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Everything You Need To Know: L5P Duramax

Everything You Need To Know: L5P Duramax

For truck lovers and automotive enthusiasts alike, the L5P Duramax was a big, big deal.

When this diesel engine was poised to debut in a completely redesigned GM Heavy Duty line of pickups, the rumor was swirling big time. GM was promising some fairly staggering numbers from the factory, and claiming that these big horsepower and torque numbers would come with an engine that was more efficient and better for the environment. Its predecessor (LML Duramax) was somewhat notorious for a pesky injection pump issue, so the pressure was on for GM to produce a motor that made great power and remained reliable.

So, did the L5P live up to all the hype? You bet it did!

We’re going to cover everything you need to know about this incredible workhorse:

  • Changes From LML
  • Performance Characteristics
  • L5P Reliability
  • Performance Upgrades

Ready to dive right into this awesome engine? Let’s do it!

Changes From LML

Let’s just get this out of the way, up front. There are so many changes to the L5P Duramax, that it can almost be considered a clean-sheet design. It may share the same displacement, and same basic architecture but this engine truly deserves the moniker of “all-new”:

Upgraded Cylinder Block:

To deal with the increase in power over the LML, the L5P Duramax utilizes a strengthened cylinder block which includes an induction hardened cylinder wall, revised casting, and 5 nodular iron main bearings, paired with 4 bolt, cross-bolted main bearing caps.

Tougher Reciprocating Assemblies:

To further strengthen engine internals, GM designed a larger diameter crankshaft with larger connecting rod journals, increased bearing surface area, fractured cap forged and sintered powdered metal alloy connecting rods, cast aluminum "remelted" pistons.

New Camshaft Design:

An enhanced camshaft profile was designed to take advantage of the LM5’s new power characteristics.

New Turbocharger Design:

GM ditched the Garrett Turbocharger, which was a Variable Vane design, for a brand new Borg Warner unit that is electronically controlled and utilizes a Variable Geometry Design.

New Fuel Injection Pump:

Ah, the troublesome CP4 pump has finally been addressed by a brand new unit designed by Japanese manufacturer Denso dubbed the HP4.

New Fuel Injectors:

To go along with the HP4 pump, GM also opted to upgrade the fuel injectors to Denso units that produce 20% better flow than the outgoing Bosch units.

Higher Oil & Coolant Flow Rates:

A high-capacity engine oil cooler boasts a 50% increase in cooling capacity, along with piston jets that also produce 50% more cooling capability and a dedicated turbocharger oil line.

Ceramic Glow Plugs:

Upgraded ceramic glow plugs are designed to reduce preheat times, as well as modify current flow to maintain a consistent temperature from cylinder to cylinder.

Upgraded Allison 1000 Transmission or Allison 10 Speed:

The Allison 1000 Transmission soldiers on from 2017 - 2019 but comes back to the table with upgrades like a beefed-up torque converter to handle the additional power. 2020 models and up with a 10 - Speed Allison transmission that adds in additional performance and towing capability.

Integrated Hood Scoop:

According to GM, 60% of intake volume is pulled through the standard hood scoop. Not only does this provide a cold air charge at a dead stop, but it also provides a Ram Air effect at speed for improved turbocharger response.

Performance Characteristics

This is why you’re really here, isn’t it? You want to know all about what kind of power the L5P puts down from the factory, and what type of power delivery you can expect from this ultra-modern diesel. Ask, and you shall receive!

Let’s start out with the raw numbers:

Horsepower: 445 HP @ 2800 RPM

Torque: 910 lb.ft. @ 1600 RPM

0-60 (Unladen): 6.5 - 6.7 Seconds (Motor Trend / Car and Driver Estimates)

Towing 10,000 pounds, acceleration from 50 to 70 mph: 10.6 Seconds

The power curve on the L5P really tells the story here with all 910 lb. ft. of torque available at a super-low 1600 RPM. This is exactly why the 3500 models with Max trailering package can pull a simply staggering 23,300 - 35,300 pounds (depending on the year and equipment) when properly equipped. Depending on the year, the L5P comes paired with either the Allison 1000 or 10 - Speed Allison transmission.

To put this all in perspective, the L5P puts out 48 MORE horsepower than the LML and 145 lb. ft. more torque than its predecessor.

