Duramax LLY Mod Guide

General Motors launched the Duramax engine in 2000, and it was the innovation the company needed to secure one of the top spots in the Diesel engine market. However, the company soon realized that it must constantly develop the engine to remain competitive. The first significant attempt to improve the first-generation Duramax engine gave birth to the LLY Duramax.

The LLY Duramax didn’t only tackle all the complaints about the previous generation but further improved the engine. It was also the first Duramax engine with the largest factory turbocharger, a 62.6MM compressor, which could support up to 560HP. Nonetheless, the engine was rated for 310HP and  605 lb/ft of torque. The engine was initially mated to a five-speed Allison transmission but later upgraded to a six-speed. Despite the engine’s stellar ratings and specifications, it wasn’t without some downsides, with the major one being the tendency to overheat while towing.

With the capability of the stock turbocharger, the LLY Duramax left tons of power on the table. Utilizing this power requires making some performance modifications to the truck. However, pushing the engine to squeeze more power out of it could easily result in catastrophic damage if wrongly done. This guide will delineate the performance modifications required to get extra power out of the LLY Duramax while using stock components and staying within tolerable design limits. We’ll further discuss how to push the engine beyond its limits by highlighting all the weak spots that need correction to achieve such a task.

Duramax LLY Horsepower Limits

Pushing the LLY Duramax to get more power out of it might seem relatively easy, but it isn’t. No doubt, the unexploited potential of the stock turbocharger usually leaves most drivers wanting more power. However, there are some drawbacks that would limit how much power you can get out of the engine while using stock components.

• The LLY Allison transmission can handle about 400whp before requiring tuning. At around 500whp it will need to be fully built. (Manual transmissions can handle over 600whp without any mods)
• The connecting rods are only good for about 600whp, so you’ll need some internal upgrades to push past these levels
• A lift pump is a great mod out of the gate, but is a must have at 500whp+ as this is about the capacity of the stock fueling system as the LLY doesn’t have a stock lift pump

Hence, regardless of the potential of the stock turbocharger, you’ll still need to make some improvements if you want heavy power.

Best Duramax LLY Performance Upgrades

• Turbo Inlet Manifold
• Exhaust System
• EFI Live Tuning
• Lift Pump
• Intake 
• Transmission
• Turbocharger

1) LLY Duramax Turbo Inlet Manifold

Circling back to the engine’s overheating problem, it was primarily due to its restrictive turbo inlet manifold. The design of the stock turbo inlet manifold, also known as the turbo mouthpiece, was restrictive and hindered airflow into the turbo. Consequently, it impacted the turbo spool as the turbocharger struggled to build up adequate boost, leading to heightened exhaust gas temperature. Replacing the turbo mouthpiece should be the first modification when improving engine performance, as making any other modification without replacing this piece is simply asking for trouble.

Any mouthpiece other than the stock mouthpiece is a better improvement. However, most drivers prefer to opt for LBZ’s stock mouthpiece because it is an inexpensive alternative that is equally as good as most aftermarket units — it improves airflow, turbo spool and eliminates the risk of overheating. The only downside of the LBZ mouthpiece is that you’ll have to cut the intake tube for it to fit. With most aftermarket units, you don’t need to modify the intake tube because they fit perfectly, but they cost more than the LBZ unit.

2) 6.6L LLY Exhaust System

Opening up the exhaust side to allow for the free flow of exhaust gas is another way to gain more horsepower. This enables the engine to breathe better and reduces the exhaust gas temperature. Not only will this improve performance, but it also eliminates the risk of overheating when you tune the engine for more power.

Downpipe

The squished stock downpipe is not an optimal design for exhaust gas flow. The exhaust system is 3.5″ with odd bends, a catalytic converter, and a muffler for noise and emission reduction. Improving the engine performance without opening up the exhaust side to allow easy exit of exhaust gas could cause back pressure, increasing the exhaust gas temperature.

Replacing the downpipe is the best exhaust modification on the LLY. Since it bolts directly to the turbocharger it has the greatest increase in backpressure reduction therefore resulting in the biggest increases to turbocharger efficiency.

Boosted Performance’s downpipe fits perfectly and comes with a protective heat shield. This replacement improves the flow of exhaust gases from the turbocharger into the exhaust system. Consequently, it enhances the turbo spool, resulting in more power, and further helps cool down the EGTs.

Exhaust Manifold

Replacing the exhaust manifold with an aftermarket one makes a huge difference. PPE’s High Flow Exhaust Manifold with Up-pipes is an excellent choice for significant power gains. While the factory up-pipes performed just fine under normal application, their expansion bellows weren’t ideal for high horsepower applications as they tend to crack and leak exhaust. However, PPE’s combo is suitable for extreme operating conditions and allows unrestricted exhaust gas flow. It also reduces exhaust gas temperatures and improves turbo spool-up.

Exhaust System

Swapping out the stock exhaust system is an additional exhaust modification that can yield some good results. The stock exhaust system is usually geared towards emission and noise reduction. This makes it hard to push out exhaust gases. Diamond Eye’s 4″ exhaust system will help open things up, reduce EGTs and significantly improve the engine performance.

3) LLY Duramax EFI Live

Opting for custom tuning after upgrading the turbo mouthpiece and exhaust assembly will bring significant power gains. You can contact a reputable dealer to write the tune for you if you don’t know how to go about it. While you are at it, remember to discuss what modifications you currently have on your truck and your future plans.

With EFI Live, you get a customized tune tailored to your needs and don’t have to worry about ruining your transmission, provided you keep things around 400HP and 650 lb-ft of torque.

