Duramax is a family of 6.6L diesel engines designed by General Motors. They are produced by the Ohio-located diesel manufacturer DMAX, which itself is a joint venture between GMC and Isuzu, a Japanese vehicle and diesel engine company.
An LML Duramax engine. Did you ever notice how much an engine look like a heart? As in, this is what a robot’s heart would look like. Sorry, just thinking aloud. Source: Wikipedia.
The central idea behind the Duramax’s design was to build a diesel engine that could fit in the same spaces as a typical gasoline engine in a full-size pickup truck. This was no small feat. Diesel engines are typically larger and heavier than their gasoline counterparts. This is because of all the additional equipment required for a diesel engine to inject fuel into a container of compressed air: a turbocharger, an injection pump, and an intercooler, among other things.
GMC solved this problem by installing fuel rails directly into the valley of the engine block and attaching the oil cooler to the left side of the cylinder block. This, alongside simplified coolant piping that was strung through the flywheel housing, reduced the overall size while ensuring that even temperatures were maintained on both sides of the engine.
Don’t worry, we won’t keep you in suspense here. The Duramax L5P, the current model first released in the 2017 model year, easily takes the crown as the best Duramax engine yet produced. We’ll discuss the many upgrades of the L5P at the end of this article. But first, let’s survey the previous Duramax models and see just how far they’ve come.
Sources: GM Corporate Newsroom; Duramax Hub.
Duramax Engines Through the Ages
Several Duramax engines were produced throughout the years to comply with ever-shifting EPA emissions standards. First up is the LB7, which first saw the light of day in 2001.
Duramax LB7
Source: Truck Trend.
The LB7 was the first Duramax engine and was made from 2001–2004. It was produced just as GMC was retiring its 6.5L Detroit diesel engine. As a 6.6L V-8, it sports a 32-valve design with 4 valves per cylinder. It also comes with high-pressure common-rail direct injection along with a composite cylinder head. It is turbocharged and intercooled, and its valvetrain has forged steel rocker arms with bridges to operate two valves.
The whole engine weighs in at 835 lbs. and is about 30x30x32” in its dimensions. Successive Duramax engines would stick close to these same size/weight parameters.
The LB7 has steel laminate head gaskets, made of three stainless-steel plates coated with fluorine. It has six 14mm head bolts per cylinder and a crankshaft with 4340 steel forging and a 90º plane twist. The pistons are high-silicon aluminum alloy manufactured via a gravity casting process, and its connecting rods are made of forge steel with a piston pin diameter of 1.36”. Some further specifications are listed below:
- Power: 300 HP at 3,100 RPM
- Torque: 560 lb-ft at 1,800 RMP
- Block: cast iron
- Head: aluminum
- Compression ratio: 17.5:1
- Valvetrain: OVH, 4 valves per cylinder.
The cooling system of the LB7 sends coolant to the back end of the engine before directing it around the cylinders towards the front. Coolant passes through a channel in the flywheel at the rear of the engine and is then sent towards the left and right sides of the cylinder block. After that, the coolant goes through a passage at the front of each cylinder head. Dual thermostats possess opening temperatures of 180º and 185º, respectively.
The LB7 was used in the Chevrolet Kodiak (GMC Topkick) as well as the Silverado (GMC Sierra HD) produced from 2001–2004.
Sources: Truck Trend Network; Duramax Hub.
Duramax LLY
Source: Duramax66.
This Duramax engine was introduced in 2004 and completely replaced the LB7 after a few months of production. It is a 6.6L turbocharged engine with 32 valves, like the LB7. However, it was also the first Duramax engine built to meet the new emissions standards on diesel trucks that were implemented in 2004. GMC’s goal was to meet these standards while keeping the Duramax engine market competitive.
The LLY has the same size and weight as the LB7 and possesses just a smidge more horsepower. It does, however, have a noticeably higher peak torque at 605 lb-ft. The key difference between the LLY and LB7 is that the LLY uses a Garrett turbocharger and includes an EGR (exhaust gas recirculation) valve.
- Power: 310 HP at 3,000 RPM
- Torque: 605 lb-ft at 1,600 RPM
- Block: cast gray iron
- Head: cast aluminum
- Compression ratio: 17.5:1
- Valvetrain: OHV (overhead valve), 4 valves per cylinder
The LB7 was used in the Chevrolet Silverado as well as the 2006 Hummer H1 Alpha.
Duramax LBZ
Source: HCDMAG.
The LBZ was a short-lived Duramax V-8 engine produced only from 2006–2007. The LBZ brought with it a substantial increase in HP and torque over the LLY and was easily the most aggressive Duramax engine yet made. Interestingly, the LBZ is sometimes called the “hot rod” engine for its high degree of performance and lack of a diesel particular filter. It was also the last instance of a Duramax engine being available with a manual transmission. However, it was ultimately replaced by emissions-compliant engines such as the LLM (see below).