At all engine speeds, in all conditions, the L5P simply trounces the LML. It’s not even a fair fight!

L5P Reliability

One of the main goals for this next-generation diesel powerplant was addressing and improving upon reliability concerns that seemed to plague the LML Duramax. If you’re not in the know on LML issues, a quick search of the internet with undoubtedly bring you to various message boards, forums, and articles on CP4 fuel injection pump issues that plague this generation. To put it bluntly, it was a major, major concern and it needed to be addressed by General Motors.

Denso Pump Upgrade


GM ditched the Bosch CP4 unit for a robust unit produced by Denso known as the HP4. This upgraded unit is built to handle the rigors of Heavy - Duty truck duty that can handle higher fuel pressure than the CP4 without succumbing to break down or failure. According to those in the know, the CP4 issues stem from a lack of lubrication in modern low-sulfur diesel fuel that leads to eventual breakdown.

The issue with the CP4 pump comes down to material durability. Debris created by cam erosion within the pump eventually passes into the injectors and then is recirculated throughout the fuel system! As the engine runs, more and more damage occurs until the lines, regulator, and injectors are completely and totally ruined. Due to the high pressure within the system, this can cause the pump to lock up, or explode!

So far, there have been very few reported issues with the Denso HP4 pump.

Performance Upgrades

The L5P Duramax puts out some seriously impressive numbers from the factory but you know, we all want just a little more power for our trucks! Upgrading the L5P is not quite as straightforward as the previous generation of Duramax but upgrades are entirely possible. One of the main roadblocks to upping the power on the L5P is the Allison 1000 transmission on 17 - 19 models which can “only” handle around 500 rear wheel horsepower for short bursts. Stock turbos, stock injectors, and internals can easily handle 600 horsepower, which means that the Allison 1000 will give up the ghost without substantial upgrades. Allison 10L1000 units are more robust but run into similar issues.

Here are some common L5P Upgrades:

Merchant Automotive Rear Housing Support:

The rear transfer case housing on Allison 1000 equipped trucks can suffer from hard-to-find cracks and once the transfer case housing cracks, it can weep fluid and eventually lead to an expensive repair or replacement.

This custom-engineered piece of equipment utilizes high-quality 3/16-inch plate steel that is cut to an exact fit on the transmission and transfer case of specific trucks. The housing and transmission are linked together with 7/8-inch 0.120-inch wall thickness DOM mild steel tubing to provide support for your rear housing, even under the most extreme of conditions, including vibration from driveline component failure.

S& B Cold Air Intake:

This awesome cold air intake S & B for the L5P is engineered to minimize air restriction and flows 43.86 percent better than stock. This means more power, better overall response times.

This kit comes with a fully enclosed airbox that protects the filter from dirt and keeps out engine heat. The L5P intake kit features two large air inlet points. If you want to customize the amount of air you take in, we include an optional box plug that can seal off one inlet point. Authorized dealers are also backed by S&B's Million Mile Limited Warranty!

For a quick performance boost, this is an awesome option.

MBRP Exhaust:

With an MBRP performance exhaust, you can make that happen with Up to 18 more horsepower and 28 ft/lb more torque not to mention lower EGTs. More pulling power, improved throttle response, greater fuel efficiency, with an MBRP diesel performance exhaust you will be amazed at what your truck can do.

Bolt-on, no weld design makes installation a no hassle one and a half to two-hour project. Everything is a direct fit to the OEM hangers and you get all the clamps and parts you will need, all in one box, no matter what configuration of truck you have.

Not only does this system look good, but it sounds great too!

Trust The Diesel Experts

We can say with confidence that the LM5 Duramax is one incredible engine. It produces horsepower and torque from the factory that was once only achievable with high levels of engine tuning. Many of the reliability issues present in the LML were handled, and the motor seems poised to go down as one of the all-time greats to ever wear a Duramax badge.

At Merchant Automotive, we’re the Duramax experts and we have some of the best products available for the LML Duramax and Allison platform. Whether you’re looking for more power, more reliability, or the best maintenance products and tools available; Merchant Automotive has you covered. No one knows Duramax diesel engines better than Merchant Automotive!

So what are you waiting for? Check out our selection of L5P products today! 

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