Pushing the Engine Beyond 500HP

You’ll need to make substantial modifications to the truck for more significant power gains beginning with the weak spots. If you intend to push the engine beyond 500HP from the start, be sure to make all the modifications pointed out above except for EFI Live tuning. You should save the customized tuning for later when you’ve made all the performance modifications you intend to do on the truck.

4) 6.6L Duramax Lift pump

Clearly, the LB7 Duramax provided the platform from which other Duramax engines emerged, making most of its successors have something in common with it. And considering that the newer LLY aimed to fix all the issues that plagued the LB7, it was surprising that the LLY still didn’t have a lift pump. Instead, the engine relied solely on the Bosch CP3 injection pump to pull the fuel from the tank.

A lift pump is an excellent investment whether you intend to keep your truck in stock form or modify it. The lift pump increases the longevity of the high-pressure pump by helping to build up fuel pressure up to the CP3 pump. Tuning the engine for big power gains without improving fuel supply would cause the engine to throw low rail pressure codes because of increased fuel demand.

While there are plenty of options on the market, Airdog has proven dependable over the years. Their DF-165 4G lift pump is a perfect fit for bone stock or modified trucks as it can support up to 1,000 HP. Unlike some units that require dropping the fuel tank to install a suction tube, this unit doesn’t, which makes its installation relatively easy. It also includes a filter to ensure that the high-pressure pump only receives pure diesel fuel. Despite all these features, the system has a quiet operation. 

5) LLY Intake

After opening up the exhaust side and increasing the fuel supply, the next step is to improve the intake air supply to balance the equation. Depending on the turbo mouthpiece you have installed, you may need to rescale the MAF sensor to add more fuel for the increased airflow. Suppose you have the LBZ mouthpiece installed and opt for LBZ intake, making it a complete LBZ intake. You will need to rescale the MAF sensor for the changes in airflow readings, or the truck will run like crap.

However, if you intend to go all out for power by pushing past the abilities of the stock turbocharger, you would need an aftermarket intake because the LBZ’s intake is only good up to 500HP. An aftermarket intake from S&B will drastically reduce restriction, allowing the engine to inhale better. You should also expect more horsepower and improved fuel economy.

6) LLY Duramax Transmission

Throughout the production run, they mated the LLY Duramax engines to an Allison transmission. The engine debuted with a five-speed Allison Transmission in 2004, which was later upgraded to a six-speed Allison transmission in 2006. These transmissions were dependable and built to last long, but only with the stock power. Although you can still do some decent modding with the stock transmission, the rule of thumb was to keep it under 90HP or increase the chances of catastrophic failures.

You’ll have to build the transmission to hold the power you want it to handle. Rebuilding a transmission is more complicated and labor-intensive than an engine, so it is best to contact an expert if you aren’t up to the task. There are several ways to go about the process — source parts individually, opt for complete kits, or take the easy way out by settling for ready-to-install units.

However, before proceeding, you should be sure about what horsepower levels you want, as this would determine what upgrades your transmission needs. Starting with upgrading the stock clutch to allow for a quicker and smoother shift, triple disk performance torque converter, and installing a Trans-Go Shift kit to increase clutch holding power.

There are various offers on the market for drop-in units, with 550HP, 750HP, and 750+HP builds being the most popular. For daily drivers looking to step things up a bit using the stock turbo and fuel system, a 550HP build should cut it. However, if you want to take performance to the next level by upgrading the turbocharger and fuel system, the 750HP build was designed for such power. Then there is the 750+HP build designed for much higher horsepower and people looking to push their truck’s limit.

7) 6.6L LLY Turbocharger

Before you decide to go this route, you need to answer the following questions — what kind of power are you looking to make, what are you using your truck for, and most importantly, are you sticking with the stock fuel system?

With the right supporting modifications, LLY’s stock turbo can handle up to 560HP. You should only consider upgrading the turbocharger for horsepower gains upward of 600HP. A 64MM turbo from Duramax Tuner or Ryan’s Diesel Services would support up to 650HP, and with the lift pump assisting the CP3 pressure pump, you shouldn’t have any issues running the engine at this power level.

However, if you decide to go beyond 650HP, you’ll need a bigger turbo, like 68MM. You’ll also have to upgrade the Y-bridge. The Y-bridge tends to blow out at high boost levels, causing boost leaks, and opting for a bigger turbocharger without making the necessary improvements to the fuel system limits the turbo’s performance. Yet, a big turbo may not be the most efficient choice in some situations — If you consistently tow with your truck, a big turbo introduces surges when hauling heavy loads at low RPM.

A compound turbo setup would be a better option for drivers who regularly tow heavy load and wants more power. You’ll also need to rebuild the engine to handle the power, and except you are running a twin setup, bigger turbochargers also experience more lag.

Duramax LLY Performance Mods Summary

Although the LLY Duramax was rated for 310HP and  605 lb/ft of torque, the factory turbo had the capacity to support way more power. However, the major limitation to pushing the engine for significant power gains was the stock Allison transmission which peaked around 400HP, followed by the fuel system.

Despite these limitations, you can still push the engine to gain considerable power by making slight modifications. By improving the restrictive turbo mouthpiece and freeing up the exhaust side, you can conveniently make 400HP while running mostly stock components. However, things get really pricey when you decide to push the engine limits because you’ll have to rebuild the transmission to handle such power.

Depending on what kind of power you are looking to make, you may also have to upgrade the turbocharger, injectors and eventually rebuild the engine. However, don’t forget to finish off with custom tuning before calling it quits to get the most outcome from all the improvements made on the truck.