1991 GMC Suburban: Doomsday Diesel Part 6 Photo Gallery
37-Inch Tires, Brutally Simple Suspension, and a Dozen Other Parts Built To Last Forever
David Kennedy –
Oct 1, 2012
Photo 1/22 | doomsday Diesel Part 6 1991 Gmc Suburban | Doomsday Diesel is a ’91 GMC Suburban 2500 that began life with a gas engine. Now it has a ’95 12-valve Cummins built by Industrial Injection between the framerails.
Photo 2/22 | doomsday Diesel Part 6 1991 Gmc Suburban | Doomsday Diesel is a ’91 GMC Suburban 2500 that began life with a gas engine. Now it has a ’95 12-valve Cummins built by Industrial Injection between the framerails.
Photo 3/22 | doomsday Diesel Part 6 cummins Engine And Tci Transmission | Mercenary Offroad’s Todd Farrand pulled the engine and TCI 6X (based on a GM 4L80E) so he could modify the pan.
Photo 4/22 | doomsday Diesel Part 6 brake Booster | We planned to keep the Suburban’s original vacuum brake booster and master cylinder, but in order to get the most out of our new four-wheel EBC disc brakes, and to make sure the injection pump won’t hit the master cylinder (arrow), we’re ditching it in favor of a Hydroboost system.
Photo 5/22 | doomsday Diesel Part 6 front Differential Hitting Oil Pan | With the Cummins swapped in and a Dynatrac ProRock Dana 60 front axle in place, we found the oil pan and pinion yoke could connect when the suspension is twisted up (arrow).
Photo 6/22 | doomsday Diesel Part 6 trimming Oli Pan | Farrand then cut the oil pan open and painstakingly trimmed the pan back to gain as much front driveshaft clearance as possible. He took care not to encroach on the Cummins’ oil pickup tube and made sure to clean the pan of all grinding dust before test fitting it to the engine.
Photo 7/22 | doomsday Diesel Part 6 welding Oil Pan | When he was happy with the clearance gained, Farrand TIG-welded the pan closed using a piece of sheetmetal he fit in place. The welds were then tested for leaks by filling the pan with water.
Photo 8/22 | doomsday Diesel Part 6 borgwarner 4470 Transfer Case | We wanted to use a ’99 to ’10 Super Duty NV 271 transfer case, but it wouldn’t fit between the Sub’s framerails. We opted for this BorgWarner 4470 (used in ’96 to ’00 1-ton GM chassis-cab trucks) that Rockland Standard Gear built us. Rockland has experience with all kinds of 4x4s, from snowplow trucks to off-road race trucks, and it deemed its Tranzillia gearbox up to the task. We like the BW 4470 because it offers a 1410 fixed-yoke rear output, PTO capability, and pressurized lubrication.
Photo 9/22 | doomsday Diesel Part 6 transmission And Transfer Case Installed | To mate the TCI 6X 4L80E to the BW 4470, we used the cast-iron GM transmission adapter that came with our Suburban and an Energy Suspension urethane mount. This truck was built with a passenger-side transfer case front output, but bolting the driver-side drop 4470 in place gained exhaust and driveshaft clearance. All we had to do was flip the factory crossmember around and drill a new hole in the truck’s frame.
Photo 10/22 | doomsday Diesel Part 6 body Lift And Bushings | The power take-off (PTO) feature adds about 6 inches to the length of the transfer case, which caused the BW 4470 to contact the floor of the Suburban. To make more room, we added Offroad Design’s complete 1-inch body lift and a new set of Energy Suspension urethane body mounts. The urethane mounts are Energy Suspension’s trademark red, but we took Offroad Design’s billet-aluminum body lift blocks and had Prime Plating hard anodize them to prevent wear and corrosion. The hard anodizing process turns most machined aluminum a dark gray color.
Photo 11/22 | doomsday Diesel Part 6 body Lift And Bushing Installed | By replacing the body mounts with new urethane pieces, we’ll be able to align the doors and fenders with their original specs. The Offroad Design 1-inch body lift on top of the urethane will give the truck valuable room for the 37-inch tires we plan to run.
Photo 12/22 | doomsday Diesel Part 6 leaf Springs | Tuning the vehicle’s ride height is going to be tricky until we finish assembling Doomsday Diesel. We began with the factory two leaf springs (front), but they couldn’t handle the extra weight of the Cummins, and the suspension bottomed out just sitting in the shop. Next, we ordered a set of Rancho’s 4-inch-lift, five-leaf, RS 86206 springs (rear) with a spring rate of 560 pounds per inch (ppi). We also tracked down a set of discontinued Rancho RS 40054S springs (middle)—which has 672 ppi—for sale on Amazon.com in case the front end gets too heavy.
