Blog / 2025

Sierra HD Engine Showdown: 6.6L Gas vs. 6.6L Duramax Diesel

When it comes to purchasing a Sierra HD, each engine option offers unique capabilities designed to suit different drivers’ needs. Let’s take a closer look at the key differences between the 6.6L V8 Gas Engine and the 6.6L V8 Duramax Turbo-Diesel Engine to help you decide which one is the best fit for you.

6.6L Gas vs. 6.6L Duramax Turbo-Diesel

The chart below shows a side by side comparison between the two different engine options.

Type:6.6 V-8 6.6L V-8 Duramax Turbo-Diesel
Fuel Type:Regular Gasoline (87 Octane)Ultra-Low Sulfer Diesel and B20 Biodiesel
Horsepower:401 HP (299kW) @ 5200 RPM*470 HP (350 kW) @ 2800 RPM*
Torque:464 lb-ft (629 Nm) @ 4000 RPM*974 lb.-ft. (1,322 Nm) @ 1600 RPM*
Emissions Control: Catalytic Converter, Three-Way Catalyst, Positive Crankcase VentilationCooled Exhast Gas Recirculation, Selective Catalytic Reduction, Diesel Particulate Filter, Intake throttle
Displacement:6564 CC (400 CI)6604 CC (403 CI)
Engine Orientation:LongitudinalLongitudinal
Compression Ratio:10.8:116.0:1
Valve Configuration:Overhead ValvesOverhead Valves
Valves per Cylinder:TwoFour
Valve Lifters:Hydraulic RollerMechanical Roller
Firing Order:1-8-7-2-6-5-4-31-2-5-7-8-4-5-6-3
Bore x Stroke:103.25 x 98.0 mm103 x 99 mm
Block:Cast IronCast Iron
Cylinder Head:Cast AluminumCast Aluminum
Intake Manifold:CompositeCast Aluminum
Exhaust Manifold:Stainless SteelCast Nodular Iron with Steel Pipe Extension
Main Bearing Caps:Cast NodularCast Nodular Iron with Steel Pipe Extension
Crankshaft:Forged SteelForged Steel
Camshaft:Billet SteelSteel
Connecting Rod:Forged Powder MetalPM Iron
*As tested in the Chevrolet Silverado Heavy Duty and GMC Sierra Heavy Duty.

After looking over the table, you’ll notice that both engines deliver impressive performance, but they’re built for different priorities. The 6.6L V8 Gas Engine offers strong horsepower and smooth, consistent power for everyday driving and lighter towing needs. On the other hand, the 6.6L Duramax Turbo-Diesel stands out with its torque and efficiency—perfect for heavy-duty towing, hauling, and long-distance performance. No matter which you choose, the Sierra HD delivers the strength and dependability you can count on.

Engine Availability

The standard or availability of these engines changes for the different trims of the Sierra HD. The chart below shows which of the two engines are either standard or available as an upgrade on each of the Sierra HD trims.

Engine6.6L V-8 Gas6.6L V-8 Duramax Turbo-Diesel
Trim: ✴️
PRO✴️
SLE✴️
SLT✴️
AT4✴️
AT4X✴️
DENALI✴️
DENALI ULTIMATE
✅ = Standard
✴️ = Available
❌ = Not available

After reviewing the Sierra HD engine comparison and availability charts, we hope you now feel more confident and informed about which engine best fits your driving needs. Whether you value the balanced strength and everyday versatility of the 6.6L V8 Gas Engine or the unmatched towing power of the 6.6L Duramax Turbo-Diesel, both options deliver the dependability that drivers expect from GMC. No matter which engine you choose, your Sierra HD is built to handle the toughest jobs with confidence and capability mile after mile.

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