Part Eight: Button Up That Rear End
If you’re an avid reader of Diesel World magazine, this truck should be pretty familiar as we enter Part 8 in the build on this 2012 2500HD LML Duramax. Purchased used in November of 2017 with 117,000 miles on the clock, this basically bone-stock truck that had no idea what it was in for. GM engineers did a great job with the 2011-2014 trucks and the LML powerplant is a solid performer, with more than 370 hp in stock form. They’ll do just about anything the average truck owner could ask, but let’s be honest—we aren’t average truck owners.
We’ve upped the power a little (an extra 127 hp/217 lb-ft to be exact) with the help of the Jammer Stage 2 kit from Edge Products. We’ve improved airflow through the engine with a high-flow intercooler pipe from Deviant Race Parts, and we extended our range between fill-ups with a 57-gallon replacement fuel tank from Titan Tanks. The suspension has been upgraded with Bilstein shocks and a leveling kit from Kryptonite Products. To better fit our needs for heavy towing in the summer, a Curt Double EZ-Lock gooseneck hitch and some airless rear suspension bags from SuperSprings were installed. Mickey Thompson Baja ATZ P3 tires and 20-inch SD-5 wheels offer an aggressive look with great on/off-road performance. With plans of adding more horsepower and torque soon with a bigger injection pump and turbocharger, this month we’ll be giving the rear end a little attention to ensure it can handle and put that power to the pavement safely.
In heavy towing situations the rear axle goes through a lot of strain and usually gets neglected. While most owners are good about changing engine oil and fuel filters regularly, it seems many forget about the differential fluid and taking care of it when they should. All that torque from the engine gets pushed through the trans and sent to the rear axle, which must send the power to the tires. Under heavy load a lot of heat can be generated within the axle, and heat can kill a fluid’s ability to keep things lubricated. Inside the differential you’ve got metal-on-metal gear sets in the ring and pinion, and of course you can expect some wear to occur. Wear over time deposits fine metallic debris in the gear oil, which can affect the longevity of the differential.
An aftermarket differential cover can increase longevity with a little extra fluid capacity and also help reduce fluid temperatures under sustained load. The aFe Power rear differential cover for the GM axle allows an additional quart of gear oil and has been cast with cooling fins to help draw the heat out of the axle housing. The magnetic plug will also trap those small metal particles and keep them from being passed through the gearset. They’ve also engineered an easy-see sight glass on the side so you can monitor fluid level and condition without having to drain the diff or remove the cover.
One negative aspect to all the extra torque we’ve added since the truck was stock (with plenty more to come in the next couple installments) is the strain it creates on the drivetrain. Extra torque puts more load on the driveline and rear axle, and under heavy acceleration it will cause the rear axle to “wrap,” or rotate, under the load, throwing off the U-joints and pinion angles and potentially leading to premature failure. It also can change the rear suspension geometry enough that the tires will lose traction at the ground, creating a shudder and wheel-hop situation when towing, racing or sled pulling.
To overcome these issues, the Duramax specialists at HSP Diesel developed a true bolt-in, budget-friendly traction bar kit that will resolve the above noted issues, while looking great underneath the truck thanks to many powdercoating options. Tractions bars are exactly what the name implies: an additional bar added under the vehicle to help locate the axle and prevent it from wrapping under load, planting the tires and eliminating wheel hop and drivetrain shudder.
The rearward axle brackets will slip right into place on your factory axle U-bolt plates using all the factory hardware. The frame-side bracket will require holes drilled in the frame, which will be the most challenging part of this install. The double-walled boxed frame is tough to drill, but patience and a good drill bit goes a long way here. Since the boxed frame won’t allow a bolt to be installed easily, HSP supplies a nifty self-holding stud that will grab the frame from the inside when positioned correctly and allow the brackets to be bolted in place. The bars themselves use a rebuildable and greaseable upper joint for full range of motion, and the threaded attachment point allows the bars to be made longer or shorter by loosening the jam nut and rotating the heim one way or the other to change preload on the axle.
After an initial test drive and a couple wide-open throttle runs, we quickly noticed how much better the transmission seemed to shift as the tires and axle stayed planted and offered a more positive feel through the drivetrain. When towing, we no longer feel that slight shudder when leaving a stoplight and we’re trying to apply all that torque to get the load moving and up to speed quickly. The traction bars made a noticeable difference in driving feel and the added fluid capacity should keep the rear differential and axles happier while towing through the 100+ degree summer days.
For the next parts of the build, we’re planning to remove the cab from the frame and swap in a whole bunch of performance goodies to take this truck to an all new level of performance and drivability. We’ll also be looking into a full transmission build soon, since we’re positive the stock 124,000-mile Allison won’t endure 600+ horsepower for long. We also plan to do a few more cosmetic upgrades with some fender flares, maybe a front bumper and some new side steps. Stay tuned as we continue to take Project Looks. Muscle. Longevity from stock truck to the ultimate daily driver and weekend tow rig.