- Power: 360 HP at 3,200 RPM
- Torque: 650 lb-ft at 1,600 RPM
- Block: cast iron
- Head: aluminum
- Compression ratio: 16.8:1
- Valvetrain: OHV (overhead valve), 4 valves per cylinder
Like its predecessors, the LBZ was used in Chevrolet Silverado and Kodiak medium-duty trucks.
Duramax LMM
Source: Diesel Experts.
The LMM was the first Duramax engine with a DPF (diesel particulate filter), which by mid-2007 was required by federal emissions standards. Since no manual transmission was available, every LMM was built with a 1000 Six Speed automatic transmission.
- Power: 365 HP at 3,100 RPM
- Torque: 660 lb-ft at 1,800 RPM
- Block: cast iron
- Head: aluminum
- Compression ratio: 16.8:1
- Valvetrain: OHV (overhead valve), 4 valves per cylinder
As you can see, the specs on the LMM largely match the LBZ. It has a touch more power and torque, but otherwise this new engine preserved the functionality of previous Duramax models while complying with new emissions regulations.
Duramax LML
Source: Truck Trend.
Introduced in 2011, this Duramax employed advanced emissions equipment such as a diesel exhaust fluid injection. This allowed it to not only meet but easily exceed federal emissions guidelines, as it reduced nitrous oxide emissions by 63% compared to the LMM engine.
- Power: 397 HP at 3,000 RPM
- Torque: 765 lb-ft at 1,600 RPM
- Block: cast iron
- Head: aluminum
- Compression ratio: 16.0:1
- Valvetrain: OHV (overhead valve), 4 valves per cylinder
This engine included the “9th injector technique” in which fuel is supplied directly to the DPF. This eliminates cylinder washing concerns and allowed the engine to run on B20 biodiesel.
Whereas previous models of the Duramax were minor tweaks on the original design, the LML incorporated a great number of new components. This included an upgraded engine block casting, a new oil pump, main bearing design, and a re-routed oil passage circuit. The result was a substantial boost in both power output and torque.
Duramax L5P: The Ultimate Duramax Engine
Released in 2017, the L5P is the latest a greatest version of the Duramax. It is substantially more durable than previous Duramax models and features an impressive 445 HP and 910 lb-ft of torque. The L5P replaced the LML in Chevy Silverado pickup trucks.
- Power: 445 HP at 2,800 RPM
- Torque: 910 lb-ft at 1,600 RPM
- Block: cast iron with induction hardened cylinder walls
- Head: aluminum
- Compression ratio: 16.0:1
- Valvetrain: OHV (overhead valve), 4 valves per cylinder, with mechanical roller lifters
We spoke to representative from Seaview Buick GMC in Lynwood, WA about the advantages of a Duramax engine. They assured us that the latest version of the Duramax “is probably the most reliable diesel engine” on the market. “The best thing the Duramax has going for it,” they added, “is that it’s hooked up to the Allison transmission.” The Allison is a highly regarded, fully automatic transmission used in medium and heavy-duty vehicles.
CLICK HERE FOR A VIDEO DEMONSTRATION OF THE “UNREAL” SOUND OF THE LP5 IN ACTION.
“All diesels have their inherit problems, lots of transmission problems, things like that,” Seaview told us. Though they conceded that older models of the Duramax experienced issues with the fuel injectors, the L5P “It doesn’t have any problems ever, they go forever,” Seaview concluded.
All in all, the L5P is a substantial improvement over not just the LML but every prior Duramax model. Key upgrades include:
- Advanced electronic and structural redesigns
- Strengthened cylinder and engine blocks with 5 nodular iron main bearings
- Electronically controlled variable geometry turbocharger with an increased maximum boost pressure
- Upgraded single-cooler exhaust gas recirculation system with integrated throttle valve
- A cylinder head casting with enhanced airflow and greater head strength
- A new torque converter for the automatic transmission
- New air intake system with hood scoop
According to Duramax Diesel Wholesale, the L5P is “a completely different animal from the LML” and all previous Duramax models. “The head, the block, the turbo chargers, the fuel systems—they’re all different.” DDW added that GMC “had to reduce the DOC aftertreatment and fix it for the new emissions standards from the EPA,” which was the impetus for several upgrades from the LML model.
A Chevy with a Duramax L5P installed. Trust us, it’s in there. Source: GMC.
As this list should make clear, the L5P is less of a sibling and more of a distant, much more successful cousin of the other Duramax engines. It’s a workhorse of an engine: tough, powerful, reliable, and (so long as you follow the instructions) easy to maintain. It is, in short, exactly what a truck diesel engine should be.
Sources: Duramax Hub; GM Authority; interview conducted with Seaview Buick GMC (06/23/2020); interview conducted with representative from Duramax Diesel Wholesale (06/23/2020).
