Of all the engines out there, nothing matches GM’s 6.6-liter Duramax in terms of longevity. Yep, I said it, not even the Cummins. GM’s revolutionary common-rail engine was introduced way back in 2001, to replace the aging 6.5-liter diesel. The new engine had high-flowing cylinder heads, a stout bottom end, and a new common rail injection system. It was everything that the 6.2- and 6.5-liter diesels weren’t. And, it came out of the box swinging with 300 horsepower—which was a lot back in 2001. Then and now simple bolt-on Duramax upgrades are an easy way to boost power.

While other companies were changing injection platforms, displacements, injection systems, and a variety of other things, GM has stuck with its trusty 6.6 Duramax over the years. Like most diesel pickups, power has been bumped up throughout the years. In fact, the new 2023 Duramax comes in at 445 horsepower and 910 lb-ft of torque. They’ve also doubled the truck’s transmission speeds since its initial five-speed Allison offering. The new trucks even come with 10-speed transmissions. Talk about always being in the powerband.

It’s amazing how much power you can make with just a couple of modifications. These runs on an L5P-powered Chevy compare stock versus tuned with an Edge Pulsar and an aftermarket intake.

As advanced as you can make a solid platform that has lasted more than two decades, there’s always room for improvement. For more oomph, we talked to some of the best in the business at Edge, Calibrated Power, Holley, KT Performance, S&B Filters. As it turns out, the newer Duramax engines can easily make more power than stock…a lot more.

Your First Duramax Upgrades: Computer Tuning

Between Ram, GM, and Ford, the GM offerings deliver perhaps the most gains with a tune-only combination. The factory exhaust system isn’t very restrictive (even with the DPF). This allows for decent gains past its stock 445 horsepower rating. If you’re talking about flywheel numbers, then you can easily boost the engines past 600 horsepower with tuning alone. This means right at or a little more than 500rwhp, all with whistle-clean exhaust.

Just like the other computer-controlled diesels, the factory leaves a little wiggle room when it comes to upping the boost, timing, rail pressure, and injector duration. All of these different factors need to be in balance with a good tune, and the aftermarket has pretty much figured it out. Companies like Edge Products and Calibrated Power offer emissions-intact tuning that still allow for power gains up to around 150 horsepower more than stock. Oh, torque is also up, and most likely fuel economy is a little bit as well.

Air intakes can do quite a bit to help the L5P Duramax with airflow. This particular unit from S&B flows a whopping 51.83 percent more than the factory version.

Step Two: Air Intakes

Although the exhaust system on the Duramax engines is fairly efficient in stock form, the intake tract leaves a bit of room for improvement. This is especially true when the boost is turned up. Aftermarket intakes can allow 50-percent (or more) of air into the engine, which allows the turbocharger to maximize its performance. Intakes are also a modification that is easy, unobtrusive, and usually 49 or 50-state legal.

We checked out S&B Filters intake for the L5P and were impressed at how far simple intakes have come these days. Materials that won’t deteriorate over time, intake box improvements, and more than 99 percent filtration were all high points of its current design. We’ve actually seen power differences on the dyno with these setups (especially when tuning is involved) to the tune of about 20 to 30 horsepower at the rear wheels.

Calibrated Power offers a complete custom-tuned TCM for those with Allison 10-speed transmissions. They especially recommend it for those folks running larger tires, a bigger turbo, or aggressively tuned trucks.

Transmission Tuning Is A Needed Duramax Upgrade

It should come as no surprise that when turning up the engine, the transmission needs a little help as well. The Duramax has been long mated to an Allison transmission, first a five-speed, then a six-speed that lasted all the way up until the 2019 model year. If you have a 2017 to ’19 model, then you have a six-speed Allison, which isn’t necessarily a bad thing. Just as you can boost the power of the engine, you can also boost the holding capacity of the transmission, through raised line pressure and revised shift strategies.

If you have a ten-speed transmission (2020 through ’23 model years) then you can also accomplish the same thing, through a revised TCM. The big difference between these two is price, as transmission tuning through the six-speed can start out as little as $300, versus a ten-speed which can run upwards of $1,400. Either way, transmission tuning can be crucial to a long transmission life, especially if you’re going to hot rod your diesel.

We talked to the folks at Calibrated Power again about its drop-in turbo selection. You can get everything from a lightly modified stocker for $1,800, all the way to a 67mm thumper that’s designed for 700 horsepower.

High-End Duramax Upgrades: Drop-In Turbochargers

One part that pushes the confines of our Duramax upgrades budget a bit is a drop-in turbocharger. Calibrated Power offers the Stealth line of turbochargers. These are capable of 41psi of boost and more than 600rwhp—all out of a stock engine with factory emissions equipment. These turbos feature higher-flowing compressor wheels that are capable of moving a lot more air than stockers…up to 20 percent more.

The other cool thing about an upgraded turbo is that it’s a passive modification that’s always working. Even on a truck with factory programming, an upgraded drop-in will outperform a bone stock turbo. “Our most basic drop-in turbo still performs especially well in the mid-range of the RPM band, and while towing,” notes Calibrated Power. Expect a lower-level upgrade to run about $1,800, and the price can go up from there to $3,000 for turbos capable of 700 horsepower. It all depends on where your budget lands you.

As farfetched as it sounds, nitrous is a perfectly legal modification to your truck. This NOS system is designed specifically for diesels and is adjustable from 60 to 150 horsepower.

Nitrous Oxide-An Out-Of-The-Box Duramax Upgrade

Not traditionally on our list of Duramax upgrades, we’re going to throw one in here out of left field for you guys. Did you know nitrous oxide is technically legal to have installed on your vehicle? No, you can’t use it on the street, but we’re told you can have it installed as long as the bottle’s valve isn’t open.

When you make your way to the dyno or the track, that’s when the fun stuff happens. We’ve seen nitrous make a 40rwhp improvement on even a stock truck with stock programming. But with a tune and nitrous, we wouldn’t be surprised to see a 100 to 150 horsepower gain. With that, you’re talking about a 650 to 700 horsepower, emissions-legal truck, which is astounding, but not necessarily reliable. Especially with a tune, we wouldn’t be surprised to see the transmission make an early exit. Unfortunately, a rebuilt transmission will cost about as much as everything else combined.

Wrapping it Up: Too Much Power?

The newest generation of the Duramax engine is still one of the most advanced powerplants out there. This, even though it was designed decades ago. A lot of people like to think that emissions regulations keep their trucks from “making power”. Fortunately, this simply isn’t the case. At nearly 700rwhp, our theoretical “kitchen sink” list of Duramax upgrades will help your truck make enough power to slip the transmission. If that doesn’t happen, you could, perhaps, even require a built bottom end. Whatever the case, the Duramax/GM platform was, and will continue to be an excellent starting point to build everything from a tow monster to a real screamer.