Duramax LLY Downpipe Upgrade Guide

When the GM Duramax LLY was launched halfway through the 2004 model year, it represented a step forward in the Duramax diesel engine family. The LLY took the place of the outgoing LB7 Duramax and improved upon some of the LB7’s shortcomings. While the Duramax LLY was an unquestionable improvement over the LB7, it still had a few kinks. One of the main complaints among LLY Duramax owners is overheating. That is especially true when towing or if you are running higher than stock power. 

Aside from the LLY’s minor issues, it is a great turbo diesel V8 with a ton of potential out of the gate. An upgraded LLY downpipe is often one of the first modifications that Duramax owners choose to update. Not only are LLY downpipes one of the most affordable mods available, but they also help the engine in multiple important ways. An upgraded Duramax donwpipe can help reduce EGTs by a significant amount, improve turbo response, help fuel economy and free up a few trapped horsepower.

In this guide, we’ll cover Duramax LLY downpipe upgrades in detail including why they’re beneficial and which ones are the best on the market currently. 

What is a Duramax LLY Downpipe?

The downpipe is one of the most crucial elements of the exhaust side of any turbocharged engine. It is the first exhaust component after the turbocharger’s turbine outlet. That means that it plays a very significant role in the exhaust characteristics of your LLY Duramax. A Duramax LLY downpipe then feeds into the catalytic converter and the rest of the vehicle’s exhaust system. 

As the first exhaust component after the turbo itself, it is important that your downpipe is optimized to get exhaust gasses away from the turbo with few airflow obstacles and at the lowest temperature possible. Well-designed Duramax LLY downpipes are also built to reduce backpressure at the turbo outlet which reduces stress on the turbine.

The factory LLY downpipe is not good in any of those regards. In fact, it is one of the main limitations to making big power from the LLY Duramax V8. The LLY’s factory downpipe is skinny and not optimized for airflow. It is called a “squished downpipe” in the LLY community, as it uses a restrictive tapered, and flattened design that is detrimental to airflow. 

LLY downpipes are very simple components. Unlike some other downpipes that introduce other variables into the equation, like catalytic converters, Duramax downpipes are pretty much exactly what they sound like; a bent pipe. Most aftermarket Duramax downpipes are nearly identical in shape, but there is a bit of variation in terms of materials.

Duramax LLY Downpipe Upgrade Benefits

• Decreased EGTs
• Increased turbo response
• Better sound at WOT and idle
• 5-10whp gain on stock turbo without tune
• 15-40whp gain with upgraded turbo and tune
• 1-5 MPG improvement

Ultimately, there are a few very notable and significant reasons that you might want to consider upgrading your LLY Duramax’s downpipe. Quite a few of those reasons revolve around turbo efficiency and turbo health. 

One of the main reasons that LLY owners opt to upgrade their downpipe is the impact that it has on exhaust gas temperatures. In the diesel truck world, EGTs are extremely important to monitor. Hot EGTs can do significant damage to any turbo diesel motor. This is a serious issue for the LLY Duramax in particular. The LLY is known to overheat easily when towing or under heavy stress. 

Aftermarket LLY Downpipes typically have better thermal properties than the stock unit. This helps reduce EGTs by up to 100 degrees fahrenheit, while also cooling faster after a period of intense heat. This not only keeps your LLY healthier but also helps improve turbo efficiency and response.

In addition to those benefits, an upgraded LLY Duramax downpipe also provides some performance and sound-related perks as well. Due to the fact that the factory LLY downpipe hampers exhaust flow, you’ll see a bit of added performance with a better-optimized downpipe. 

It is important to note the performance gains that you see from an upgraded LLY downpipe scale with other modifications. While you might pick up 5-10 horsepower from a stock-turbo, untuned LLY V8, you’ll see far better improvements when paired with an upgraded turbo and a tune. 

LLY Duramax Downpipe Considerations

With so many different LLY Duramax downpipe options on the market, there are a number of considerations that you should take into account before pulling the trigger. Beyond that, to get the most out of an upgraded LLY Duramax downpipe, you should also consider pairing it with some additional modifications.

