The 5 Best Duramax LML Performance Mods

Five simple bolt-on mods can take the Duramax LML past 500whp and 1000wtq. However, the engine does have some weak points from a tuning perspective that must be addressed if you plan on adding more than just an intake and tune.

We are going to cover: ECM and TCM tuning, cold air intakes, exhaust systems, downpipes, and intercoolers. These upgrades are going to be the best bolt-on modifications for easy power gains. Additionally, we will discuss engine power limits and supporting mods for various power goals.

Duramax LML Power Limits

• The stock block/bottom end can handle about 700whp, above that requires a built bottom end. Pistons can crack at any point above 600whp
• The CP4.2 needs a lift pump pretty much immediately and will need to be swapped around 600whp
• 450whp is the end of the reliability mark on the Allison 1000 transmission with TCM tuning. At 500whp reliability will dip and you’ll eventually need a torque converter upgrade
• The factory turbo taps out around 525whp, making it one of the most restrictive components

CP4.2 & Fueling

The biggest hurdle with the LML is the CP4.2 fuel pump. The CP4.2 flows about 20% less fuel than the predecessor CP3. The fuel pump will tap out around the 450whp mark. Additionally, it is already prone to premature failure, so adding a lift pump is an important first step. Over 600whp and you’ll want to consider swapping the CP4.2 with a CP3. If you can afford doing it sooner we absolutely recommend it as blowing a CP4.2 can run you $10k+ for repairs.

Add a lift pump before you implement any of the 5 mods mentioned below.

Allison 1000 Transmission

Once you reach the 500whp mark you run into the second hurdle: the Allison 1000 transmission. Transmission tuning will help prolong the life of the Duramax A1000 but it will not stay reliable for too long when consistently pushing 500whp+. The first step is upgrading the torque converter which will handle about 750whp before the Allison needs to be fully built.

Stock Turbo

Out of all Duramax engines the LML actually has the worst turbo. The stock turbo is good for around 525whp before it needs to be upgraded. For ~650whp you’ll want to upgrade to an aftermarket variable geometry turbo. Pushing 700whp+ you’ll want to get a fixed geometry turbo.

Stock Block

While the block, rods, and pistons were strengthened over the LBZ and LMM engines, the LML isn’t bulletproof. It can handle about 100whp more than those engines, making it good for about 700whp. Above those power levels the bottom end of the block including the pistons need to be upgraded. However, the pistons can crack about the 600whp mark, although it is more commonly seen on cars pushing beyond 650whp.

Best Duramax LML Performance Mods

• Cold air intake
• Tune & TCM tuning
• 4″ exhaust system
• 3″ downpipe
• Larger intercooler

Supporting Mods:

• Lift pump (recommended immediately)
• TCM tuning (for 450whp+ but good mod regardless)
• Built transmission (500whp+)
• CP3 fuel pump conversion (550whp+)
• Larger injectors (600whp+)

Other Noteworthy Mods:

• Turbo upgrade
• Intake horn/ Y-bridge upgrade
• Turbo up-pipes
• Cold side intercooler pipe

If you would rather consume this content via a video, check out our “How to Build a 500 WHP Duramax LML video below:

1) Cold Air Intake Upgrade

The best bang-for-the-buck mod, and best mod in general, on our list is a tuner. However, we put a cold air intake first on our list because it’s highly complimentary with a tune and will will amplify the power gains of both the tune and intake.

Adding an intake to a tuned LML can result in 20whp and 60wtq gains in the lower end of the power curve and ~10whp and ~20wtq peak power gains. We recommend and prefer open intake systems as they provide more air flow over closed box systems and therefore provide better power gains.

Check out our in-depth Duramax LML Intake Upgrade Guide for more details on open vs closed intake systems. Additionally, it includes a few of our favorite intake systems.

LML Intake Upgrade Benefits

• Up to 20whp and 60wtq power gains when tuned
• Peak power gains around 5-10whp and 10-20wtq
• Faster turbo spool / better throttle response
• 30%-50%+ increases to air flow
• 1-2mpg improvement
• Cool intake sounds

Boosted Performance 4″ Open Intake

The best bang-for-your-buck intake on the market is Boosted Performance’s 4″ open intake system for the LML. It increases airflow by over 60% compared to the factory system making it one of the best intakes on the market from a performance standpoint.

They use high quality S&B Filters that are Made in the USA along with 4″ CNC mandrel bent aluminum piping that is finished in a wrinkle black coating to prevent heat transfer and add durability. Overall, this is the intake we prefer to install on all the LML’s we work with.

Price: $299

Buy Here: Boosted Performance 4″ LML Intake

2) ECM & TCM Tuning

Out of all the Duramax LML performance mods, a tuner is hands down the best. Tuners offer the biggest power gains and the lowest cost per hp. A tuner alone can add over 100whp and over 200wtq with simple off-the-shelf maps.

The two options for engine tuning are using a plug-and-play tuner such as SCT, Edge, Bully Dog, etc. or opting for custom tuning. Plug-and-play tuners come with pre-built maps built for performance, towing, fuel economy, and more. They are easy to use and offer the ability to change between maps easily depending on what you are using your Duramax for.

