LML Duramax Modifications: Building a Rocket Ship

Imagine driving a brand new Duramax with twice the factory horsepower. Obviously, your rig wouldn’t roll out of GM like that, but that doesn’t mean you couldn’t build it yourself. Thanks to a host of high quality, well-refined products being offered throughout the diesel aftermarket, it’s not hard to get a 2011-2015 LML Duramax-powered truck up to speed in a hurry. We recently sat in on a start-to-finish build at Randall’s Performance in Gladstone, Illinois, where a ’15 GMC Sierra Denali 2500 HD was fitted with some of most reputable power adders in the industry.

The weeklong build took the bone-stock, 3,500-mile GMC from 350 hp at the wheels to a tire shredding 700-plus. Key upgrades included: a compound turbocharger arrangement, CP3 conversion, injector up sizing, lift pump, and a built Allison transmission. Along the way—and despite more than doubling the truck’s original power output—crucial characteristics such as longevity, reliability, and drivability were in no way compromised. If you’re looking to turn your late model Duramax into the perfect truck, here’s some inspiration. DW

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1. For some baseline horsepower and torque numbers, the Denali was run on a SuperFlow chassis dyno at Randall’s Performance. At 349 hp and 656 lb-ft. of torque, this is what a typical dyno graph looks like for a bone-stock ’11 to ’15 LML Duramax.

2. To ensure the six-speed Allison would survive the stress of sending 700 hp to the pavement on a regular basis, the case was emptied of its contents and refitted with a GT6 overhaul kit from Goerend Transmission. This all-inclusive kit included Raybestos clutches, steels, frictions, RatioTek valvebody kit, TransGo shift kit and pump kit, PTO covers for added C-3 clutch pack cooling, and a host of OE components such as select apply plates, bearings, and thrust washers.

3. Engine-to-transmission power transfer was left in the hands of Goerend Transmission as well, which supplied a triple disc torque converter with a billet front cover. This top-of-the-line converter came with an 1,800-rpm stall speed and a billet stator (which is guaranteed to never break) to handle boosted launches and drag racing.

4. An added insurance measure offered by Goerend Transmission for the Allison is its cast-aluminum PTO covers. The PTO covers feature internal fins that route lube oil directly to the C-3 clutches. This provides added cooling to what is arguably the Allison’s hardest working (and most failure prone) clutch pack.

5. The final touch on the Allison transmission was this high capacity pan from Goerend. The lightweight, CNC-machined cast-aluminum pan came with a new internal filter, billet-aluminum filter lock, magnetic drain plug, and holds 3 quarts of additional ATF over stock.

6. Further bulletproofing entailed the addition of a Merchant Automotive transfer case brace. Made from 7/8-inch diameter, 0.120-inch wall DOM tubing and utilizing ¼-inch thick plating, this brace keeps the transmission’s rear housing from cracking due to driveline vibration (such as a failing U-joint or bent rear drive shaft).

7. Prior to reinstalling the transmission, the guys at Randall’s bolted these high flow up-pipes to the factory exhaust manifolds. Made by Wehrli Custom Fabrication, the symmetrical pipes measure 2-inches in diameter, feature smooth bends, and are made from 11-gauge stainless steel. The primary reason for upgrading to the 2-inch up-pipes is due to them having stronger bellows (the OE bellows can blow out) and the fact that they help decrease exhaust gas temps.

8. With a reputation for offering some of the best compound turbo kits in the industry, Wehrli Custom Fabrication’s S400 over stock twin-turbo system was the only choice for this build. Based on the ease of the installation process, the quality and craftsmanship of the parts included, and the overall performance gain, the techs at Randall’s Performance swear by these kits and install them religiously on 2001-2016 Duramax mills. To our knowledge, this is the only Duramax twin kit on the market with a one-piece cold side intercooler pipe between the turbos. This adds rigidity to the entire system and keeps the atmosphere turbo from bouncing (which can lead to broken piping, mounting brackets, or boost leaks). As standard equipment, both the downpipe and hot-pipe are coated in high-temperature black.

9. While a BorgWarner S475 turbo (with a cast compressor wheel) comes standard on the Wehrli Custom Fabrication kits, this truck’s owner opted to upgrade to a billet wheeled 80mm version. Even though we’re not upgrading the stock Garrett VVT turbo in the valley, this billet S480 will provide enough airflow up top to easily clear 700 hp. The key benefits of retaining the stock VVT turbo are: lightning quick spool up and great towing performance, not to mention that it keeps the overall cost of the twin kit down.

10. The billet S480 also features a T6 turbine inlet flange, 1.32 A/R exhaust housing, and the proven 96mm turbine wheel. The compressor cover was powder coated Sparkle Electric Blue to match the 3-inch intercooler plumbing and Y-bridge.

11. Opening up airflow into the engine is a fully fabricated 3-inch Y-bridge, which comes standard in the Wehrli Custom Fabrication twin kit. For added strength over its competitor’s products, a machined aluminum bead roll is welded onto the end of the Y-bridge. This ensures the pipe doesn’t crush or oval (and wind up leaking) after tightening the T-bolt clamp.

12. The issue with the LML Duramax having piezoelectric injectors is while the stock injectors have the potential to make as much as 650 hp with the right amount of airflow, the Bosch CP4.2 can’t supply enough rail pressure to make 500 hp. This is why Randall’s replaced it with this 10mm stroker CP3 from Exergy Performance. When used as a single pump (as it is in this build) Exergy’s 10mm CP3 can support in excess of 900 hp on LML Duramax applications.

13. To bolt a CP3 in place of the factory CP4.2, a CP3 conversion kit had to be employed. This system from Wehrli Custom Fabrication comes with everything needed to perform the swap, including a pump flange plate, new high-pressure fuel line, fittings, and all necessary hardware. Once the CP4.2 is removed, the supplied flange can be installed on the CP3, followed by the factory drive gear.

14. In terms of flow, the Exergy 10mm CP3 (right) vastly outperforms the Bosch CP4.2 (left). At a test speed of 3,000 rpm and 180 MPa (26,000 psi) of rail pressure, the 10mm Exergy CP3 flows 54 percent more fuel than the stock CP4.2. And because Exergy’s pumps don’t fall off after 3,000 rpm like the factory CP4.2 does, the Exergy pump flows an impressive 72 percent more volume at 3,500 rpm.

15. With the pump flange plate’s O-ring oiled, the 10mm CP3 was installed exactly where the CP4.2 used to reside (at the front of the lifter valley, mounted to the block). Once installed, the guys at Randall’s started connecting the factory fuel supply and return lines that would be reused.

16. Supplying fuel pressure to the Exergy Performance 10mm CP3 was left to a Velocity 200 system from Fuelab. However, due to being preset at a higher pressure (18 psi) the guys at Randall’s opted to run a system intended for ’98.5-to-present Cummins applications (PN 30303) over the Duramax version, which comes preset at 8 psi. This was done as an added safeguard for the Exergy 10mm CP3.

17. Because the truck had just 3,500 miles on it, the factory piezoelectric injectors were essentially brand new—making them good candidates to receive modifications. So, they were pulled and shipped to Exergy Performance to be modified. And thanks to Exergy’s precise quality control and state-of-the-art test facility, the injectors were balanced within 2 percent of each other before being returned to us.