To Smoke A Muscle Car…L5P Duramax Edition

• January 23, 2021
• Story By
  Mike McGlothlin

It might still displace the same old 6.6 liters, but the L5P Duramax represents the most radical departure from all other renditions of GM’s diesel V-8 that came before it—and nearly all of it benefits the horsepower junkie. First, a higher-flowing Denso HP4 high-pressure fuel pump outperforms the previous power plant’s Bosch CP4.2. Second, the potent HP4 is supported by an electric in-tank lift pump, the first lift pump GM has ever included on its Duramax-powered HD’s. Third, the heads not only provide more airflow than ever before, but equal airflow to each cylinder. Fourth, stronger pistons reside at the top of the rotating assembly (and within a heat-treated block) and finally, an electrically controlled and actuated variable geometry turbo from BorgWarner makes for ultra-quick spool up and big torque down low.

In bone-stock form, a ’17-’19 L5P Duramax-equipped GM sends 400 hp to the rear wheels, along with 850 lb-ft of torque. That’s not a bad start! Tack on another 200 ponies with some keyword wizardry and poof, your late-model work truck can squeeze into the 12’s. For more on the impressiveness of a tune-only L5P, keep scrolling. This could be you!

A Different Kind of ECM Infiltration

To say GM’s E41 ECM is on lockdown from the factory would be an understatement. It took the aftermarket two years to infiltrate it and, to date, the only company that provides the means of recalibrating the L5P ECM is HP Tuners. You either purchase a modified ECM from HP Tuners or send in your original ECM to be exchanged with a modified version. From there, you have free reign to tune the ECM however you wish.

Emissions-Friendly Tuning (550+ HP)

Thanks to the revised heads, Denso common-rail injection system and state-of-the-art VGT, a ’17-newer L5P Duramax is capable of producing the most tune-only horsepower the industry has seen since the compound turbocharged 6.4L Power Stroke was around. Better yet, with the EGR, DPF and SCR system still intact, DuramaxTuner.com has shown that an emissions-friendly 550 to 570rwhp can be made. However, unlocking the most tune-only horsepower in Duramax history comes at a cost. By the time you pay to unlock the ECM and then reflash it with a performance file, you’ll have $3,500 to $4,000 invested.

Off-Road Tuning (600+ HP)

Competition-only tuning for the L5P can push a ’17-newer HD to 600rwhp and 1,300 lb-ft of torque. The price of admission is similar in that you’ll have to fork over $3,500 to $4,000 for tuning alone, but then, you’ll have at least that much (and probably more) invested in an ’01-’16 Duramax to get to the same power level. That’s pretty good for a factory turbocharger with a 61mm compressor wheel inducer, hardly the largest offered in the Duramax lineage.

Turbo Intake Horn

Similar to the intake restrictions found on previous Duramax engines, the L5P’s factory turbo intake horn chokes airflow, bottlenecking things down to just 2.4-inches in diameter. Chassis dyno testing has shown that replacing the factory unit with a 3.5-inch diameter version (like the one from WCFab shown here) can yield a 25hp gain. A direct, drop-in replacement piece, it’s designed to integrate perfectly with the stock turbo, accommodates the factory PCV line and retails for $219 to $239.

Transmission Tuning

To optimize the shift points of the Allison 1000, transmission control module (TCM) tuning is highly recommended, especially if the transmission is stock. Not only can an improved shift schedule, lockup strategy and operating pressure help get the most out of your 600rwhp ECM tuning, but it may also help preserve the life of a factory Allison. Like the L5P’s ECM, TCM tuning works on an exchange program where cores are needed. You’ll either have to send in your truck’s TCM first or buy a tuned TCM and send yours in once you receive the modified one.

The Inevitable Allison Build

Like all previous generations of the Allison-equipped Duramax, the commercial-grade automatic will still need to be beefed up in order to cope with fast-shifting and big torque in the mix. Once again, the Allison will provide hundreds of thousands of trouble-free miles at the factory horsepower and torque level, but once you give it an additional 200-plus horsepower and 500 lb-ft to deal with, you venture into no man’s land, where it may live a while or only last a week. Enjoying your power reliably is the name of the game with any high-horsepower Duramax/Allison combination.

12’s

Given the average weight of a typical crew cab, short bed 4x4 Chevrolet Silverado or GMC Sierra HD, 600 to 625rwhp allows you to inflict plenty of pain on the street-car crowd at the track. A tuned ’17-’19 L5P with a built transmission and four-wheel drive can pull off high 12-second E.T.’s with regularity. Lighter weight, regular cab trucks are capable of running 12.50s or quicker with a sound, boosted 4x4 launch.

Buy Tie-Rod Sleeves

Just like the LML that came before it (and the ’01-’10s that came prior to that), you’re going to need tie-rod sleeves to keep stress off of the tie-rods and tie-rod ends during hard four-wheel drive launches. Tie-rod sleeves will eliminate the infamous front toe-in issues GM trucks face in four-wheel drive. They install by threading over the factory tie-rods, effectively increasing the diameter (and strength) of the tie-rod itself. Of course, any higher horsepower diesel truck will experience rear axle wrap with substantially more torque in the mix. It behooves you to install a quality set of traction bars before you get too carried away at the track.