Duramax Horsepower & Torque by Model Year and Engine Variation
The Duramax 6.6L V8 turbo-diesel engine is nearing its 20th year of production, after debuting in 2001. Over the engine’s lifespan it has gone through 6 different engine variations: LB7, LLY, LBZ, LMM, LML, and L5P.
From the first generation to the one currently in production today, the 6.6L Duramax has nearly double its horsepower and torque, from 235hp and 500lb-ft. in 2001 to 445hp and 910ft-lb. today. And along the way the engine has become significantly more emissions friendly.
For those looking at purchasing a Chevy or GMC with the Duramax, we wanted to put together a chart outlining its history and provide a variation comparison so you can determine which model of the 6.6L is right for you.
Duramax Engine Spec Comparison by Engine RPO Code
Which Duramax 6.6L Engine is the Best?
Before we get into “which one is the best?” we’re going to walk through each engine variation by RPO code and talk about the differences and key features between them. Between all of the variations, there are really two key trends: (1) more power and performance, and (2) increased emissions restrictions and fuel efficiency.
Engine technology has drastically improved in the last 20 years at the same time as emissions regulations have gotten stricter. Fortunately, today’s engines are able to offer drastic horsepower improvements while still meeting regulations. With the horsepower and torque of the Duramax nearly doubling from 2001 to 2020, this is certainly true for this 6.6L V8 engine.
The LB7 is the original production 6.6L Duramax V8. Following success with the performance and reliability of the LB7, this engine became the building block of the Duramax line all the way up until today. Fuel economy on the LB7 was stout at around 20mpg and the IFS suspension included on the HD’s of the time made these cars a great ride compared to Ford and Dodge.
Outside of LB7’s sold in California, these engines did not have any emissions control devices such as an EGR (exhaust gas recirculator) or a DPF (diesel particulate filter). The lack of emissions control devices helped boost reliability of these engines as these emissions parts are frequent fault areas on a lot of diesels. Additionally, it had a fixed geometry turbo which saved it from any vane stick or turbo actuator issues. The one crux of the Duramax LB7 to be aware of is the injectors. Injectors on the LB7 were prone to cracking or leaking, which in most cases requires an injector replacement which can run $3k-$4k.
Outside of the injector issues, these engines are rock solid. Fortunately, by now, any old LB7 will probably have had the injector problem fixed. The block and internals are strong and the engine itself is certainly capable of surviving the 300k mile mark. Overall, a great engine, a comfortable ride, and a decent amount of tune-ability for folks looking for additional performance.
With the Garrett GT3788VA, the LLY Duramax has the largest turbocharger in it out of any other 6.6L Duramax variation. Given the larger turbo and various other small engine improvements, the LLY has the biggest horsepower gains from a tune only compared to any other variation, with the exception of the L5P Duramax.
The LLY engine is very similar to its predecessor. However, the LLY has traded common problems swapping out injector issues for overheating issues and head gasket problems. The LLY is known to overheat easily while towing heavy loads and high-mileage ones are known to blow head gaskets pretty easily. Fortunately, head gaskets are easy and cheap fixes and overheating is only common when towing very heavy things.
One other potential downside is that the Allison automatic transmission is rather weak when it comes to added power. Once you toss an extra ~100hp onto the LLY, you’re going to start running into transmission issues. Outside of these limited issues, again a very strong engine capable of surpassing the 300k mile mark with ease.
Perhaps the shortest lived of the 6.6’s, the LBZ Duramax is one of the most desirable amongst diesel enthusiasts today. The LBZ has the best performance and horsepower potential with bolt-on mods and was paired with a stronger Allison transmission than found in the LLY Duramax. Additionally, the LBZ was the last engine model (RPO) to not have all the problematic new emissions features with it (with the exception of an EGR). The LBZ’s lack of emissions-controlling technology is actually the sole reason for its short lifespan. As regulations became more strict the LBZ was forced to be replaced with a more emissions-friendly Duramax.
For most folks, the LBZ is a dream of an engine and will do whatever you’d like it do while maintaining its reliability. However, for those diesel power enthusiasts looking to push engines to massive horsepower numbers, the LBZ has its crux. The pistons on this engine are known to crack above the 600rwhp mark. With 360hp at the crank from the factory, and plenty bolt-on mods for added power, 600rwhp shouldn’t be necessary for most. Plus it’ll cost you about $20k in mods to get there.
One of the most reliable and most power-capable Duramax’s made, the LBZ will continue to be the diesel enthusiasts Duramax of choice.
The LMM is virtually the same engine as the LBZ with more emissions control. The LMM features a diesel particulate filter (DPF), an oxidation catalyst, and a more savvy EGR. DPF’s essentially capture and hold any leftover diesel particulate, preventing it from being released into the atmosphere. The DPF and the emissions regeneration cycles are known to be longevity killers. Additionally, these parts are known headaches / failure points.
Fuel mileage decreased from this year forwards as the diesel exhaust treatment requires fuel to function properly. However, outside of the emissions technology, the LMM is virtually the same as the LBZ. The internals and block remained unchanged, meaning the engine is reliable with great performance potential.
A new body style gave these trucks a nice fresh and modern look, a look that I still love to this day. Additionally, the trucks got a stronger Allison transmission. Still prone to cracking pistons with too much power, and now prone to emissions system problems, the LMM is slightly less reliable than the LBZ.
The LML Duramax took a big performance leap to nearly 400hp and to 765lb-ft. of torque. To accommodate for the power increases, Duramax strengthened the block, pistons, rods, main bearing, and a higher flowing fuel pump. For emissions purposes, GM abandoned the CP3 high-pressure fuel pump which ultimately limited the potential power gains from bolt-on mods. Therefore, this engine ranks lowest on our list for tuning/power upgrade potential. However, with the strong power numbers out of the box, we’re not sure additional tuning is needed.
With its fully-boxed frame and upgraded IFS suspension, the Duramax LML-featured trucks are the best towing trucks out of any of the other Duramax’s on this list.
The only two downsides of this engine are its emissions controls and the new CP4.2 HPFP that’s used. With all the emissions systems you can add, the LML has a number of additional failure points such as the diesel particulate filter which are known to be problematic. Secondly, the new CP4.2 bosch injection pump was bad and is known to fail from a lack of fluid. When the pump fails, it can take all of the injectors out with it which results in a costly repair.
Overall: best towing truck, good reliability minus the HPFP, strong power but limited tuning potential.
L5P: 2017 to Present
Baddest of them all, the L5P puts out 445hp and a monstrous 910lb-ft. of torque. The most notable changes to the L5P Duramax over the LML is: new cast-aluminum heads which carry more air, upgrade to Denso HP4 HPFP over the Bosch CP4.2, ditching the Garrett turbo for an electric BorgWarner turbo, and adding a lift pump. A lift pump is a huge improvement; it provides additional fueling capabilities and helps relieve stress on the main HPFP, creating a more reliable fueling system and one capable of more horsepower.
With the engine upgrades, all signs are pointing to the L5P having strong tuning and performance potential. However, these engines are still new and its potential has yet to be fully realized. Same goes for the engine from a reliability perspective. My gut tells me GM made some great improvements to help reliability, but L5P’s haven’t been on the road quite long enough to give a definitive answer on this.