There’s something special about the L5P Duramax. It’s efficient, it’s tough, it makes serious power — and it’s one of the most capable diesel engines GM has ever built.
Found in 2017 and newer Chevrolet Silverado HD and GMC Sierra HD trucks, the L5P isn’t just a small update from earlier Duramax generations — it’s a total redesign that changed the game for diesel owners.
At FASS Motorsports in Washington, Missouri, we work on a ton of L5P trucks — and we get why people love them. They’re reliable, powerful, and respond incredibly well to upgrades.
Here’s everything you need to know about what makes the L5P Duramax so awesome.
A Quick History of the L5P Duramax
The L5P was introduced for the 2017 model year as the next evolution of the 6.6L Duramax V8. After years of incremental updates, GM decided to start fresh — and the L5P brought over 90% new components compared to the outgoing LML engine.
Right off the showroom floor, the L5P produced 445 horsepower and 910 lb-ft of torque — a massive jump over previous generations. It was paired with the legendary Allison 1000 6-speed automatic (and later a 10-speed* in 2020+ models), making for one of the smoothest and strongest diesel drivetrains on the market.
*The 10-speed offered since 2020 is not actually an Allison transmission, according to GM news sources.
What Makes the L5P Duramax So Popular
The L5P quickly earned a reputation as the most well-rounded Duramax yet — here’s why owners can’t get enough of them:
- Big Power from the Factory: The L5P makes serious torque and horsepower without breaking a sweat. You can tow, haul, or make passes at the drag strip — all in the same day.
- Rock-Solid Reliability: With its new Denso HP4 fuel system and stronger block, the L5P can handle high cylinder pressures with ease. It’s built to last.
- Refined Drivability: It’s powerful, but smooth. The L5P delivers linear throttle response and quiet operation — a huge step up from earlier generations.
- Massive Tuning Potential: Once unlocked, these engines can push well past 550+ RWHP with tuning and supporting mods.
It’s a diesel that can do it all — work hard, play harder, and still handle a long road trip in comfort.
Technical Highlights
- 6.6L V8 Engine: Cast-iron block with aluminum heads for strength and weight savings.
- Denso HP4 Injection Pump: A major upgrade over the CP4 — more reliable and efficient.
- Bigger Variable Geometry Turbo: Spools faster and delivers stronger mid-range power.
- New Pistons & Rods: Forged internals handle higher boost and pressure levels.
- Improved Cooling & EGR Design: Keeps temps down and reliability up during towing or high load.
All of this means the L5P not only runs harder than older Duramax engines — it’s also far more refined and reliable when properly maintained.
The Good and the (Few) Bad
Every diesel has its quirks, and while the L5P is arguably the most dependable Duramax to date, there are still a few things to keep in mind:
- ECM Encryption: Early on, tuning was locked down due to encrypted engine control modules (ECMs). Luckily, tuning solutions are now widely available, though often more costly than older platforms.
- Fuel Filters: Regular fuel filter changes are critical — even more so if you’re running questionable diesel fuel (we recommend upgrading to a FASS Fuel System).
- Transmission Limitations: The Allison 1000 can handle a lot, but the 10-speed introduced in 2020 has become infamous as a failure point.
Still, for most owners, these are minor trade-offs for an engine that delivers so much performance and longevity right out of the gate.
Common L5P Duramax Upgrades
Want to make your L5P even better? These are the upgrades we see most often here at FASS Motorsports — all proven to boost performance, reliability, and drivability:
- FASS Fuel System: The ultimate first upgrade. It filters diesel down to 2 microns, removes air/vapor, and protects injectors and the high-pressure pump.
- Cold Air Intakes: Improve airflow, optimize performance.
- Tuning & ECM Unlocks: Professional tuning unlocks major power potential — especially paired with supporting mods.
- Turbo & Intercooler Upgrades: For owners chasing higher power, aftermarket turbos and intercoolers improve spool and cooling.
- Suspension & Leveling Kits: Carli, BDS, and ICON kits transform ride quality and stance.
- Lighting, Wheels & Tires: Because looking good matters, too.
Even basic bolt-ons can make a noticeable difference — turning an already great truck into a powerhouse.
What They Cost in Today’s Market
The L5P’s reputation for reliability and performance keeps resale values high. As of 2025, here’s what you can expect to pay for one:
- 2017–2019 Models: $35,000–$50,000 depending on mileage and condition
- 2020+ Models: $50,000–$65,000+
- High-end trims (Denali, High Country): $60,000–$80,000+ for clean examples
With their strong build quality and tuning potential, clean L5Ps remain some of the most sought-after used diesel trucks on the market.
Why Diesel Enthusiasts Love the L5P
At the end of the day, the L5P Duramax offers that perfect blend of modern engineering, brute strength, and tuning potential that diesel fans crave.
It’s tough enough to tow 20,000 pounds, refined enough for a daily driver, and still wild enough to make 600+ rear-wheel horsepower with minimal mods.
Whether you’re running it stock or fully built, the L5P represents everything diesel trucks are supposed to be — powerful, reliable, and just plain fun to drive.
FASS Motorsports: Your Local Duramax Experts
At FASS Motorsports in Washington, Missouri, we help L5P owners get the most from their trucks with fuel systems, turbo kits, suspension upgrades, lighting, and expert installation.
We serve Franklin County and the greater St. Louis area with trusted brands, local service, and diesel knowledge you can count on.
👉 Shop L5P Duramax parts online
👉 Visit our showroom in Washington, MO for professional installation and support
Questions? We’re Here to Help.
- Phone: (636) 429-7020
- Email: info@fassmotorsports.com
FASS Motorsports
25 Town and Country Drive
Washington, MO 63090
