Quick Facts:
- Engine leaving: 6.6L L5P Duramax turbo-diesel V8, sold in 2017 to 2026 model-year HD trucks.
- Engine arriving: All-new 8.3L Duramax turbo-diesel V8, RPO code L6P, nicknamed Mongoose.
- 8.3L output: 555 hp and 1,230 lb-ft, per GM.
- Outgoing L5P output: 470 hp and 975 lb-ft on 2024 to 2026 trucks.
- Max towing, 2027: Up to 20,000 lb conventional and 38,000 lb gooseneck on properly equipped trucks.
- Trucks: 2027 Chevrolet Silverado HD and GMC Sierra HD.
- Dealer arrival: Late 2026. Pricing not announced.
- Best for: HD diesel shoppers choosing between a leftover 2026 and a first-year 2027.
 7 min read
In This Article
The L5P Duramax Bows Out After 2026
The 6.6L L5P Duramax ends its run with the 2026 model year. On September 24, 2026, GM detailed the diesel for the 2027 Chevrolet Silverado HD and GMC Sierra HD. The replacement is an all-new 8.3L V8. GM rates it at 555 hp and 1,230 lb-ft. The 2024 to 2026 L5P makes 470 hp and 975 lb-ft, so the jump is 85 hp and 255 lb-ft.
Three days earlier, GM confirmed only the displacement and the Mongoose nickname. We covered what GM confirmed about the 8.3L Duramax at the time, when output was still a rumor. Now the numbers are official. For this reason, the shopper’s question has changed from “how big?” to “should I buy the old engine or the new one?”
Specifically, the news matters most to two groups. First are buyers who need an HD diesel truck this winter and spot a leftover 2026 on a dealer lot. Second are current L5P owners who wonder what the change means for parts, service and resale. Both groups get a direct answer below.
Service Parts for the Trucks Already on the Road
If you own an L5P or plan to buy a leftover, routine service does not change. Fuel filters, oil filters and engine air filters are the items you replace on schedule. Match each part to your truck’s model year, because the oil filter changed for 2020 and the air filter changed for 2020 and 2024. The grid below lists three L5P service parts to keep on the shelf, and the oil filter shown fits 2020 and later trucks, while the air filter fits 2020 to 2023.
L5P service parts (oil filter 2020 and later, air filter 2020-2023)
GM Genuine Parts TP1015 (13539108) Fuel Filter Kit with Seals
Doc's Diesel Oil Filter D11878 | Compatible with Chevrolet/GMC 6.6L…
ACDelco GM Original Equipment A3248C (84554703) Air Filter
How We Know GM Dropped the L5P Duramax
GM never used the word “discontinued” in its announcement. Instead, the evidence comes from GM’s own documents. In a September 24 column, GM President Mark Reuss wrote the 2027 HD trucks get “two available powertrains.” Those two are the 8.3L diesel and an updated 6.6L gas V8 rated at 412 hp and 472 lb-ft. The L5P is not one of them.
Our second piece of evidence is GM’s fleet order guide. The 2027 HD pages went live on September 15. Their engine listings named an L8S gas V8 as standard, while the L6P Duramax appeared as the diesel option. Meanwhile, the L5P code survived on only one 3500 HD configuration, a crew cab 4WD long bed. As a result, the L6P Duramax is the only diesel a 2027 buyer orders in practice.
The naming has confused plenty of shoppers, so here is the short version. Megamax Duramax was the rumor name a 2024 report attached to this engine. Mongoose is GM’s own nickname. L6P is the order code. All three describe the same 8.3L engine, and it is a clean-sheet design rather than a bored-out L5P.
L5P Duramax vs. 8.3L L6P: Key Specs Side by Side
The table compares the outgoing engine with its replacement, using GM’s published figures. Where GM has not released a number, the table says so.
| Specification | 6.6L L5P (2017-2026) | 8.3L L6P (2027) |
|---|---|---|
| Horsepower | 470 hp (2024-2026); 445 hp (2017-2023) | 555 hp |
| Torque | 975 lb-ft (2024-2026); 910 lb-ft (2017-2023) | 1,230 lb-ft |
| Transmission | Allison 10-speed (2020-2026); Allison 6-speed (2017-2019) | Allison 10-speed 10L1000 |
| Block and internals | Cast iron | Compacted graphite iron, forged crank and rods, steel pistons |
| Max gooseneck towing | 36,000 lb (2026 3500 HD) | 38,000 lb |
| Max conventional towing | 20,000 lb (2026) | 20,000 lb |
| Engine weight and payload | Engine weight not listed; payload published per truck | Not announced |
| Built at | DMAX, Moraine, Ohio | DMAX, Brookville, Ohio |
Two rows matter more than the headline power. First, conventional bumper-pull towing stays at 20,000 lb, so travel-trailer owners gain pulling power, not a higher rating. Second, GM has published no engine weight. A bigger diesel usually weighs more, and every added pound comes out of payload. If you haul a truck camper, read our guide to payload and GVWR before you order.
