Released in 2017, the 6.6L Duramax L5P is the latest Chevy & GMC turbo-diesel engine. The L5P produces 445hp and 910lb-ft. of torque from the factory. When dyno’d the L5P produces about 400whp and 850wtq.
You can’t ask for much more power from the factory. However, for those who are looking for some extra power and performance a handful of simple bolt-on performance upgrades can take the L5P past the 550whp and 1,200wtq territory. Getting to and beyond these power levels will require some additional supporting mods which we’ll discuss below. Fortunately, the L5P has a lot less performance inhibitors compared to previous Duramax diesels.
This guide is going to cover the 5 best L5P performance upgrades. Additionally, we’ll cover L5P power limits, and some of the supporting components that need to be upgraded to achieve various power levels. Pushing past the 600whp mark gets very expensive so while we will discuss necessary mods for these power levels, our 5 best upgrades are going to focus on the less expensive and more achievable bolt-on mods.
Duramax L5P Power Limits
Upgrading the L5P is fairly easy, however, there are a few important hurdles to discuss. Fortunately, the L5P has less restrictions and weak points than previous Duramax’s such as the LB7 or LML. We’ll cover these items in a bit more depth below, but here are the biggest considerations:
- The Allison 1000 transmission will need to be built around 550whp, getting it to handle 500whp will require some mods/tuning
- 550whp is pushing the limit on the stock VGT BorgWarner turbo
- The ECM is tough to crack which means tuning options are a bit more limited and expensive
- The stock fueling system can handle 750whp+ (will need larger injectors)
While the ECM is tough to crack, there are still a handful of tuning options we’ll discuss in our mods list. Additionally, the stock fuel pump, lift pump, and injectors are surprisingly good for 750whp which saves a lot of money and headache when it comes to modding.
A1000 Transmission Limits
As is true with any Duramax truck, the Allison 1000 transmission in the L5P is the biggest weak point. The stock A1000 runs at 80psi of line pressure which is really low. This impacts it’s ability to hold power and put it to the ground. The transmission can handle 550whp before needing to be built, but you will need to address line pressure prior to this.
At 500whp or anywhere near it we recommend tuning the transmission (TCM tuning) and installing an EPC Solenoid Fooler which will allow the transmission to constantly run over 200psi of line pressure. This will help the A1000 hold power and maintain some reliability at 500whp.
At 550whp, and maybe even a bit before, you’ll need to build out the Allison which will cost about $5,000.
BorgWarner VGT Turbo Limits
The stock L5P turbo is a BorgWarner 61mm variable geometry turbo. At 550whp the turbo is pretty much completely maxed out. I’ve seen some claims of 600whp on the stock turbo, however this would be the absolute limit. Additionally, at those power levels you are at a big risk of blowing the turbo and potential damaging the internals of the engine.
Therefore, we recommend replacing it a 550whp. And if you plan on running a consistent 550whp it is also worth replacing as reliability is questionable at those power levels. Most opt for an upgraded S300-series or S400-series fixed geometry BorgWarned turbo for upgrades.
Best Duramax L5P Performance Upgrades
- ECM Tuning
- Cold air intake
- 5″ exhaust
- Intake horn & resonator pipe
1) Duramax L5P Tuning
Engine tuning is the single best L5P performance upgrade. It offers the biggest power gains at the lowest cost per horsepower. However, tuning on the L5P isn’t as simple as hooking up a flash tuner such as SCT. Fortunately, there are still a few options on the market.
The two options are custom tuning or getting a plug-and-play piggyback tuner. This topic is a bit more confusing since the tuners for the L5P aren’t the same as the flash tuners we are used to for pretty much all other diesels. If you are just looking for a bit of extra power get a piggyback tuner. If you need precision and maximum horsepower gains then you need to modify the ECM and go the custom tuning route.
