If you've been looking to swap an hx35 turbo for 6.5 diesel engines, you probably already know that the stock GM-X series turbos leave a lot on the table. The factory turbo on the old Detroit 6.5L was "fine" for its time, but let's be honest—it's a bit of a hair dryer. It generates a ton of heat, creates massive backpressure, and starts to choke out just when the engine feels like it's finally getting into the power band.
Replacing that factory unit with a Holset HX35 is one of the most popular mods for these trucks, and for good reason. It's not just about chasing high horsepower numbers; it's about making the truck more efficient, lowering your exhaust gas temperatures (EGTs), and actually enjoying the drive when you've got a trailer hooked up behind you.
Why the stock turbo struggles
The stock turbos, like the GM-4 or GM-8, were designed with a very small turbine housing. This was great for quick spooling in city traffic, but it's a nightmare for performance. When you're pulling a grade, the engine is trying to push out a lot of exhaust, but that tiny housing acts like a cork in a bottle.
This restriction causes backpressure to skyrocket. On a stock 6.5, it's not uncommon to see a 2:1 or even 3:1 ratio of backpressure to boost. That means your engine is working way harder than it needs to just to move air. All that trapped heat eventually leads to cracked heads or blown head gaskets, which are the two things every 6.5 owner fears most.
Enter the HX35 solution
The Holset HX35, typically found on the 12-valve and early 24-valve 5.9 Cummins engines, is almost the "Goldilocks" turbo for a 6.5 Detroit. It has a larger 12cm² exhaust housing (usually) that allows the engine to breathe much better than the stock GM unit.
When you run an hx35 turbo for 6.5 diesel applications, you're basically giving the exhaust a bigger exit door. This immediately drops your backpressure. Instead of that 3:1 ratio, you get something much closer to 1:1. The result? Your EGTs drop significantly—sometimes by 150 to 200 degrees under load—and the engine doesn't feel like it's fighting itself at higher RPMs.
Spool-up and driveability
One concern people have is whether a bigger turbo will be "laggy." If you're used to the instant, almost electric-like spool of a tiny GM-4, you might notice a slight difference, but it's barely there. The HX35 is still a relatively small turbo in the grand scheme of things. It starts building boost early enough that you won't feel like the truck is a dog off the line, but once it hits, it pulls much harder and much longer than the stocker ever could.
What you need for the swap
Now, it's not a "bolt-on and go" project in the simplest sense. You can't just unbolt the GM-8 and slap the Holset on the factory manifold because the flanges are totally different. The 6.5 uses a weird rectangular flange, while the HX35 uses a standard T3 flange.
The adapter plate
Most guys go one of two ways: they either buy a pre-made adapter plate or they buy a custom manifold. The adapter plate is the budget-friendly route. It bolts to your factory manifold and provides a T3 mounting surface for the HX35. Just keep in mind that this raises the turbo up an inch or two, which can lead to some tight clearances with the hood or the air intake plumbing.
Oil lines and cooling
The 6.5 turbo is oil-cooled, and so is the HX35. However, the fittings are different. You'll need to get some custom oil feed and drain lines made up. Most hydraulic shops can whip these up for you, or you can find kits online specifically for this swap. One nice thing is that you don't have to worry about coolant lines for the HX35, which simplifies the plumbing a bit compared to some other aftermarket options.
The wastegate situation
This is where things get a bit "choose your own adventure." The stock 6.5 uses a vacuum-actuated wastegate controlled by the truck's computer (the PCM). The HX35 uses a mechanical pressure-actuated wastegate.
You've got two real options here: 1. Mechanical Spring Gate: You can build or buy a simple spring-loaded tensioner (often called a "turbomaster" style) to hold the wastegate shut until a certain boost pressure is reached. 2. Boost Controller: You can use a manual boost controller to reference the actual boost pressure coming off the turbo to open the gate.
Whatever you do, you'll likely need to get a "boost bolt" or tap your intake manifold to give the wastegate (and your boost gauge) a clean signal.
Dealing with the downpipe
The exhaust outlet on an HX35 is different from the GM series. You're going to need a custom downpipe or an adapter. Most people find that a 3-inch or 4-inch diamond eye downpipe can be modified to work, but expect to do a little bit of cutting and welding to get the clocking just right. It's a bit of a chore, but having a full 4-inch exhaust behind an hx35 turbo for 6.5 diesel is where you really start to see those EGT benefits.
Tuning and fueling
Can you run an HX35 on a bone-stock 6.5 with a stock tune? Technically, yes. The mechanical injection pump (DB2) or the electronic pump (DS4) will still move fuel, but you won't be getting the most out of the turbo.
If you have the electronic DS4 pump (found in 1994+ trucks), the computer might get grumpy when it sees boost levels climbing higher than the factory parameters. It might even throw the truck into "limp mode." To really make this swap shine, you'll want a custom tune. A good tune will adjust the fuel maps to match the increased airflow, making sure you aren't running lean and that you're actually making use of that extra oxygen.
For those with the mechanical DB2 pump (pre-1994 or converted trucks), you'll just want to turn the fuel screw up a flat or two. Just watch your smoke levels and your EGT gauge.
Is it worth the effort?
If you use your truck for anything other than a grocery getter, then absolutely. The 6.5 diesel is a great engine, but it was suffocated from the factory to meet emissions and noise standards of the 90s. By installing an hx35 turbo for 6.5 diesel setups, you're essentially letting the engine work the way it was meant to.
The biggest "win" isn't the extra 30 or 40 horsepower you might pick up; it's the peace of mind. Knowing that when you're pulling a long hill in the middle of July, your EGTs aren't climbing into the "melt my engine" zone is worth every bit of fabrication work. Plus, let's be honest—the whistle of a Holset turbo just sounds a whole lot cooler than the stock setup.
It's a weekend project for most guys with basic tools and a bit of mechanical patience. You'll spend some time hunting for fittings and maybe a bit of time swearing at the downpipe bolts, but once you feel that mid-range pull and see those lower temperatures on your gauges, you'll wonder why you didn't do it years ago.