Duramax LMM Downpipe Upgrade Guide

General Motors released the Duramax LMM 6.6 liter diesel engine midway through the 2007 model year as the successor to the Duramax LBZ. GM mainly introduced the LMM for emissions purposes, as it included a diesel particulate filter (DPF) and other new federally required emissions equipment. While the Duramax LMM makes more power than the outgoing LBZ, there is still a ton of room for even more. That’s why Duramax LMM downpipe upgrades are some of the most popular LMM mods available. Read on to find out the best Duramax LMM downpipe and other key LMM downpipe information.

What is a Duramax Downpipe?

Let’s start off by explaining what a Duramax downpipe is. The downpipe is the section of exhaust that bolts directly to the turbocharger. There can be some confusion on the Duramax, since a few manufacturers will refer to the downpipe as the turbo direct pipe and the next part of the exhaust with the catalytic converter as the downpipe. However, for our purposes, when we say downpipe we are referring to the part of the exhaust that connects directly to the turbocharger.

The purpose of the downpipe is to collect the spent exhaust gasses exiting the turbocharger and direct them through the rest of the exhaust system. It is a relatively small piece of pipe that is catless, but it plays a very important role. The stock downpipe is sometimes referred to as a crushed downpipe because it is skinny. This is mainly out of necessity, as its straight design gets in the way of the engine bay, so it gets crushed a little on its side to fit.

Most aftermarket Duramax LMM downpipes are going to be 3-3.5” in diameter. This is the ideal size for a turbocharged diesel engine. It is not too small that it will still be a restriction, but it’s not too big where you would lose exhaust velocity and hurt scavenging. For most people, a 3” downpipe will be more than fine.

Reasons to upgrade the Duramax LMM Downpipe

There are many reasons to upgrade the Duramax LMM downpipe, but the most important is the increased exhaust flow. By using a larger and less restrictive piece of piping, the exhaust air can more easily and more quickly travel out of the engine and through the exhaust. The faster this process happens the more power and engine will make.

The cause of the increased exhaust air velocity is the reduction in backpressure from using a larger pipe. Engines rely on pressure differential to operate, because air moves from areas of high pressure to low pressure, like say moving from a high pressure engine to the lower pressure atmosphere. The greater the pressure differential the faster the exhaust gas will move, and the bigger the downpipe the greater the pressure differential.

However, there is a point where using too large of an exhaust will actually hurt exhaust flow and cause velocity to decrease. That’s why, as we mentioned, the most ideal exhaust sizing for a turbo-diesel making under 1,000 horsepower is usually a 3-3.5” downpipe.

GM 6.6 Diesel Cat Delete and DPF Delete

Since the Duramax LMM downpipe does not have a catalytic converter in it there is no need to do a cat delete on the LMM downpipe. This helps many companies make their LMM downpipes emissions legal by getting CARB certification. It does mean that the LMM downpipe upgrade will make less power than other engines’ downpipe upgrades, but the difference should still be noticeable for the LMM.

Another common upgrade on the LMM when doing the downpipe is to delete the diesel particulate filter (DPF). On the LMM exhaust, the DPF is located directly after the catalytic converter and before the muffler. The DPF is the main difference between the Duramax LMM and LBZ, as the LBZ does not have it. It is an emissions piece that helps reduce diesel soot particulates.

Unfortunately, the early years of DPFs were not always great, and they are one of the most common Duramax LMM failure points. For those looking at doing a DPF delete along with their downpipe upgrade, we have a fantastic DPF Delete Guide. Importantly, removing the DPF can run afoul of emissions regulations, so keep that in mind before making any decisions.

Aluminized vs Stainless Steel Diesel Downpipes

For the most part, Duramax LMM downpipes are made from one of three materials: Either aluminized steel or T304/T409 stainless steel. All of these different types of steel have their benefits and disadvantages, but all of them will make suitable downpipes.

The cheapest option for LMM downpipes is aluminized steel. The reason they are cheaper is because the aluminized steel pipes will not hold up as well under harsher climates that have a lot of snow. This is because most of these areas use rock salt on the roads to help with snow and ice control.

While it is effective for helping with the roads, it absolutely destroys and degrades aluminized metal, causing premature corrosion and rust. Aluminized metal downpipes are only for those in areas that do not see road salt and that have relatively mild winters.