Since the stock ECM is pretty much uncrackable, custom tuning requires you to pull your ECM and send it to HP Tuners for modification. HP Tuners can modify the ECM so that it can then be custom tuned. This service costs about $700 and does not include the tuning. You also have the option to buy a new ECM for $1,500 if you don’t want to send in your ECM.
This is the best option for anyone that needs custom tuning. Unless you are planning on upgrading the turbo and shooting for crazy power levels then you are probably best off opting for a piggyback tuner. Not to mention you will probably end up running $2,000 to $2,500 if you go this route as custom tuning is expensive for the L5P.
L5P Piggyback Tuners
Most diesel tuners you are familiar with, like Edge/SCT/Bully Dog and so on, are flash tuners. They are handheld devices that plug into the OBDII port and allow you to alter the ECU by flashing new maps into it. Piggyback tuners work a bit differently. They connect directly to the ECM, so they don’t come with any handheld devices. Additionally, rather than re-writing or re-flashing the stock tune they instead trick the ECM by adjusting certain signals and sensor readings.
There are a few options on the market like the Banks Power Derringer and the Edge Pulsar module. Both options are around the $800 mark and are simple plug-and-play.
One potential downside to a piggyback tuner is that they don’t offer the ability to do custom tuning. However, most tuners will still come with multiple maps which can easily by changed on the fly via the steering wheel controls.
Piggyback vs. flash tuning is a more complicated topic so check out this article if you want to learn more.
L5P Piggyback Tuner Performance Benefits
- ~100hp gains
- ~150lb-ft. torque gains
- Improved gas mileage
- DME remains stock
- Easy install
- Allows for on-the-fly map changes
2) Duramax L5P Cold Air Intake Upgrade
Upgrading the L5P intake is our second favorite mod since it’s highly complimentary with a tune. Adding an intake with a tune will help maximize power gains from both the tune and the intake. Additionally, it will help reduce stress on the turbo and allow the engine to breathe better.
Tunes increase power from increasing the boost or psi of the turbo. When boost is increase, the intake air is compressed even further. With the air being compressed even further it’s important to bring more air into the intake system as the turbo needs to compress a greater volume of air for the engine. Not only will adding an intake result in some decent power gains, it will also help improve gas mileage, throttle response, and turbo spool.
We prefer open intakes which don’t have enclosed box designs since they flow more air. An open intake will usually increase airflow by about 50% whereas closed systems will provide increases closer to the 30% range. The increased airflow will provide better power gains and less restriction.
If you are in California and aren’t allowed to upgrade your intake, you can instead upgrade the intake resonator piping. This is fully legal in California and reduces the restriction created by the stock intake piping. The stock pipe has a silencer in it which results in significant air restriction.
6.6L L5P Cold Air Intake Performance Benefits
- Up to 20hp and 40tq with a tune
- Better turbo spool
- Quicker throttle response
- 1-2mpg increase
- Cool intake sounds
The power gains quoted are when used in conjunction with a tune. Additionally, these power gains are generally max power gains throughout the power curve and not gains at the peak of the curve. Banks quotes 21whp when combined with their Derringer tune.
Best Upgraded Intakes
3) 6.6L Duramax 5″ Exhaust System
With an intake and tune, we are now flowing a lot more air into the engine. Therefore, we need to increase the outflow of the exhaust air as it leaves the engine. Unfortunately, you can’t install a turbo-back exhaust system without deleting the DPF and SCR systems. You can upgrade the downpipe which we do recommend and will cover later. The next best option to a full turbo-back exhaust is a 5″ DPF-back exhaust system.
A freer exhaust system will reduce exhaust backpressure. Backpressure is bad for turbo diesels. It increase the amount of force against the turbine wheel which impacts the turbo’s ability to spool. This requires the turbo to work harder and overall has a negative impact on turbo reliability. Additionally, it results in higher EGTs.
A larger L5P DPF-back exhaust system will help reduce backpressure and lower EGTs. This results in a healthier and more reliable engine.