Custom tuning is best for people looking to maximize performance gains. Unless you are planning on going full bolt-on, upgrading your turbo, or aiming for aggressive power gains then you likely don’t need custom tuning. However, since custom tunes are built specific to your exact engine and mods they offer the best power gains and are usually the most reliable.

TCM tuning is transmission tuning. Adding a TCM tune will make the A1000 pretty reliable at 450whp and will help prolong the life of the transmission at 500whp.

Tuning Benefits

• Over 100whp+ and 200wtq+ power gains possible
• Multiple applications like towing, fuel economy, performance, etc.
• Better towing, improved fuel economy, etc.
• Cheapest horsepower per dollar spent
• Improved transmission reliability with TCM tuning

For 99% of readers, adding an intake and tune is probably all you need. Both of these will take you to 500whp, albeit from a tune on the more aggressive side. However, getting past the 500whp mark is when modding the LML starts to get really expensive.

3) 4″ Exhaust System

Now that we have an intake and tune, we are bringing significantly more air into the engine. To help with exhaust air flow we recommend upgrading to a 4″ or even 5″ exhaust system. An LML is an emissions-heavy diesel so the emissions equipment does create additional exhaust restriction. However, without getting into deleting these systems upgrading the exhaust system will provide the biggest benefit.

An upgraded exhaust will reduce backpressure, allowing for better turbo spool, a more efficient turbo, and therefore more horsepower. One additional benefit is lower exhaust gas temps, or EGTs. Lower EGTs improves the health and reliability of the engine and turbo. Running a stock exhaust with significant mods will result in very high EGTs which can destroy the turbo easily.

You can opt for budget exhaust systems made out of aluminum or go for stainless steel systems. Stainless steel will last more than a decade, even in rust prone areas, whereas the aluminum exhausts will only last 4-5 years, but are considerably cheaper.

Performance Benefits

• 5-10whp and 10-15wtq power gains
• Reduced exhaust backpressure
• Increased turbo efficiency
• Lower EGTs

4) Downpipe Upgrades

Although we have this right below an exhaust system, the downpipe is actually the most restrictive exhaust component. The stock downpipe is very skinny which makes it highly restrictive. Because the downpipe bolts up directly to the turbocharger it creates the most exhaust backpressure. Therefore, it is the most restrictive.

Earlier model downpipes connect to the turbo with a v-band. 2015.5 and later models have a 3-bolt flange. This is purely important for fitment purposes. Both downpipes are equally as restrictive.

As mentioned with exhaust backpressure earlier, it increases stress on the turbo and leads to higher EGTs. Higher EGTs decrease turbo (and internal) reliability which is why upgrading the downpipe is crucial for tuned and modified LMLs. While it is a power-adding mod it is also a good reliability mod for anybody who wants to preserve their turbo.

A 3″ downpipe will increase exhaust flow by about 20%. From a power perspective, it will add about 20whp and 25wtq. In total, an upgraded downpipe is one of the most cost-efficient Duramax LML Performance Mods.

If you are interested in learning more about Duramax LML downpipe upgrades, check out our Duramax LML Downpipe Upgrade Guide.

Downpipe Upgrade Benefits

• 20whp and 25whp gains (when tuned)
• Reduced exhaust backpressure
• Lower EGTs
• Improved turbo reliability

5) Larger Intercooler Upgrade

The factory intercooler is actually quite good but an upgrade will add reliability and provide some power gains in addition to lower EGTs and a healthier engine. We love intercooler upgrades since they help reduce engine temps which is great for reliability.

At high power levels, especially when doing multiple back-to-back pulls or driving aggressively, the stock intercooler can become overwhelmed and lose its ability to effectively cool the air before entering the engine. This leads to heat soak which reduces power and decreases engine reliability.

While power gains aren’t great for the price, the reliability factor is worth it in our mind. While it might not add a lot of power it does prevent the loss of a lot of power caused by heat soak. Lower EGTs, more reliability, consistent performance, and a cooler engine make it worth the cost. It’s also a great mod for anyone who tows heavy things frequently for long distances.

At 500whp do you need an intercooler? Probably not. But, it still doesn’t hurt.

Intercooler Upgrade Benefits

• Lower EGTs
• Healthier, more reliable engine
• Consistent performance, no power loss from heat soak
• 30%+ air capacity and air flow increases
• Slightly better MPGs, although not material enough to get an intercooler just for better MPG

Summary

A simple cold air intake and tuner can give you a 500whp LML. The downside is that modifying it beyond 500whp gets very expensive and requires significantly more modifications.

Getting to 500whp with a tune and intake will require a decently aggressive tune. Therefore, our recommendation is to add an intake, downpipe, exhaust system, and intercooler and then run a more modest tune to help improve reliability. We didn’t cover lift pumps in depth, but the CP4.2 is a death machine so please either swap it for a CP3 or add a lift pump before you add a tune.

Above 500whp and you’ll need to start looking into turbo upgrades, built transmissions, fueling upgrades such as injectors and CP3 conversions, just to name a few. There are also some solid mods that we didn’t include here that are good in the early stages such as a y-bridge upgrade, high-flow up-pipes, and intercooler piping.

Overall, the LML gets a solid grade for tune-ability. Getting to 500whp is super easy and inexpensive but pushing past that is really only for the people dedicated to high horsepower builds.