram, ORD high-clearance arms/ARB Air Locker
Rear: GM 14-bolt, ORD disc brake conversion, Ruff Stuff diff cover, DIY4X U-joint and yoke guard/Detroit Locker
Ring and pinion: 4.10:1
Wheels: 17x9 TrailReady beadlock w/rock ring
Tires: 37x12.50R17 Cooper Discoverer STT