Best 12v 5.9 Cummins Performance Upgrades

The 5.9 Cummins 12v earned a legendary reputation for reliability. The engine is also extremely strong, making it a prime candidate for performance modifications and upgrades. However, these engines are mechanically driven which makes them a bit more challenging to modify. While it isn’t as simple as just dropping a tune on it, there are 6 “free” mods that you can do to add some solid power gains.

Additionally, there are four bolt-on upgrades worthy of a mention as well. In this article we’re going to cover all 6 of the free mods as well as a few bolt-on upgrades in detail. Whether it be a VE44 or the more desirable p-pump 5.9 Cummins, these upgrades are great options to add a bit more horsepower and torque to these strong and reliable engines.

Free 12v 5.9 Cummins Upgrades

Before we get into the bolt-on mods below, first we’ll give you the low-down on six free upgrades you can do to the 5.9 Cummins 12v easily by yourself. These engines are mechanically and conservatively fueled, so it’s easy to increase fueling and gain some extra ponies and torque. In addition, you can also easily find a way to run more boost and seriously extend your powerband. Depending on the specific mod, you can gain as much as 50 horsepower without spending a dime.

The air-fuel-control (AFC) mod: The first free mod is the air-fuel-control mod, also known as the AFC mod. This applies to those with the 1994+ P7100 injection pump, of which the AFC controls the low-boost fueling. By sliding the AFC assembly to the all the way forward position, you can increase the fuel pressure under low-boost, This seriously increases horsepower at lower-loads and speeds. 

Pre-Boost screw adjustment: Located on the AFC assembly, the pre-boost screw can be modified to increase fueling during low-rpm conditions and under boost. Using a 8mm allen key, you simply back out the screw at small increments until you find the right balance. What this does is allow the P7100’s fuel rack to open further so it can squirt out more fuel.

Mack rack plug mod: This another mod for those with the 1994+ P7100 injection pump, though it costs about $15. The Mack rack plug once again increases the amount the fuel rack can open by 3 mm. This allows it to push 70 cc worth more fuel into the plungers. Importantly, on manual transmission 6BTs from 1996–1998 this mod is not recommended as it can actually decrease fueling. 

More Free Mods

Removing fuel plate: The next mod is another specific to the P7100 injection pump. Removing the fuel plate allows for the fuel rack to completely travel and open for maximum fueling. To access the fuel plate you need to remove the AFC assembly and tamper resistant bolt. There, you can then access and remove the final two bolts and the plate itself. 

Boost elbow mod: This mod goes right alongside the fuel plate mod is recommended to take advantage of other fueling mods. This one is for the HX35W Holset turbos from 1995+. By removing the wastegate and replacing it with a brass boost elbow, you can effectively run double the amount of boost as stock (35+ PSI vs 18–22 PSI). Without running more boost you won’t see much performance increase, making this very important and effective.

Governor spring kit: The final (practically free) mod that we will cover is the 3,200 or 3,000 rpm governor spring kit. This is again for the P-pump owners. The stock governor kicks in at 2,700 rpm, but the P-pump cuts fueling as early as 2,400 rpm. Installing the 3,000 or 3,200 rpm governor spring raises the governor and allows for more fueling up top. Broadening the power band. Note: Stiffer valve springs are recommended for the 4,000 rpm governor springs. 

Best 12v 5.9 Cummins Performance Upgrades

• Cold Air Intake
• Intake Manifold
• Upgraded Downpipe
• Larger Lift Pump

Now let’s talk about the top 4 12v 5.9 Cummins upgrades in terms of bolt-ons. We would suggest doing the above free mods to easily add 50 horsepower, but after that going to bolt-ons is the next step. For your first bolt-on, we suggest going with a cold air intake. Cold air intakes are a great way to add up to 15 horsepower and torque to the 12-valve Cummins. They are also very easy to install. A performance intake will massively increase the amount of airflow going into the turbo. Thus allowing it to compress more air and make more power. 

Next, we suggest upgrading the intake manifold on the 6BT. Like a cold air intake, upgrading the intake manifold will increase the amount of air that can enter the engine. Though this will be post-turbo instead of before. The stock manifold is known for being restrictive, and a larger one is a great way to boost the powerband. 

Next up we recommend getting some exhaust work done. The stock exhaust is too restrictive, and a full four inch downpipe is a great way to open things up. On turbocharged engines, the exhaust connects directly to the turbo. Adding a larger downpipe will massively increase flow, allowing the turbo to work more inefficiently and produce more power. 

