Picking up an hx35 upgrade kit is easily one of the most satisfying projects you can tackle if you want to wake up your mid-90s to early-2000s diesel. If you're running a 12-valve or a 24-valve Cummins, you already know the Holset HX35 is basically the tank of the turbo world. It's reliable, it handles heat like a champ, and it's survived decades of abuse. But let's be real—by modern standards, it's a little lazy. It takes its sweet time to spool, and once you start throwing bigger injectors or a more aggressive tune at it, that stock compressor wheel just can't keep up with the airflow demands.
That's where the magic of a rebuild and upgrade comes in. Instead of dropping two grand on a completely new S300 or a fancy variable geometry turbo, you can spend a fraction of that and transform the charger you already have. It's about working smarter, not just throwing money at the engine bay and hoping for the best.
Why the stock HX35 needs a little help
The factory HX35 was designed back when 160 to 235 horsepower was considered "plenty" for a heavy-duty pickup. Fast forward to today, and most of us are looking for double that just to keep up with modern traffic while towing a trailer. The main bottleneck is the compressor wheel. The stock cast wheel is heavy, and its blade geometry is, frankly, pretty outdated. It moves enough air for stock fueling, but as soon as you step things up, it starts to get inefficient, which leads to sky-high exhaust gas temperatures (EGTs) and a whole lot of black smoke that isn't doing anyone any favors.
When you install an hx35 upgrade kit, you're usually swapping that old cast wheel for a billet aluminum version. This is where you'll notice the biggest difference. Billet wheels are machined from a solid chunk of aluminum, which allows the blades to be much thinner and more aggressive. Because they're lighter, they have less inertia, meaning your turbo starts spinning—and making boost—much earlier in the RPM range.
Breaking down the components of the kit
So, what are you actually getting when you order one of these? Most kits aren't just a single part; they're a bundle designed to refresh the entire center section of the turbo while boosting its capacity.
The Billet Compressor Wheel
As I mentioned, this is the star of the show. You'll often see these categorized by their inducer size, like a 60mm or 62mm upgrade. A popular choice for a daily driver is the 60mm wheel. It's a significant jump over the stock 54mm or 56mm sizes without being so big that it creates massive turbo lag. You get more "lb/min" of airflow, which basically means more oxygen for your fuel to burn, resulting in a cleaner, more powerful combustion cycle.
The Rebuild Component
You can't just throw a high-performance wheel onto a tired, high-mileage bearing housing and expect it to last. A proper hx35 upgrade kit includes a full rebuild set: 360-degree thrust bearings, journal bearings, O-rings, and seals. The 360-degree thrust bearing is a huge deal. The stock version is usually a 270-degree bearing, which is fine for stock boost levels, but when you start pushing 35 or 40 PSI, you need that extra surface area to handle the increased load and keep the shaft stabilized.
The Compressor Cover
Depending on how big of a wheel you choose, some kits come with a new compressor housing (the "cold side" snail shell) or require you to have yours machined. If you're going for a direct bolt-on experience, many guys opt for the kits that include a pre-machined cover. It saves you a trip to the machine shop and ensures the tolerances are tight, which is crucial for efficiency.
The spool-up and the "Whistle"
Let's talk about the fun stuff: the sound and the feel. We all love that distinct Cummins turbo whistle, and a billet wheel upgrade usually turns that volume knob up to eleven. Because the blade profile is different and the wheel is moving more air, the acoustics change. It's that crisp, sharp siren sound that lets everyone know you're not exactly running a stock setup.
But more importantly than the sound is the throttle response. If you've ever pulled a heavy trailer up a grade and felt like you were waiting forever for the boost to climb so you could finally gain some speed, you'll appreciate the faster transient response. A billet-equipped HX35 feels much "snappier." You touch the pedal, and the boost gauge starts moving almost instantly. It makes the truck feel five hundred pounds lighter than it actually is.
Controlling the heat
One of the biggest struggles with the 5.9L engine when you start adding power is managing EGTs. If your turbo can't move enough air to burn the fuel your injectors are spraying, that unburnt fuel just turns into heat. That heat soaks into your manifold, your head, and your pistons.
By upgrading the compressor side, you're forcing more cool air into the cylinders. This extra air leans out the air-fuel ratio in a good way, helping to "blow out" the heat. Many guys report a 100 to 200-degree drop in peak EGTs after installing an hx35 upgrade kit, even while making more power. That's the kind of reliability insurance that pays for itself, especially if you use your truck for actual work.
Is it a DIY job?
A lot of people wonder if they can do this in their own garage. The short answer is yes, but you need to be patient. You aren't just swapping a bolt; you're diving into the "guts" of a precision machine that spins at over 100,000 RPM.
The biggest hurdle for most is the balancing act. Technically, the individual components are component-balanced at the factory, but many enthusiasts recommend getting the entire rotating assembly (shaft, turbine, and new compressor wheel) balanced by a shop once it's put together. However, plenty of people have had great success with "cloning" the balance or relying on the component balance of high-quality kits.
You'll need some basic tools, a good set of snap ring pliers (the big ones!), and a very clean workspace. Dirt is the absolute enemy of turbo bearings. If you take your time, keep everything surgical-room clean, and follow the torque specs religiously, it's a very doable weekend project.
Making the most of your upgrade
Once you've got your hx35 upgrade kit installed and the turbo is back on the truck, you might need to look at a few supporting mods to really see the benefits. Since you can now safely run more boost, you might want to look at an adjustable wastegate actuator or a "boost elbow." This allows you to keep the wastegate closed a bit longer to take advantage of that new, high-flow compressor wheel.
Also, don't forget about the intake and exhaust. If you're pushing more air out of the turbo, it needs to be able to get into the turbo easily. A high-flow intake filter and a 4-inch exhaust are the perfect teammates for a modified HX35. They basically complete the circuit, allowing the engine to breathe in and out without any restrictions.
Final thoughts on the upgrade
At the end of the day, an hx35 upgrade kit is about balance. It's for the guy who wants more power and better towing performance without turning his truck into a dedicated drag racer that's a nightmare to drive on the street. It keeps the legendary reliability of the Holset frame but gives it the lungs it needs to compete with much newer trucks.
It's an affordable, effective, and honestly fun way to get more out of your rig. Whether you're chasing that sweet turbo whistle or just want to keep your temps in check while hauling a load, this is one of those upgrades you'll feel every time you step on the gas. It breathes new life into an old legend, and really, that's what keeping these classic diesels on the road is all about.