2027 Chevy Silverado HD Order Guide Is Out: L6P Duramax and L8S Gas Engine Changes Confirmed
Chevrolet’s 2027 Silverado 2500HD and 3500HD order guide is now available, and it gives us our first concrete look at what GM is changing for the new model year.
The biggest news is under the hood.
For 2026, the Silverado HD order guide lists the familiar L5P 6.6L Duramax Turbo-Diesel V8 and L8T 6.6L gasoline V8. For 2027, those RPO codes have changed to L6P for the Duramax diesel and L8S for the gasoline V8.
What the new order guide does not tell us yet is arguably more interesting: GM still has not published horsepower, torque, displacement or detailed technical specifications for either new engine in the material we've reviewed.
So instead of speculating about what the L6P Duramax is, we compared the 2026 and 2027 Silverado HD order guides directly to see what GM has actually changed.
The Big Change: L5P Becomes L6P
The 2026 Silverado HD order guide identifies its diesel engine as:
L5P — Duramax 6.6L Turbo-Diesel V8
The 2026 engine/axle tables pair the L5P with the Allison-branded 10-speed automatic and the GU6 3.42 rear axle ratio.
For 2027, that engine code changes to:
L6P — Duramax Turbo-Diesel V8
In the portions of the 2027 order guide we've reviewed, GM identifies the new engine as a Duramax Turbo-Diesel V8 but does not provide the familiar “6.6L” designation alongside L6P.
That does not prove that displacement has changed. It simply means the order guide isn't giving us enough information yet to say exactly what the L6P is.
At this point, we still don't have confirmed horsepower or torque figures from the order guide either.
The Gas Engine Changes Too: L8T Becomes L8S
For 2026, Silverado HD uses the L8T 6.6L gasoline V8. The 2026 guide specifically identifies it as a 6.6L V8 and pairs it with the GT4 3.73 rear axle.
For 2027, the gasoline engine becomes L8S.
Interestingly, the 2027 order guide describes L8S as a:
V8 with Direct Injection and Variable Valve Timing, gasoline
So GM is changing the RPO for both Silverado HD engines for 2027, rather than simply introducing a new Duramax alongside the existing gas engine.
That's one of the strongest indications in the order guide that 2027 represents a meaningful powertrain update despite the truck itself largely carrying over.
The Transmission and Axle Strategy Looks Familiar
One thing that does not appear to be changing dramatically is the basic powertrain layout.
For 2026, the engine/axle tables show the L8T gasoline engine with a 3.73 axle and the L5P Duramax with a 3.42 axle. The guide also lists the 10-speed automatic transmission configurations, including PTO-capable applications.
The 2027 guide continues that same basic strategy with L8S and L6P.
That matters because there was always a possibility that a substantially revised diesel would bring a different axle ratio, transmission arrangement or other obvious drivetrain change. We're not seeing evidence of that in the order guide.
At least from an ordering standpoint, the new engines appear designed to fit into the existing Silverado HD architecture.
Electrical System Doesn't Reveal Much Either
We also compared the alternator configurations because supporting hardware can sometimes reveal changes that aren't obvious from the engine description.
The 2026 guide lists the KW5 220-amp alternator as included with the L5P Duramax and available with the L8T. It also offers a dual-alternator configuration consisting of a 220-amp primary and 170-amp auxiliary alternator.
The 2027 order guide retains essentially the same arrangement, except the engine references change to L6P and L8S.
In other words, there isn't an obvious electrical-system change here pointing toward radically different accessory requirements for the new Duramax.
GVWR Structure Appears to Carry Over
The 2026 Silverado 2500HD already offered numerous GVWR configurations depending on cab, drivetrain, engine, wheel/tire configuration and equipment.
For example, the 2026 guide contains diesel configurations ranging into the 11,000-plus-pound GVWR range, including an 11,450-pound configuration shown in the Double Cab specifications.
The 2027 guide continues to show L6P-specific GVWR configurations extending through similar territory.
We're not seeing an obvious wholesale chassis-rating change that would immediately indicate the L6P was introduced alongside a significantly uprated 2500HD platform.
That doesn't tell us what the new engine makes for power. It does tell us that GM appears to be integrating it into essentially the same HD chassis structure.
What Has Actually Changed So Far?
After comparing the two guides, the most important confirmed differences are surprisingly concentrated around the engines.
L5P → L6P Duramax
L8T → L8S gasoline V8
L6P is identified as a Duramax Turbo-Diesel V8, but detailed specifications are not provided in the order-guide material we've reviewed
L8S is identified as a gasoline V8 with Direct Injection and Variable Valve Timing
The familiar 3.42 diesel / 3.73 gasoline axle strategy remains
The existing 10-speed transmission architecture remains
Major supporting items such as alternator configurations appear substantially unchanged
Many packages, colors, tires and appearance options that might initially appear new are actually 2026 carryover items
The 2026 guide is explicit about the outgoing L8T/L5P powertrain arrangement, giving us a solid baseline for this comparison.
What We Still Don't Know About the L6P Duramax
The order guide confirms that L6P is real. It confirms that it replaces L5P in the Silverado HD ordering structure. But it does not answer the questions everyone actually wants answered.
We still need GM to confirm the L6P's displacement, horsepower, torque, turbocharger configuration, fuel-system changes, emissions-system changes and internal engine revisions.
There has been plenty of discussion and speculation surrounding the next Duramax, but those details should remain separated from what GM has actually published.
For now, the order guide gives us something much more concrete:
The 2027 Silverado HD is moving from L5P to L6P, while the gas engine simultaneously moves from L8T to L8S.
And despite those new engine codes, GM hasn't dramatically rearranged the rest of the truck around them—at least not in a way that's obvious from comparing the 2026 and 2027 ordering information.
That makes the missing engine specifications even more interesting.
The engines are officially in the ordering system. Now we're waiting for GM to tell us exactly what they are.
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