Upgrading the downpipe is also highly recommended as it reduces the most exhaust backpressure outside of deleting the emissions systems.
L5P 5″ Exhaust Performance Benefits
- 5-10whp and 10-15wtq gains
- Less exhaust backpressure
- Lower EGTs
- Healthier and more reliable engine and turbo
Reducing EGTs and backpressure is important since both of these items have negative affects on the turbo and its reliability. Adding a tune, intake, and exhaust system will add about 100whp. Considering the turbo is only capable of handling about 550whp it will be getting close to its upper limit with these few mods added. Therefore it’s important to reduce backpressure and lower EGTs as much as possible to help maximize reliability.
4) L5P Intake Horn Upgrade
An intake horn is a common term for the intake manifold. The stock intake horn is plastic and only has a small 2.4″ diameter. Once an upgraded cold air intake is added, the small diameter stock intake horn becomes restricted. Upgrading the intake horn to a 3.5″ diameter steel pipe will increase air flow and therefore increase power.
Wehrli Custom achieved a 25whp gain with their 3.5″ intake horn upgrade, but note this was on an aggressive race tune and heavily modified truck with an upgraded turbo. You can expect about 10whp on a normal tune and lightly modified truck. Their intake horn will fit with any intake system and doesn’t require any additional mods or tuning.
Upgrading the intake horn is a simple, cheap, and effective way to increase air flow and add a solid 10whp to your L5P.
Intake Horn Upgrade Benefits
- Approx. 10whp with a tune and 15-20wtq
- Improved air flow
- Easy bolt-on install
- Looks cool under the hood
5) 6.6L L5P Downpipe Upgrade
Despite putting this last on the list, it’s actually our favorite exhaust upgrade. A big 5″ exhaust system will add good sound, but a downpipe will actually provide better power gains and the most backpressure reduction. A downpipe bolts up directly to the turbocharger and connects with the emissions portion of the exhaust (the diesel oxidation catalyst specifically). Since it bolts to the turbo it has the most impact on backpressure.
The stock downpipe is pretty small and skinny which restricts airflow. Upgrading to a larger 3.5″ downpipe will help improve exhaust flow and therefore reduce backpressure. This will allow the turbo to spool more freely, improving its reliability and increasing power. Additionally, it will have a big impact on lowering EGTs.
No tuning is required for an upgraded downpipe. Although, as with any mod on this list, adding a piggyback tuner or going the custom tuning route will help amplify the gains achieved. Upgrading the downpipe on a stock L5P will have some small benefits like slightly lower EGTs, but the biggest gains and benefits come when you are running a tune and intake modifications.
L5P Downpipe Upgrade Benefits
- Approx. 10whp and 10-20wtq
- Reduced exhaust backpressure
- Improved turbo reliability
- Lower EGTs
Duramax L5P Performance Mods Summary
Five bolt-on upgrades can add 100-125whp to a 6.6L L5P Duramax. A stock L5P will dyno around the 400whp and 850wtq range. Adding a tune, cold air intake and horn, exhaust, and downpipe can easily push the L5P into the 500whp and 1,000wtq power range. Fortunately, the L5P doesn’t require many supporting mods as its fuel system can handle in excess of 750whp and the turbo is good for about 550whp.
The one concern with the L5P is the Allison 1000 transmission which is only capable of handling about 500whp. Even if you only plan on adding a tune we’d recommend doing the EPC Solenoid Fooler mod and getting TCM tuning to improve transmission reliability. If you plan on upgrading the turbo, you’ll need to build out the Allison transmission and get custom ECM tuning.
If you’re looking to keep things on a budget and want to aim for slightly more power and better MPG, then drop a Derringer tune and cold air intake on the L5P. For the majority of performance folks this will be enough. If you keep it on a modest tune you shouldn’t have any issues with the A1000 transmission.
What mods are you running on your 6.6L L5P Duramax?