Downpipe Material

One of the biggest considerations when shopping for an upgraded LLY downpipe should be the material used. Ultimately, most Duramax downpipes are made of steel. But the kind of steel used makes a big difference in terms of longevity, rust resistance, and corrosion. Most LLY downpipes are made of either aluminized steel, 409 steel, or 304 steel. Each material has its own benefits and shortcomings. 

Aluminized steel is the least expensive of the bunch to both manufacture and purchase. However, aluminized steel holds up the worst over time. That is especially true if it comes into contact with road salts used for melting snow in the winter. With enough exposure, aluminized steel will corrode and rust.

If you live in an area that has harsh winters and salted roads that you can’t avoid, stainless steel downpipe options are the way to go. 409 stainless steel is the next step beyond aluminized steel in terms of price and corrosion resistance. While slightly more expensive, 409 downpipes will undoubtedly hold up better over an extended period of abuse. 

Lastly, we have 304 stainless steel. Of all of the most common materials used to build downpipes and other exhaust components, 304 stainless steel is by far the best in terms of durability. Its material makeup makes it nearly impervious to rust or corrosion. With that being said, it is the most expensive to produce and buy.

Getting the Most From an LLY Downpipe

While there are obviously considerations that you have to take into account while choosing a downpipe, maximizing the performance of an upgraded downpipe is another story. As we briefly mentioned earlier, the performance gains from a new downpipe scale with other modifications. 

While a turbo upgrade might be a bit extreme for most people, there are some other bolt-on modifications that also improve the gains from an upgraded Duramax downpipe. Upgrading the rest of your LLY’s exhaust system is a good place to start. Depending on how extreme you want to go and the emissions requirements in your state, a cat-back or straight pipe exhaust will help carry the benefits of a freer-flowing downpipe all of the way through the exhaust system. This will untangle some power and make a notable improvement to the sound of your Silverado or Sierra. 

Duramax engines can benefit a ton from tuning. Whether you go with a plug-and-play tuner like an Edge CTS3 or a custom tuner like an EFI Live Autocal, the performance gains are massive. Most tuners increase horsepower by adjusting boost parameters. As a result, a less restrictive downpipe will help with turbo efficiency, garnering even more performance. This is compounded even further with a full LLY exhaust. 

Best LLY Duramax Downpipes

Since the LLY, and the entire Duramax family for that matter, is so popular, there are quite a few decent upgraded LLY downpipes on the market currently. It also helps that the LLY was released over 15 years ago. That has given big names in the diesel exhaust space plenty of development time. Here are our recommendations for the best LLY Duramax downpipes available currently:

• Boosted Performance Duramax LLY Downpipe
• MBRP 3”
• Diamond Eye 3”
• ProFab 3” LLY/LBZ/LMM

Due to the fact that no LLY downpipes come with catalytic converters, the difference between the products from the frontrunners is somewhat minimal. With other downpipes that use high-flow cats, the flow characteristics of the catalytic converters can play a big role in performance. While that isn’t the case for LLY downpipes, build materials, thermal properties, and flow characteristics do all still matter. 

2) MBRP 3” 6.6 Duramax V8 Performance Downpipe

Price: $189.99

Purchase Here: MBRPexhauststore.com

Digging through the LLY Duramax forums, it would be nearly impossible to not scroll past multiple mentions of MBRP and their performance exhaust parts. The team at MBRP has been around the block a few times. Founded in 1996, the MBRP team has rapidly made a name for themselves through satisfied customers and quality products. Their family-owned, Canada-based, operation was built on the principle of quality first and remains so to this day.

Their LLY Duramax 3” downpipe is designed to be as efficient as humanly possible. MBRP utilizes a mandrel bending process that ensures the smoothest internal surface possible. This allows their multi-use 6.6 Duramax downpipe to flow without any unnecessary turbulence. Their Duramax downpipe comes fully heat wrapped to benefit EGTs without adding too much additional bulk. As part of their BLK series, the MBRP downpipe is constructed from 16 gauge aluminized steel and coated in black ceramic coating. 

