These are the first 4 upgrades you should make to your 2003-2007, 5.9L Cummins.

If you're just getting into the diesel game with a third-generation Cummins 5.9L, you're on the right track. The 12V motors are great for handling high power and supporting many upgrades. The 24V is also a very solid choice for the same reasons. With more hp from stock than the 12V motors and some beefy internals, like forged rods that can handle over 800hp. There is lots of potential to be had. 

It's a great truck from the factory, but if you're anything like us, you're not interested in keeping it stock. Especially with the number of bolt-on upgrades available for the 5.9L. Why not make a great truck even better? Today we will show you the first 4 mods you should do to your 5.9L Cummins turbo diesel. 

Before you start your modifications, it's always a good idea to:

• Check for boost leaks
• Make sure all engine codes are dealt with
• Do a leak-down test to check compression
• Have an end goal for horsepower or torque 

1. Aftermarket Intake 

One of the best mods to get on the truck right away is a cold air intake. The stock cold air intake is lacking, from the way it seals to the mildly restrictive airflow and filter. Let's just say there's a bit of room for improvement. With so much room to improve, any aftermarket intake on the market should be an improvement. Some brands we recommend considering include Wehrli Custom Fab and S&B. They all improve multiple parts of the intake system, ensuring better performance for the 5.9. 

Cold air intakes reduce the amount of work the engine has to do when taking in air thanks to the less restrictive airflow. Along with improving intake air temperatures by sealing more effectively. Sucking in less hot air from under the hood. Taking less effort to suck in the air will improve the engine's efficiency and, in turn, provide more power and better fuel economy. Cooler intake air temps also mean more air enters the combustion chamber. Which should give you more power and a cleaner burn.

Cold air intakes are among the more affordable upgrades you'll want to do to your truck. They are also very easy to install, which makes them a great first place to start when modifying your truck. 

2. Tuning 

Tuning is the most cost-effective way to bring your diesel truck to life. If you're starting stock, tuning will get you the most horsepower out of any mod you can install. Plus, if you have any other bolt-on upgrades installed, like a cold air intake. A custom tune will also bring out the maximum potential of those mods. Stock or modified, custom tuning is always what you'll want. It can bring out the maximum performance your engine has to offer. Along with providing the best overall design and support to keep your truck operational if something goes wrong.

Another important reason to go custom is the ability to utilize switch-on-the-fly tuning. This allows the driver to switch through multiple different custom tunes while driving. Tunes can either provide you with tons of extra power while sacrificing fuel efficiency and EGTs or provide you with great fuel efficiency while sacrificing some power. You can see that it's not always a best-of-both-worlds situation, which is why you'll want to go with a company that offers switch-on-the-fly custom tuning, even if you're going with a single tune. Because you may and most likely will want to upgrade in the future. 

Our favorite devices to use for installing custom tuning are the MM3 if you have an 03-05 Cummins because of the great compatibility, or if you have an 06-07 Cummins, we highly recommend using EFI Live due to the added features and benefits their product will be able to offer for your truck.

3. Transmission 

The stock transmissions that come with the Cummins 5.9L are a major weak link when trying to build these trucks to handle major power. The stock automatic can handle about 400 horsepower before it starts slipping, and the clutch that comes with the manual transmission can't handle much more than that either. So if you plan to push your truck past 400hp, which we know can be done easily. Then upgrading to a built transmission is something you are going to want to do. 

4) Turbos 

The stock turbo in the third-generation 5.9L Cummins engines is a great performer at stock power levels. However, it often suffers at higher power levels, running out of air at the top of the rev range.

We've dyno'd the stock 5.9L Cummins with just a tune installed and were able to get about 400hp and 720 lb-ft of torque out of the engine before the turbo couldn't keep up with the rate at which air was being consumed by the engine, and horsepower started to drop. This means that upgrading the turbo is a must if you want to see anywhere north of 400hp or 720 pound-feet of torque.

Luckily, dropping in a larger turbo is pretty easy, and it will easily get you into much higher horsepower figures with just a tune. To give you an idea of what kind of power you can get out of a turbo upgrade. Calibrated Power tested their Stealth 67 turbo on a 2006 Cummins sporting 100% over Exergy Injectors with a 12mm Exergy CP3 pump. The truck made over 800 RWHP. Now you won't be seeing that kind of power without installing some other supporting mods, but the point is that upgrading the turbo can go a very long way in reaching those kinds of numbers and is ultimately a limiting factor when it comes to breaking past 400hp with a stock engine.

The great thing about upgraded turbos is that they deliver higher peak power across the entire operating range. Installing the turbo alone will only net you around 30-40hp, which is why we've listed this modification 4th on our list. You will need custom tuning to get actual performance from an upgraded turbo, along with a built transmission to handle the extra power.

Here are some things to consider when upgrading to a larger turbo:

• What is your end horsepower goal? 
• Where in the rev range do you want the power to be?
• Is upgrading to a twin kit a possibility in your future?
• Custom tuning is a must
• Having a built transmission is a must

Overall, while they do have a few shortcomings, the Cummins 5.9L engines are a great platform for building more power. With so many modifications on the market, it's practically impossible to keep the truck stock.