If you're looking to push past the limits of your stock truck, an l5p s400 turbo swap is probably at the top of your wishlist. It's one of those modifications that completely changes the personality of the 2017 and newer Duramax engines. For a long time, the L5P was considered a bit of a "locked" platform because of the complex ECM encryption, but now that we've cracked that nut, the sky is pretty much the limit. Bolting on a fixed-geometry S400 frame turbo is how you turn a heavy-duty workhorse into a legitimate street beast.
Why Move Away From the Stock VGT?
The stock variable geometry turbo (VGT) on the L5P is actually a pretty impressive piece of engineering. It's designed to provide instant boost off the line, help with exhaust braking, and keep things efficient while towing. But let's be real—it has its limits. Once you start asking for more than 550 or 600 horsepower, that stock unit starts to become a bottleneck. It builds up a ton of backpressure, the EGTs (Exhaust Gas Temperatures) start to climb, and it just runs out of breath at the top end.
Moving to an l5p s400 setup means ditching that VGT in favor of a fixed-geometry BorgWarner S400 chassis. You lose the moving vanes inside the turbo, which simplifies things significantly. While you might lose a tiny bit of that "instant" spool-up you get at a stoplight, you gain a massive amount of flow. It's the difference between breathing through a straw and breathing through a sewer pipe. When that S400 hits, it hits hard, and it keeps pulling all the way to the redline.
Choosing the Right S400 Size
Not all S400s are created equal. Depending on what you're doing with your truck, you'll have to choose between a few different compressor wheel sizes. The most common ones for the L5P platform are the S464, S467.6, and the S472.
The S464 is great if you still want to use your truck as a truck. It spools relatively quickly and can still handle towing duties without making you miserable. If you're looking for a solid 650-700 horsepower range, this is a fantastic "daily driver" turbo.
Then you've got the S467.6, which is arguably the most popular choice for the l5p s400 conversion. It's the sweet spot for most guys. You get enough air to support well over 700 horsepower, the sound is incredible, and the lag is manageable once you get the tuning dialed in. It turns the L5P into a different animal on the highway.
If you're going for broke and want to see 800+ horsepower, you're looking at an S472 or even larger. Just keep in mind that at this size, you're going to need supporting mods like bigger injectors and a better stroker pump to actually feed that much air.
What Goes Into the Installation?
You can't just bolt an S400 directly to the factory manifold and call it a day. Because the factory turbo is a VGT and has a specific mounting footprint, you're going to need a full conversion kit. This usually includes a new T4 pedestal, a custom downpipe, and all the oil feed and drain lines.
The plumbing is usually the trickiest part. You'll need a new intake horn and likely some custom intercooler piping to bridge the gap between the new turbo outlet and your factory intercooler. Most high-quality kits on the market today make this pretty much "bolt-on," but it's still a big job. You're looking at a full day or a weekend in the garage, especially if you're doing it on jack stands.
One thing people often forget is the cooling system and the firewall clearance. The S400 frame is significantly larger than the stock Garrett unit. It's a tight squeeze back there, and you'll want to make sure everything is heat-shielded properly so you don't melt any wires or hoses.
The Role of Tuning
You absolutely cannot run an l5p s400 setup on a stock tune. Since the truck is expecting to see a VGT turbo with an electronic actuator, it'll go into limp mode the second it realizes that actuator is gone. You need custom tuning (usually via HP Tuners) to tell the ECM that it's now running a fixed-geometry turbo.
Tuning for a fixed turbo is a bit of an art form. You have to balance the fuel delivery to help the turbo spool up without creating a massive cloud of black smoke. Because you don't have those VGT vanes to "pinch" the exhaust flow at low RPM, the tuner has to use fueling strategies to get the turbine spinning. Once it's dialed in, though, the power delivery is incredibly smooth. It's a more linear, mechanical feeling than the "on-off" switch feeling of some VGT setups.
Driving Experience and Sound
Let's talk about the sound, because honestly, that's half the reason people do this. The "whistle" of an S400 is iconic. While the stock L5P is pretty quiet (even with a straight pipe), an S400-equipped truck has that deep, gutteral howl at idle and a high-pitched scream when you're under boost. It sounds like a jet engine taking off.
On the road, the driving dynamics shift. You'll notice that you have to be a little more intentional with your right foot. If you're at a low RPM in a high gear, you can't just floor it and expect instant torque. You might have to wait a second for the boost to climb, or wait for the transmission to downshift. But once you hit about 10 to 12 pounds of boost, the truck just rockets forward. For many, that trade-off is well worth it for the top-end power gains.
Is it Reliable for Daily Driving?
A lot of guys worry that going to a big "racing" turbo will ruin the reliability of their L5P. In reality, it can actually be the opposite. Fixed-geometry turbos are incredibly simple. There are no vanes to get stuck with soot, and no electronic actuators to fail. BorgWarner's S400 series is known for being "old reliable" in the diesel world.
The main thing to watch is your EGTs if you're still towing heavy. Since you don't have the VGT to help with low-end boost, you can see higher temps if you're lugging the engine up a hill. As long as you keep an eye on your gauges and use common sense, an l5p s400 setup can easily last over 100,000 miles. It's all about the quality of the kit and the quality of the tune.
Supporting Modifications to Consider
If you're going through the trouble of doing a turbo swap, you should probably look at a few other things while you're under the hood.
- Lift Pump: The L5P finally came with a factory lift pump, but if you're pushing big power, an aftermarket FASS or AirDog system is still a good idea for better filtration and consistent pressure.
- Transmission: The Allison 10-speed (or the older 6-speed depending on your year) is tough, but it's not invincible. If you're pushing 700+ hp with an S400, you're on borrowed time with a stock torque converter and clutches.
- Head Studs: While the L5P bottom end is incredibly stout, it's never a bad idea to throw some head studs in if you plan on running high boost levels (over 45-50 psi).
Final Thoughts
At the end of the day, switching to an l5p s400 isn't just about the numbers on a dyno sheet. It's about making the truck yours. It changes the way it sounds, the way it pulls on the highway, and the way it responds to your input. It takes away some of the "nanny" feel of the modern VGT system and gives you back that raw, mechanical diesel experience.
Whether you're building a dedicated track truck or just a really fast daily driver that can surprise some sports cars at a stoplight, the S400 swap is a proven path to getting the most out of the Duramax platform. It's a big investment, but the first time you hear that turbo spool up and feel the truck settle into its power band, you'll know exactly where that money went.