Holset HX35 vs HX40: Which Turbo Is Right for You?

Holset HX35 Turbo Rebuild Service →  |  Holset HX40 Turbo Rebuild Service →

Quick Overview

The Holset HX35 and HX40 are two closely related turbochargers that show up constantly in the Cummins world, but they serve slightly different purposes. The HX35 is the smaller frame of the two and is the most common factory turbo found on 12-valve and early 24-valve 5.9L Cummins trucks. The HX40 is a larger frame unit that came as original equipment on Cummins ISC medium-duty trucks and has become one of the most popular upgrade swaps for owners pushing past what the HX35 can comfortably support.

The core tradeoff is simple. The HX35 spools quickly and feels responsive but runs out of breath at higher power levels, while the larger HX40 supports significantly more airflow at the cost of slightly slower spool. Choosing between them comes down to how you use your truck and where you want to land on the response versus airflow scale.

Which Spools Faster?

The HX35 is the quicker spooling turbo of the two. Its smaller frame lets it build boost sooner, which makes it feel snappy and responsive in everyday driving. The larger HX40 spools slightly slower as a direct result of its bigger frame and greater airflow capacity. That added size takes a touch more exhaust energy to get moving, so you trade a small amount of low end responsiveness for the ability to flow more air up top. For most drivers the HX40 spool difference is noticeable but not dramatic.

Which Supports More Power?

The HX40 is the clear winner when it comes to supporting more power. It is a larger frame turbo that supports significantly more airflow than the HX35, which is exactly why it is such a common upgrade swap on performance builds. The HX35, while excellent in stock and lightly modified form, becomes flow-limited at higher power levels. Once you start pushing past what the smaller frame can move, the HX40 gives you the additional airflow headroom needed to keep making power.

Which Is Right for Your Application?

For daily driving, towing, and mostly stock trucks, the HX35 is hard to beat. Its quick spool makes it responsive off the line and easy to live with, and it handles factory and mild power levels without complaint. If your goal is a dependable work truck or tow rig, the HX35 frame is a natural fit.

For performance and higher horsepower builds, the HX40 is the better choice. Because it supports significantly more airflow, it is the go-to upgrade swap for owners who have outgrown the flow ceiling of the HX35. You give up a little spool, but you gain the airflow capacity that performance and racing applications demand. If you are building for power and willing to accept slightly slower response, the HX40 is the smarter long term pick.

Rebuild Considerations

Both turbos come from the same Holset HX family and share similar construction, so they rebuild in much the same way using comparable core components and procedures. Neither is dramatically harder or more expensive to service than the other. The main practical difference is that the HX40 uses a larger frame and bigger wheels, so parts are sized accordingly, but the overall rebuild process and effort are very similar between the two.

Quick Comparison

How to Tell Them Apart

The reliable way to tell a Holset HX35 from a HX40 is the identification tag, not appearance alone. The housings on adjacent frame sizes within the same family can look similar at a glance, so confirm the model designation and assembly number stamped or plated on the unit before ordering rebuild parts. Exact configuration (wheel trim, housing A/R, calibration) varies by specific part and assembly number even within the same model name, so the assembly number is what actually determines which parts are correct for your unit.

For the full process, see our How to Identify a Holset Turbo guide.

Frequently Asked Questions

PT Turbo Warranty

• 12-month, unlimited mileage warranty against defects in parts and workmanship
• Final quality inspection on every rebuild: leak test, visual inspection, actuator or wastegate function check
• In-house CIMAT VSR balancing with a balance report included
• Nationwide mail-in service — you don’t need to be local

Pricing

Most rebuilds require inspection before final pricing. Standard rebuild pricing depends on the turbo model, parts availability, wheel condition, shaft condition, housing damage, and whether upgrades are requested. PT Turbo inspects every turbo and sends a quote before any work begins — no surprise charges.