Finally, our last recommendation is to get a larger fuel lift pump. This is exclusive to the 1994+ P7100 6BT Cummins. Upgrading it with a higher flowing pump is a great way to make more horsepower. If you don’t want to do the above free fueling mods, modding the stock unit with an aftermarket lift pump will provide adequate fueling for whatever you need. 

1) Cold Air Intake

Upgrading the intake on the 12v 5.9 Cummins is the first mod that many do when beginning their build. The stock air intake is adequate at stock power levels and boost levels, but when you increase fuel and boost you’ll soon find it a restriction. It’s not entirely hard to see why, as many of them have a nasty bend right before they connect to the turbo, which is horrible for airflow. 

Upgrading to a performance exhaust removes the restriction and increases airflow by as much as 30–50%. This is crucial if you plan on running elevated boost levels with your Cummins, and also adds some extra airflow even at stock levels. There are both open and closed-box performance intakes, and we usually suggest open box designs. They offer better airflow and are usually cheaper and easier to install.

The most popular options are from Sinister Diesel and S&B. Though most of them perform pretty much the same as each other. Make sure to check out our 5.9 Cummins air intake upgrade guide for a full rundown on the best options. 

2) Upgraded Intake Manifold

Now that you increased airflow pre-turbo, let’s talk about maximizing it post-turbo, too. Getting a larger and more efficient intake manifold will ensure that you are getting the most out of your air intake and turbocharger. It doesn’t matter how effective your Holset turbo is working if air runs into a bottle neck before the engine. 

In addition to increasing airflow, intake manifolds can also lower the exhaust gas temperature (EGT) and provide NPT ports. NPT ports allow you tap in for boost gauges and also for water-methanol injection. While a water-meth system is usually reserved for racing applications, they can be used on moderate builds to make some extra power while fighting against engine knock. 

Make sure to check out our 5.9 Cummins intake manifold upgrade guide. There we have a full breakdown of the best designs and selections. 

3) Upgraded Downpipe

After upgrading the intake system on the 12v 5.9 Cummins, your next step is to open up the exhaust some more. The stock exhaust is very restrictive once you get past stock power levels, meaning it has to be upgraded for any other mods to see any noticeable gains. You can still see some power increases with fueling and air intake, but everything will still be hamstrung and bottle-necked by the stock exhaust.

On turbocharged engines, the exhaust connects directly to the “hot side” of the blower. This part is known as the downpipe, because the exhaust usually leads down from the turbo to the undercarriage of the truck. Increasing the diameter of the downpipe to a full 4 inch system is a great way to reduce the back pressure your turbo is experiencing. In a nutshell, engines operate due to pressure differentials, and reducing back pressure is the best way to increase the differential and increase air flow.

As far as exhausts go, the MBRP 4 inch downpipe for the 1994+ 5.9 Cummins is a great option for the 12-valve. MBRP are one of the top diesel exhaust manufacturers for a wide range of engines, and they have a great reputation for quality. 

4) Upgraded Lift Pump

Our final recommendation for 12v 5.9 Cummins upgrades is to get a larger lift pump. As you’ve probably guessed by now, the weakest link for performance in the 12v 5.9 Cummins is the fueling. While you are pretty limited if you have the VE44 Rotary Injection Pump, the P-Pump can be modified for improvement.

Adding a larger lift pump supplements the P7100 as does numerous things, most important of which is to increase fuel pressure to the engine. This results in increased horsepower, torque, and creates a wider power band. It also makes the throttle more responsive on and off boost. 

By far, the most popular lift pump for the 5.9 Cummins is the AirDog FP-150 4G. It is the newest offering from AirDog, and is a revision of their previous model. It’s not the easiest install and it’s the priciest mod on our list, but it is the best way to increase fueling. 

Conclusion

The 12v 5.9 Cummins has long been regarded as one of the top turbo-diesel engines ever made, and it’s not too hard to see why. They are extremely durable and reliable, and they sound glorious. One knock on them is that by today’s standards they are pretty underpowered, but that can be largely rectified with just a few mods. For those with the 1994+ P-pump, there are several free mods you can do to increase fueling on the 12-valve Cummins, which can add as much as 50 horsepower.

There are also several bolt-on mods, starting with a cold air intake. Adding a performance intake will bump horsepower and torque and allow your turbo to work more efficiently. A performance intake manifold will help improve airflow post-turbo, which can be just as important. 

After taking care of the intake system, upgrading the downpipe to a full four inch diameter is the next step. Upgrading the exhaust will decrease back pressure and increase airflow, adding horsepower and torque. Finally, upgrading the p-pump by adding a larger lift pump is the best way to add fueling on the 12v, hands down.