409 vs 304 Stainless Steel

The next step up is 409 stainless steel. Compared with aluminized steel, 409SS is much more superior in terms of resisting corrosion and rust. It is also much more expensive than aluminized metal, too. For the most part, high performance exhaust systems are made largely out of 409, because it presents a good balance of cost and corrosion resistance. It also polishes nicely, and is much cheaper than the next step up, 304 stainless steel.

304SS is the top dog for mass produced performance exhaust material without using titanium. 304SS looks sleek, polishes very nicely, and is even more corrosion resistant than 409SS – hence the increased cost. Compared with 409SS, 304SS has double the amount of chromium, and more than 20x the amount of nickel, which is what makes it so corrosion resistant. It’s often referred to as marine steel because it even holds up to repeated use in saltwater. It can turn gold after repeatedly being subjected to high heat.

For most people, a 409SS exhaust will be a good compromise of price and performance/longevity. For most people in most climates, 409SS will be more than fine for lasting several years without succumbing to any corrosion or rust.

Considering the placement of the downpipe on the Duramax LMM, running alongside the engine instead of below it, even an aluminized steel exhaust will probably be fine. Still, if price is the same, usually the best option will still be a 409SS downpipe, with 304SS being nice but somewhat overkill.

Duramax LMM Downpipe Upgrade Benefits

The top benefits to upgrading the Duramax LMM downpipe are:

• +10–20 horsepower
• +10–20 lb-ft of torque
• Decreased back pressure
• Increased exhaust flow velocity
• Increased engine and turbocharger efficiency

The most noticeable benefit of upgrading the Duramax LMM downpipe is going to be the increase in power. With a freer-flowing downpipe, you can see gains of 10–20 horsepower and 10-20 lb-ft of torque. If you were removing the catalytic converter you would see even bigger gains, but the LMM stock cat is located downstream in the exhaust from the downpipe.

Most of these gains will be from the decreased back pressure of a larger downpipe, which increases exhaust flow velocity (like we explained above) due to the pressure differential. The decreased back pressure lets the engine and turbocharger breathe easier, allowing them to work more efficiently.

Best Duramax LMM Diesel Downpipe

There are dozens of downpipe options on the market. From high-quality US made products to overseas value options that range in pricing. The number of options can make it hard to decide what downpipe is right for you. At the end of the day it is a piece of exhaust piping, but there are a few important factors to consider:

1. Materials
2. Quality
3. Fitment

304 stainless steel is the best material for a downpipe. A lot of the cheaper options on the market are aluminized steel or other cheaper materials that will rot and rust over time. Stick to SS304 for best quality. Outside of material quality there is also weld quality – most of the flanges are welded on and poor welds can lead to issues with the piping down the road as it is exposed to a lot of heat. Lastly is fitment, a lot of value products were never developed on real vehicles which can create fitment problems.

Overall, you aren’t going to get stainless steel, good quality, and good fitment on the super budget options.

Boosted Performance LMM 3″ Downpipe

Price: $219
Material: 304 Stainless Steel
Buy Here: Boosted Performance LMM 3″ Downpipe

Our favorite downpipe for the LMM comes from Boosted Performance. It’s made of 304 stainless steel, it has a nice heat shield on it, fitment is great and it works on both factory and aftermarket exhaust systems, and it doesn’t require any tuning or additional modifications. Weld quality is great and it comes in at a good price point.

At 3″ this downpipe is a lot less restrictive than the factory unit and therefore will have a big impact on backpressure reduction and improved performance. At $219 it isn’t the cheapest option on the market but it is one of the best values when you consider materials, quality, and fitment.

Duramax LMM Downpipe Upgrade Summary

You cannot go wrong using any of our recommended LMM downpipe upgrades. They all provide about the same power and performance gains of 10–20 horsepower/torque, and they all look relatively similar – except the MBRP which is black. Many people will say that an exhaust pipe is just a pipe and craftsmanship doesn’t really matter. And while that is true to a point, it’s still a good idea to invest in a domestic company that will still be there to stand behind their product years later.

From our list, the two CARB approved options are the PPE and Diamond Eye downpipes, so if you need to be emissions compliant those are your two options. The ProFab and MBRP downpipes are not CARB compliant, but they are still excellent pieces that will perform very well. As far as actual emissions output goes, it’s very unlikely the PPE and Diamond Eye are actually better, they just applied for and got the CARB certification. So, if you really care about emissions it doesn’t really matter which you choose – so long as you leave the catalytic converter in place.

Let us know your thoughts on upgrading the Duramax LMM downpipe in the comments below!