If you're hunting for the best l5p upgrades to finally wake up your 2017–2023 Duramax, you already know these trucks are solid from the factory but definitely have some untapped potential. The L5P engine brought a lot of improvements over the older LML, like a stronger block and better heads, but GM still left a lot on the table when it comes to throttle response and pure pulling power. Whether you're looking to tow a heavy fifth-wheel without breaking a sweat or you just want that "throw you back in the seat" feeling, there are a few specific parts that actually make a difference.
Starting with Airflow and Intake
Most people start their build with a cold air intake because it's a relatively cheap and easy afternoon project. While the stock airbox on the L5P isn't terrible, it's designed more for quiet operation than maximum flow. If you're planning on adding more fuel or a tune later, you're going to need more air to keep those EGTs (exhaust gas temperatures) in check.
An S&B or Banks Ram-Air intake is usually the go-to here. These systems open up the airway and let the turbo breathe. You'll notice a little more turbo whistle—which let's be honest, we all want—but more importantly, it smooths out the power delivery. It's not going to give you 50 horsepower on its own, but it lays the foundation for every other modification you do down the road.
The Tuning Hurdle and ECM Unlocking
We can't talk about the best l5p upgrades without addressing the elephant in the room: the locked ECM. Unlike older Duramax engines where you could just plug in a tuner and go, the L5P's engine control module is locked down tighter than a drum. To actually tune these trucks, you usually have to send your ECM out to be unlocked or buy a pre-unlocked one from a company like HP Tuners.
It's a bit of a hurdle and a chunk of change, but it's the single biggest "night and day" difference you can make. A good clean tune can add anywhere from 50 to 150+ horsepower while actually improving your fuel economy if you can keep your foot out of it. It also fixes that annoying dead pedal feeling where you hit the gas and wait a full second for the truck to decide it's time to move.
Why You Need a Lift Pump
One of the weirdest things about the L5P from the factory is that it doesn't have a lift pump in the tank. It relies on the high-pressure injection pump (the Denso HP4) to suck fuel all the way from the tank to the engine. While the HP4 is a great pump and way more reliable than the old CP4, asking it to do all that work creates a lot of stress and can lead to fuel starvation when you're pushing the truck hard.
Installing a FASS or AirDog lift pump is one of those "insurance" upgrades. It sits on the frame rail and pushes a steady, pressurized flow of clean, air-free fuel to the engine. Not only does this take the load off your expensive injection pump, but it also adds extra filtration. Modern diesel injectors have tolerances thinner than a human hair, so keeping dirt and water out of them is the best way to avoid a $5,000 repair bill.
Better Fuel Filtration
Speaking of fuel, these aftermarket lift pumps use filters that are much easier to find and cheaper to replace than the OEM ones. Plus, they usually have a water separator that's much more effective. If you live in a cold climate or buy fuel from questionable stations, this is easily one of the best l5p upgrades for long-term reliability.
Fixing the Exhaust Bottleneck
If you're keeping the emissions equipment intact, you're a bit limited on what you can do with the exhaust, but there are still ways to improve flow. The factory downpipe on the L5P is notoriously restrictive. It's got some ugly bends and collapses that create backpressure right at the turbo outlet.
Swapping in a high-flow downpipe helps the turbo spool up faster and helps drop those EGTs when you're hauling a heavy load up a grade. It's a tight squeeze to install, but your turbo will thank you for it. If you want to go a step further, look into a "stealth" or "screamer" turbo. These are drop-in replacements that look stock on the outside but have upgraded internals to move way more air without requiring a bunch of custom fabrication.
Strengthening the Transmission
The Allison transmission behind the L5P is legendary, but it isn't bulletproof—especially once you start adding 100+ horsepower with a tune. For the 2017-2019 trucks with the 6-speed, you really have to be careful about "limping" the transmission if you get too aggressive.
One of the best l5p upgrades for the drivetrain is a TCM (Transmission Control Module) tune. This increases the line pressure inside the trans, which helps the clutches hold tighter and prevents slipping. It also changes the shift points so the truck isn't constantly hunting for gears. If you have the 2020+ models with the 10-speed Allison, tuning is still an option and makes the truck feel much more athletic and less like a heavy work horse.
The MAP Sensor Spacer
This is a small, cheap mod that most people overlook, but it's actually a genius fix. Because of the way the EGR (Exhaust Gas Recirculation) works on these trucks, the MAP sensor—which measures boost pressure—tends to get caked in oily soot. When that sensor gets covered in gunk, it sends bad data to the computer, and your performance suffers.
A MAP sensor spacer moves the sensor out of the direct path of that soot. It's a 10-minute install and costs very little, but it keeps your engine running the way it's supposed to for much longer. It's probably the best "bang for your buck" reliability mod on the market for an L5P.
Upgrading the Intercooler Pipes
The factory intercooler pipes on the L5P are made of plastic and use quick-connect clips that are prone to leaking or even blowing off under high boost. If you've ever had a boot blow off while towing, you know it sounds like a gunshot and leaves you stranded with zero power.
Swapping these out for heavy-duty silicone boots and stainless steel pipes is a smart move. Not only do they look a lot better under the hood, but they also eliminate the risk of a boost leak. Most aftermarket kits are larger in diameter than the stock ones, too, which further helps with that goal of getting as much air into the engine as possible.
Handling the Heat
If you do a lot of heavy towing in the summer, you might also want to look at a larger aftermarket intercooler. The L5P runs hot by nature, and keeping those intake air temperatures down is key to maintaining power. When the computer sees the air getting too hot, it will pull back the fuel to protect the engine, which means you lose power right when you need it most on a steep climb.
Conclusion
Building the perfect Duramax doesn't mean you have to do everything at once. If you're just starting out, grab an intake and a MAP sensor spacer to get things moving. When you're ready for the big gains, look into the ECM unlocking and a lift pump. The best l5p upgrades are the ones that balance performance with longevity. These trucks are expensive, and while we all want more speed, keeping the engine healthy so it can hit 300,000 miles is the real goal. Focus on the airflow and fuel delivery first, and the rest of the build will fall right into place.