Against rivals, the 8.3L now leads on paper. Ford’s High-Output 6.7L Power Stroke makes 500 hp and 1,200 lb-ft, while Ram’s High-Output Cummins makes 430 hp and 1,075 lb-ft. For a closer look at the Ram side, see the 6.7L Cummins in the Power Wagon.
What L5P Owners Say After Years of Towing
We dug deep into the Duramax owner forums and the RV towing forums to see how the L5P holds up. The praise lands in the same places again and again. Owners pulling heavy fifth-wheels describe towing without drama, and several report stock trucks past 200,000 miles with mostly sensors and wear parts replaced.
After reading those threads, I rank calm, uneventful heavy towing as the L5P’s biggest strength. Owners bring it up first, too. The ride also earns steady praise, since the independent front suspension feels smoother than the solid front axles on Ford and Ram HD trucks.
However, the complaints are consistent as well. Emissions faults top the list, including NOx sensors, DEF heaters and soot-clogged sensors, and these faults trigger reduced-power warnings. Next come injector and wiring connector failures, which owners say carry long labor times. One more weak spot I would flag on 2020 and later trucks is the 10-speed transmission, since valve body complaints show up across many owner threads.
What This Means for You
GM’s announcement tells you what the new engine makes. It does not tell you which truck to buy this winter. The right call depends on how soon you need a truck and how much you tow.
Buy a Leftover 2026 L5P If You Need a Truck Now
A leftover 2026 makes sense if you tow every month and need a truck before 2027 prices are public. The L5P has ten model years of owner history behind it, while the 8.3L has none. On the forums, many owners refuse to be test drivers for a first-year engine. Price is the other unknown. GM plans to build the next GMC Sierra HD in Canada, which brings tariffs into the math. Our look at truck tariffs and the January 1 deadline explains the pricing risk.
Wait for the 8.3L If Payload and Price Allow It
On the other hand, waiting fits gooseneck buyers near today’s 36,000 lb limit on a 3500 HD. The 2027 Silverado HD adds the 38,000 lb gooseneck rating, upgraded exhaust braking and rack-and-pinion electric steering. Still, wait for two numbers first: the price and the payload of the exact truck you want. GM has released neither for the 2027 Silverado HD.
Towing Gear for Either Truck
No matter which truck you choose, the trailer side of the rig needs care too. A trailer tire pressure monitor, heavy rubber wheel chocks and a lock for a 2-5/16 inch coupler protect a heavy rig. Here are our picks for all three.
Towing gear for a heavy rig
MaxxHaul Solid Rubber Wheel Chock 2-Pack | 8" x 4" x 6" Heavy-Duty …
Proven Industries Model 2516 Heavy-Duty Trailer Coupler Lock for 2 …
Already Own an L5P? Nothing Changes Today
Your truck does not lose support because a new engine arrived. Dealers and independent diesel shops still service Duramax engines going back to 2001. On resale, forum owners say 2017 to 2019 trucks with the 6-speed Allison hold their value well. For 2020 to 2026 trucks, the threads show no clear resale trend yet. Finally, any remaining factory warranty on your truck stays in force.
Final Thoughts
The L5P Duramax leaves as a known quantity. It tows hard, rides well for an HD truck and runs to high mileage for many owners. Its weak spots are also well documented, from emissions sensors to the 10-speed valve body.
If you need a heavy-duty diesel before 2027 prices are public, a leftover 2026 L5P is the proven choice. However, if your trailer sits near the top of today’s ratings, the 8.3L deserves a look once GM publishes payload and pricing. I drive a Chevy Colorado ZR2 rather than an HD, so I lean on owner reports for heavy-duty advice. The 8.3L has none yet, and I would wait for them before signing for a first-year engine. Numbers on paper say little about year-three reliability.
If neither GM truck feels right, Ford’s High-Output Power Stroke and Ram’s Cummins both deserve a test drive. Either way, match the truck to your real trailer weight, not the headline torque figure.
Frequently Asked Questions
Is the L5P Duramax being discontinued?
Yes, in practice. GM lists two engines for the 2027 Silverado HD and Sierra HD: the 8.3L diesel and a 6.6L gas V8. GM’s fleet order guide replaces the L5P code with L6P on nearly every 2027 configuration.
What years did the L5P Duramax come in?
The L5P powered Chevrolet Silverado HD and GMC Sierra HD trucks from the 2017 model year through 2026. Trucks from 2017 to 2019 used a 6-speed Allison, while 2020 and later trucks use the Allison 10-speed.
How much horsepower does the 8.3L Duramax make?
GM rates the 8.3L Duramax at 555 hp and 1,230 lb-ft of torque. The outgoing L5P makes 470 hp and 975 lb-ft in 2024 to 2026 trucks.
Is the Megamax Duramax the same engine as the Mongoose?
Yes. Megamax Duramax was a nickname from a 2024 rumor report, while Mongoose is GM’s own name. The order code is L6P, and all three names describe the new 8.3L V8.
When does the 8.3L Duramax go on sale?
GM says the 2027 Chevrolet Silverado HD and GMC Sierra HD arrive at US dealers beginning in late 2026. Pricing has not been announced.