L6P Duramax: Everything We Know About GM’s New Diesel Engine

The next generation of GM heavy-duty diesel power is beginning to take shape.

For the 2027 model year, General Motors’ ordering information identifies a new L6P Duramax Turbo-Diesel V8 for the Chevrolet Silverado HD and GMC Sierra HD. The new engine takes the place of the familiar L5P in the ordering structure.

But there is a much bigger story developing around what GM hasn’t told us yet.

The material currently available identifies the engine as the L6P Duramax Turbo-Diesel V8, but does not provide the displacement, horsepower or torque figures we’ve been waiting for.

That leaves us with an unusual situation: we know the next Duramax has arrived in GM’s ordering system, but some of the most important details about it remain unknown.

Here is everything we know so far—and, just as importantly, what we don’t.

What Is the L6P Duramax?

L6P is the RPO code appearing for the new Duramax Turbo-Diesel V8 in 2027 Silverado HD and Sierra HD ordering information.

It replaces the L5P, the 6.6-liter Duramax V8 currently used in GM’s heavy-duty pickups.

That distinction is important.

This isn’t simply an internet rumor about a future engine. The L6P designation itself is now present in GM ordering information.

What remains uncertain is exactly how different the L6P will be from the L5P it replaces.

What GM’s 2027 Information Tells Us

The 2027 ordering information gives us several useful clues about how the L6P fits into the truck.

The diesel continues to be paired with an Allison-branded 10-speed automatic transmission, and the familiar 3.42 rear axle ratio remains associated with the diesel powertrain.

Other diesel-related equipment also continues into the new configuration, including the exhaust brake and diesel-specific supporting equipment.

This suggests that GM has designed the L6P to integrate into the existing Silverado HD and Sierra HD architecture rather than requiring an obviously different drivetrain layout.

That doesn’t tell us what changed inside the engine.

But it does give us a starting point.

L5P vs. L6P

Here’s what we can establish right now:

L5P Duramax

6.6-liter turbo-diesel V8
470 horsepower
975 lb-ft of torque
Allison-branded 10-speed automatic
3.42 rear axle configuration in current HD applications

L6P Duramax

Turbo-diesel V8
Appears in 2027 Silverado HD and Sierra HD ordering information
Replaces L5P in the ordering structure
Allison-branded 10-speed automatic remains
3.42 rear axle configuration remains
Displacement not yet specified in the material we’ve reviewed
Horsepower not yet specified
Torque not yet specified

That missing displacement number is one of the biggest reasons the L6P has attracted so much attention.

Why Is the Missing Displacement Interesting?

The outgoing engine is normally identified very clearly as the 6.6L Duramax Turbo-Diesel V8.

The new ordering information we’ve reviewed identifies L6P as a Duramax Turbo-Diesel V8 without giving us that familiar displacement figure.

That does not prove the engine is larger than 6.6 liters.

It could simply reflect how the early ordering information was written.

But until GM publishes the actual specifications, it leaves open an obvious question:

Is the L6P still a 6.6-liter Duramax, or is GM increasing displacement?

For now, the correct answer is that we don’t know.

Is the L6P the Rumored 8.3L “MegaMax”?

This is where confirmed information and internet speculation need to be separated.

Reports of a significantly larger GM heavy-duty diesel predate the L6P designation. One of the most widely circulated reports described an 8.3-liter Duramax under development.

The nickname “MegaMax” was created in media coverage surrounding that rumored engine. It is not an official General Motors engine name.

Now that an actual new Duramax carrying the L6P code has appeared, it is natural to ask whether the two stories are connected.

But there is currently an important missing link:

We do not have confirmation that the L6P is an 8.3-liter engine.

The L6P could turn out to be the rumored larger-displacement Duramax.

It could also be a substantially revised engine retaining the existing 6.6-liter displacement.

Until displacement is published or independently verified, those possibilities need to remain separate.

What About the 600 HP and 1,200 LB-FT Rumors?

Another frequently repeated claim surrounding the rumored larger Duramax involves output approaching 600 horsepower and 1,200 lb-ft of torque.

Those numbers should currently be treated as rumors, not L6P specifications.

For comparison, the outgoing L5P produces:

470 horsepower at 2,800 rpm

and

975 lb-ft of torque at 1,600 rpm.