While aluminized steel isn’t the most resistant material to rust and corrosion, there’s no doubt that MBRP’s LLY downpipe is a high-quality product that will last years without issue. MBRP even backs that claim up with a 3-year warranty on any exhaust component damage. 

3) Diamond Eye Turbo Direct 3” LLY/LBZ/LMM Downpipe

Price: $203.29

Purchase Here: dieselpowerproducts.com

Diamond Eye is yet another leading name in the Duramax performance parts space. They carry Duramax parts ranging from full performance exhausts to exhaust clamps. Diamond Eye takes great pride in their Oregon-based facility stocked full of state-of-the-art manufacturing equipment. With mandrel bending technology and an on-site research and development facility, Diamond Eye is committed to their mission of providing the Duramax community with the very best. 

Like the other entries on our list, the Diamond Eye downpipe boasts a complete 3″ pipe. It is also heat wrapped to minimize exhaust gas temperatures (EGT). Crafted from aluminized metal, this downpipe might not be the strongest or most corrosion-resistant here. Regardless,  it does still get high praise in the Duramax community. Diamond Eye puts their facility to work, using precise CNC machining to ensure a superior fit and features mandrel bending for optimal airflow. Similar to its counterparts, the Diamond Eye downpipe seamlessly fits the LLY and LBZ Duramax diesel engines.

For an aluminized steel part, the Diamon Eye LLY Duramax downpipe is a bit on the pricey side. For $213, you’d expect to get a 409 stainless steel part at least. With that being said, there are very few complaints about the Diamond Eye downpipe in the forums. 

ProFab 3” Duramax LLY Downpipe

Price: $185.07

Purchase Here: profabperformance.com

Rounding out our list, we have an LLY Duramax downpipe from another all-American manufacturer that prioritizes value and performance. ProFab Performance Parts has been in the Duramax tuning game for a while. Starting as a repair shop outside of Kansas City, ProFab has grown to embrace the manufacturing side of the automotive business. Their bread and butter is Duramax performance parts.

The result is a part that is quality made with in-house mandrel bending and CNC equipment. The ProFab 3” LLY downpipe is constructed from aluminized steel, ceramic coated, and surrounded by a heat wrap. Due to the excess steps that ProFab has taken in their manufacturing process, their downpipe is one of the best in terms of lowering EGTs. 

Many enthusiasts in the Duramax and LLY community have had a great experience with the ProFab 3” Duramax downpipe. However, some say that its design and heat shielding can make it difficult to install in such a tight space. 

Duramax LLY Downpipe Upgrade Guide Summary

Upgrading the downpipe of your Chevrolet Duramax LLY diesel engine can provide significant benefits and improve the overall performance of your vehicle. The stock LLY downpipe is known for its restrictive design. As a result, it hampers exhaust flow and leads to increased exhaust gas temperatures (EGTs) and decreased turbo efficiency. By installing an upgraded downpipe, you can address these issues and unlock the true potential of your Duramax engine.

One of the primary advantages of an upgraded LLY downpipe is the reduction in EGTs. Diesel engines are particularly sensitive to high EGTs, which can cause damage to the turbocharger. Aftermarket downpipes with better thermal properties can lower EGTs by up to 100 degrees Fahrenheit. That plays a larger role in improving turbo efficiency and response while also ensuring the longevity of your engine.

Additionally, an upgraded downpipe can provide performance gains by optimizing exhaust flow. The factory LLY downpipe restricts airflow, limiting the engine’s power potential. With a freer-flowing downpipe, you can experience improved horsepower, torque, and fuel economy. However, it’s important to note that the performance gains will be more significant when paired with other modifications such as an upgraded turbo and a tune.

There are a number of reputable manufacturers in the LLY Duramax downpipe space. Boosted Performance, MBRP, Diamond Eye, and ProFab are all known for their commitment to the Duramax family and all provide downpipes that are among the top choices in the market. Each of these options offers unique features and advantages, such as superior material quality, thermal properties, and fitment.