Photo 13/22 | doomsday Diesel Part 6 leaf Spring Shackle Mounts | The leaf-spring suspension is a proven technology that’s been around since the horse-drawn wagon. While the springs themselves rarely ever fail, the bushings that attach them to the vehicle take a beating. Fortunately for ’73 to ’91 GM truck owners, Offroad Design has removed these weak links from the truck with its greasable urethane spring bushing kit. The biggest increase in durability comes from its heavy-duty Front Upper Shackle Hanger kit. These new pivot points replace the factory 1 1⁄8-inch frame pivot bushings (bottom) with massive 1 1/2-inch pieces (top)— virtually ending front bushing wear.
Photo 14/22 | doomsday Diesel Part 6 drilling For Upper Shackle Mount | Installing Offroad Design’s Front Upper Shackle Hanger kit required us to drill out some rivets in the frame and then use a hole saw to enlarge the openings.
Photo 15/22 | doomsday Diesel Part 6 upper Shackle Installed | The Offroad Design Front Upper Shackle Hanger kit was then bolted in using Grade 8 hardware. We also upgraded the front with Offroad Design’s heavy-duty zinc-plated shackles to replace the thin stamped-steel factory versions that can bend from extreme use.
Photo 16/22 | doomsday Diesel Part 6 rancho Rs 9000 Xl Shocks Installed | To dampen the 520-ppi-spring-rate, 4-inch-lift Rancho leafs, we stepped up to dual Rancho RS 9000 XL adjustable shocks using upper Quad Shock mounts from LMC Truck (arrow). Using dual adjustable shocks will let us tune the ride to be soft—while still having enough capacity to prevent the shocks from fading. To compensate for the suspension lift, we fitted Offroad Design’s extended brake lines to the front of the truck. These DOT-brake lines feature a Teflon inner core wrapped in a layer of Kevlar braid, a layer of stainless braid, and then the protective outer vinyl shield.
Photo 17/22 | doomsday Diesel Part 6 front Drag Link Installed | Changing the suspension’s ride height meant we’d also have to correct the Suburban’s steering geometry. Offroad Design’s crossover steering kit allowed us to use a heavy-duty draglink to connect a two-wheel-drive steering box to an ORD’s high-steer arm mounted on the passenger-side Reid Racing steering knuckle. Then a 1 1/2-inch-diameter tie rod was used to connect the two steering knuckles. Greasable tie-rod ends were used instead of rod ends for the longest possible service life.
Photo 18/22 | doomsday Diesel Part 6 steering Box Installed | Since we added a 1-inch ORD body lift, had to change the steering box, and just didn’t trust the 21-year-old factory steering shaft, we bolted in this Borgeson telescoping steel steering shaft. It features two billet-steel U-joints to provide smooth steering action and eliminates the rubber coupler used by the factory steering shaft that can deteriorate over time. From this angle, you can also see the Hydroboost brake booster (from a ’91 6.2L diesel Suburban) and the brake master cylinder (from a ’96 GMC P-30 Step Van) that we’ve tied into the Sub’s brake system using LMC Truck’s complete stainless-steel brake line kit. We’re still dialing in the brake hardware, so check back next month to see our final combination.
Photo 19/22 | doomsday Diesel Part 6 saginaw Steering Box | We had Lee Manufacturing rework our Saginaw power steering box for extreme use. Tom Lee builds these boxes to aerospace-specs, and he fitted ours with a billet upper sector shaft cover, a two-wheel-drive sector shaft (to allow us to use crossover steering), and modified the box to control a remote ram assist.
Photo 20/22 | doomsday Diesel Part 6 hutchison Beadlock | We’ve used Hutchinson two-piece, double beadlock wheels on many of the vehicles we’ve built. For this project, we’re reusing a set of 17x8 1/2-inch billet-aluminum Rock Monster wheels we’ve had since 2002. These wheels are rated for 3,850 pounds each, use steel wheel-lug inserts, and were originally powdercoated silver. For this build, we disassembled the wheels, had the powdercoating chemically stripped, and had the wheel faces polished.
Photo 21/22 | doomsday Diesel Part 6 hard Anodized Hutchison Beadlock Wheel | We then sent the wheels to Prime Plating, where they were given a Type III hard-anodized finish (mil-spec-A-8625). This finish is similar to what can be found on high-end cookware and provides an abrasion- and chemical-resistant matte-gray surface that’s literally part of the wheel.
Photo 22/22 | doomsday Diesel Part 6 bfg Baja T A Tire Installed | There’s no other tire on earth that has the survivability track record of the BFGoodrich Baja T/A. This is the tire that’s won the grueling Baja 1000 off-road race more than any other—and it’s the ultimate 37x12.50R17 zombie-crushing tool.