All modern diesels are beasts, but the L5P Duramax in GM’s ’17-present heavy-duty trucks is exceptional. From the factory, the latest 6.6L diesel V8 produces 445hp and 910 lb-ft of torque at the crank. And thanks to the efficiency of the Allison transmission it’s mated to, most chassis dyno tests reveal that 400 hp and 850 lb-ft successfully makes it to the ground right off the showroom floor. As you can imagine, 400rwhp is an amazing starting point for any modern pickup, but (predictably) that’s not enough for the gear heads amongst us. Once you break into the ECM and reflash it, the 550rwhp (or more) that’s on tap will eventually take its toll on the Allison. But even after you’ve beefed up the Allison, you’re not out of the danger zone. At 550 to 600rwhp, the L5P’s factory turbocharger is begging for mercy and needs to be upgraded—for both turbo and engine reliability going forward. We’ll talk about all of that and more below.
Roadblock #1: Factory Allison Transmission
With each new version of the Duramax, the Allison is improved at the OE level in order to deal with the always-increasing horsepower and torque levels, so it goes without saying that the six-speed version behind the ’17-’19 trucks is tough. Unfortunately, under most conditions the stock Allison operates at a lax 80-psi of line pressure—not good for holding power. While line pressure spikes to 220-230 psi during shifts, most of the time all the Allison sees is 80-psi.
Transmission Solution #1: EPC Solenoid Fooler
Turns out, addressing the Allison’s lack of line pressure can help it hold up to higher horsepower. You can ramp up line pressure by opting for transmission control module (TCM) tuning when you reflash the ECM or for $50 to $60 you can install an EPC solenoid fooler plug. The simple, aftermarket piece installs in place of the factory EPC solenoid and provides a constant 230 psi of line pressure (the fooler offered by Ryan’s Diesel Service is pictured above). The higher pressure will help the factory clutches hold up much longer with added power in the mix than they otherwise would.
Transmission Solution #2: Performance Rebuild
As far as basic and budget-friendly transmission-saving mods go, TCM tuning and the EPC solenoid fooler can only take you so far if you’re dead-set on sending more than 500rwhp through the stock Allison. Like every generation of this transmission that came before it, the factory Allison can’t handle everything the factory injectors, high-pressure fuel pump and turbocharger can throw at it. If you want ultimate peace of mind, you’ll have to pull it, tear it apart and rebuild it with stronger, more efficient components. Expect to pay $4,200 to $5,000 for a properly-prepped Allison capable of handling high horsepower for the long-term.
Roadblock #2: Tuning the ECM
While 550rwhp or more is possible on the stock turbo, injectors, lift pump and high-pressure fuel pump, HP Tuners offers the only way to infiltrate the factory ECM (which comes from the factory in full lockdown/tamper-proof trim). HP Tuners’ software affords custom calibrators the ability to flash their own custom files into the ECM—and many tuners are proficient enough to squeeze in excess of 550rwhp and 1,200 lb-ft out of the L5P platform. Custom tuning is the only way to get this kind of power, but it inevitably forces the next weak link to surface: the turbo.
Budget Power-Adder
If you’re on a tighter budget and can’t fork over the $5,000 for a built transmission or the $2,500-plus for custom tuning, the Derringer Tuner from Banks Power is the best option on the market. Independent tests have proven the Derringer capable of adding 94rwhp and 141 lb-ft of torque and it’s completely plug-and-play, meaning you can remove it for warranty work or service visits to the dealership (and we’ll also note that it leaves no fingerprint in the ECM). The Derringer is also 100-percent emissions-compliant and arguably the safest tuning you can run on a daily driven L5P Duramax, boasting a 19:1 air/fuel mixture for less soot accumulation in the DPF and better fuel economy.
Roadblock #3: Factory Turbo
At 550rwhp, the factory 61mm BorgWarner variable geometry turbo is way out of its map, drive pressure is creeping toward the alarming 2:1 (drive to boost) ratio we all fear and EGT is hot—hot enough to melt pistons. Though an all-out tuning effort from a custom calibration can force 550 to 570rwhp through the stock turbo, there is no guarantee how long it will last, especially if it’s seeing the aforementioned pressure differential and 130,000 rpm worth of shaft speed on a regular basis. If you’re going with the “big tune” because you’ve built the Allison to handle it, it’s time to also think about a turbo upgrade sooner rather than later.
Fixed Geometry BorgWarner
To cool off the engine, introduce even more horsepower and last longer while doing it, switching to a fixed geometry, S300 or S400 frame BorgWarner turbocharger is a sound path for improved performance. One company that arguably has the most R&D in different turbo systems for the L5P Duramax is Wehrli Custom Fabrication. Its S300 turbo install kit comes with a ceramic coated billet T4 adapter and mounting hardware, a freer flowing (and heat-wrapped) 3.5-inch diameter downpipe, 3.5-inch turbo intake horn and your choice of an SX-E series turbo (be it BorgWarner’s S364.5 SX-E, S369 SX-E or S372 SX-E). When properly spec’d, fixed geometry turbochargers typically last longer (due to their simplicity) than VGT’s and are easier on the engine thanks to the lower drive pressures they produce.
The Stock Injection Pump Can Support 770rwhp!
Speaking of the folks at Wehrli Custom Fabrication, not only are they building fixed geometry turbo systems for the L5P, they’re campaigning the quickest and most powerful version in the country right now. The company shop truck, an ’18 Silverado 3500HD standard cab 4x4, has made 766rwhp on the chassis dyno and run a very clean 11.87 at 116 mph in the quarter-mile. The Duramax benefits from a prototype S400 turbo kit with an S472 SX-E, 100-percent larger injectors from Exergy Performance and believe it or not, the factory lift pump and the stock HP4 Denso high-pressure fuel pump.
Things weren’t all rosy for the previous generation of the Duramax, either. Discover the performance roadblocks you’ll come across with the ’11-‘16 LML right here.