That gives GM considerable room to increase output with the L6P, but we shouldn’t turn expected improvements into specifications before GM publishes the numbers.

Why Would GM Develop a New Duramax?

Peak horsepower and torque are only part of the equation.

A new heavy-duty diesel gives engineers an opportunity to address several objectives simultaneously, including:

Power and torque
Towing performance
Thermal management
Fuel efficiency
Combustion control
Emissions performance
Durability
Turbocharger response
Fuel-system capability
Long-term compliance requirements

A displacement increase could potentially be one tool engineers use to achieve those objectives, but it isn’t the only option.

Turbocharger changes, revised combustion hardware, injection-system improvements, cooling changes, calibration and aftertreatment revisions could all substantially change an engine without a dramatic displacement increase.

Until we see the hardware, we should keep those possibilities open.

The L6P Isn’t the Only New Engine

There is another interesting part of the 2027 story.

GM is also changing the gasoline HD engine designation from L8T to L8S.

That means both major Silverado HD and Sierra HD powerplants receive new RPO codes for 2027.

The outgoing L8T is the familiar 6.6-liter gasoline V8. The new L8S appears alongside L6P in the 2027 ordering information.

Seeing both engine codes change makes the 2027 powertrain story bigger than simply replacing one diesel engine.

What Doesn’t Appear to Be Changing Dramatically?

The surrounding truck architecture provides another clue.

So far, the 2027 ordering information doesn’t show an obvious wholesale redesign of the drivetrain around L6P.

The diesel retains the familiar 3.42 axle strategy and Allison-branded 10-speed transmission configuration. Many of the existing HD chassis and equipment arrangements also continue.

That could mean GM has engineered the L6P specifically to work within the current-generation HD platform.

It also means we shouldn’t assume a new engine code automatically means an entirely new truck.

What We Still Need to Learn

Several major L6P questions remain unanswered.

We’re watching for confirmation of:

Engine displacement
Bore and stroke
Compression ratio
Horsepower
Peak torque
Torque curve
Turbocharger configuration
Fuel-injection system and pressure
Internal engine changes
Pistons and connecting rods
Cylinder-head changes
Cooling-system changes
Oil capacity and specification
Emissions and aftertreatment changes
Maximum towing ratings
Fuel-economy implications
Service procedures and maintenance intervals
Production timing and application availability

Those details will tell us whether the L6P represents an evolution of the current Duramax architecture or something considerably more substantial.

Confirmed, Reported and Rumored

Because so much information is circulating about this engine, MegaMaxDiesel.com will separate developments into three categories.

CONFIRMED

The L6P designation appears in 2027 GM heavy-duty ordering information.

It is identified as a Duramax Turbo-Diesel V8.

It replaces the L5P in the 2027 Silverado HD and Sierra HD ordering structure we’ve reviewed.

The Allison-branded 10-speed transmission and 3.42 diesel axle strategy remain.

REPORTED

Automotive publications have reported that GM has been developing an updated or potentially larger-displacement heavy-duty Duramax.

Prototype Silverado HD and Sierra HD trucks have also been observed undergoing testing, although seeing a prototype alone does not establish exactly which engine or component is being validated.

RUMORED / NOT CONFIRMED

An 8.3-liter displacement has been reported but has not been confirmed as an L6P specification.

The MegaMax name is a media nickname, not an official GM engine designation.

Claims of approximately 600 horsepower and 1,200 lb-ft should also remain in the rumor category until supporting information becomes available.

The Most Important Thing We Know Right Now

The biggest development isn’t an 8.3-liter displacement or a 1,200-lb-ft torque figure.

Neither of those has been established as an L6P specification.

The significant development is much simpler:

There is a new Duramax designation in the 2027 HD ordering information, and it is L6P.

Now we have an actual engine code to follow.

The next pieces of information—particularly displacement, horsepower, torque and technical changes—will tell us whether the L6P is an evolution of the 6.6L Duramax or the beginning of something much bigger.

MegaMaxDiesel.com will continue updating this page as additional L6P information becomes available.

Last Updated: September 20, 2026

MegaMaxDiesel.com is an independent publication and is not affiliated with, sponsored by, endorsed by, or operated by General Motors, Chevrolet, GMC or Duramax. “MegaMax” is a media-created nickname associated with reporting about a possible future larger-displacement Duramax and is not currently an official General Motors engine name.