Cummins ISX15 Turbo Upgrades: HE400VG, S400 and Compound Kits
Posted by Will on May 23, 2025
Cummins ISX15 Turbo Upgrades: HE400VG, S400 Conversion and Compound Kits
The stock turbocharger on the Cummins ISX15 is one of the most commonly replaced components on high-mileage and performance-built Class 8 trucks. Whether the goal is reliability restoration, performance improvement, or maximum output from a deleted engine, there are now three distinct upgrade paths available from Diesel Power Source® — each serving a different operating need and power target.
This guide explains each option, what it delivers, who it is right for, and how they compare to each other.
The ISX15 Stock Turbo Problem
The factory ISX15 turbo is sized conservatively for the average duty cycle. On a truck running moderate loads at moderate altitudes on flat terrain, it performs adequately. On a truck that regularly tows heavy GCW at altitude, runs sustained high-output routes, or is being pushed toward the higher end of its tuned power range, the stock turbo becomes the limiting factor — high EGTs, restricted airflow, and premature failure are the common results.
The factory HE400VG uses a variable geometry turbine housing controlled by an electronic actuator. Over time and high mileage, these actuators develop the same failure modes seen on the 6.7L Cummins in pickup applications — position faults, stuck vanes, and eventually reduced boost control. On a high-mileage ISX15 the turbo replacement is often the right time to evaluate whether a direct replacement or a meaningful upgrade better serves the truck's needs.
Option 1: HE400VG Direct Replacement
The Diesel Power Source® HE400VG replacement turbo for ISX15 and X15 is a direct-fit replacement for the factory VGT unit. It uses the same mounting pattern, the same actuator interface, and the same sensor provisions as the OEM turbo — it installs exactly where the factory unit was with no manifold changes or system modifications required.
The DPS HE400VG is available in 67mm and 70mm compressor wheel configurations. The 67mm is a direct match to the factory wheel diameter and restores stock performance with improved bearing life and durability over the original unit. The 70mm provides a meaningful airflow increase within the VGT housing — more top-end flow, modestly improved EGTs under load, while maintaining full factory functionality including the actuator-driven variable geometry and emissions system compatibility.
For trucks running emissions-compliant configurations or operating in jurisdictions with active emissions enforcement, the HE400VG replacement is the correct option. It preserves full emissions system functionality and requires no modifications to the factory setup. The HE400VG turbo and manifold kit is also available as a package for trucks replacing both components simultaneously.
Best for: Emissions-equipped trucks, reliability restoration, moderate performance improvement, trucks where factory functionality must be fully preserved.
Option 2: S400 Single Turbo Conversion
The S400 complete turbo conversion kit for ISX15 and X15 replaces the factory VGT system entirely with a fixed-geometry S400-series turbo and the DPS T6 exhaust manifold. This is the appropriate path for deleted ISX15 trucks targeting meaningful performance increases beyond what the HE400VG can deliver.
The S400 frame is substantially larger than the factory VGT - it flows more air, supports higher horsepower, and eliminates the variable geometry actuator and vane system as potential failure points. On a properly tuned deleted ISX15 the S400 conversion delivers significantly better sustained airflow under high-load conditions, lower EGTs at elevated power levels, and more robust turbo hardware than the factory unit.
The tradeoff of a large single turbo on a 14.9-liter engine is spool response at lower RPM, a large S400 needs more exhaust energy to spin up than the smaller factory VGT, which can make the truck feel less responsive at light loads and lower RPM. For trucks spending most of their miles at road speed under load this is a manageable tradeoff. For trucks that need immediate response from low RPM - frequent stops, grades from a standing start under load, variable speed work - the compound kit addresses this tradeoff more effectively.
Best for: Deleted trucks targeting meaningful power and EGT improvement over stock, straightforward single-turbo installation, trucks where top-end airflow is the priority.
Option 3: Compound Turbo Kit
The Diesel Power Source® ISX15 / X15 Compound Turbo Kit is the newest and most capable option in the ISX15 turbo lineup. It uses two BorgWarner S400-series turbos in a compound (series) configuration - a high-pressure S472 or S475 turbo feeding compressed air into a large atmospheric S488 - to deliver 550 to 700 HP with faster spool, broader power band, and cooler operating temperatures than any single turbo setup.
This is the same compound technology Diesel Power Source® has developed and proven across thousands of Cummins 5.9L and 6.7L pickup truck installations, engineered and scaled specifically for the 14.9-liter ISX15. The engineering approach is identical - matched turbo pairing, application-specific inter-stage piping, all components in one complete kit - applied to the demands of Class 8 commercial use.
Two configurations are available. The S472/S488 uses a 72mm high-pressure compressor for faster spool and stronger low-RPM response, targeting 550 to 650 HP - the better choice for towing-focused applications. The S475/S488 uses a 75mm high-pressure compressor for higher peak airflow, targeting 600 to 700 HP for trucks with more aggressive power requirements.
Both configurations preserve Jake brake functionality, mount in the factory position, and include all components needed for installation. ESN verification is required at order to confirm fitment for each specific engine configuration.
Best for: Deleted trucks targeting maximum performance, trucks needing both quick low-RPM response and high sustained airflow, owner-operators and fleets wanting the best available compound technology for ISX15.
Comparison: Which Option Is Right for Your ISX15
| Factor | HE400VG Replacement | S400 Conversion | Compound Kit |
|---|---|---|---|
| Emissions compatible | Yes | Deleted trucks only | Deleted trucks only |
| Jake brake preserved | Yes | Yes | Yes |
| Power range | Stock to modest gain | Meaningful increase | 550 to 700 HP |
| Low-RPM response | Factory VGT spool | Slower than VGT | Fast — HP turbo spools quickly |
| EGT reduction | Moderate | Good | Best — two-stage compression |
| Installation complexity | Direct replacement | New manifold required | Complete system — all included |
The Exhaust Manifold Factor
Regardless of which turbo upgrade path is chosen, the factory ISX15 exhaust manifold is worth evaluating at the same time. The OEM manifold uses seal-type joints between its three sections that degrade over time, leading to exhaust leaks and reduced turbine drive pressure. A cracked or leaking manifold directly limits what any turbo upgrade can deliver - the exhaust energy that should be driving the turbine is escaping before it gets there.
Diesel Power Source® offers the Non-EGR ISX15 exhaust manifold for deleted trucks and the EGR ISX15 exhaust manifold for emissions-equipped applications. Both use press-fit 3-piece HSMD ductile iron construction that eliminates the seal-type joint failures of the OEM design and delivers up to 22% more exhaust flow. For trucks undergoing any turbo upgrade, replacing the manifold simultaneously is the most efficient approach - the work is already halfway done.
For S400 conversion and compound kit installations, the T6 exhaust manifold for ISX15 is included in the conversion kit or available separately. The compound kit includes the DPS high-flow manifold as part of the complete package.
Frequently Asked Questions: ISX15 Turbo Upgrades
What is the best turbo upgrade for a Cummins ISX15?
The best ISX15 turbo upgrade depends on the truck's emissions status and power goals. For emissions-equipped trucks the HE400VG replacement in 67mm or 70mm is the correct path - it maintains full emissions compatibility and factory functionality. For deleted trucks targeting meaningful performance improvement, the S400 single turbo conversion delivers significantly better airflow and EGT reduction. For trucks targeting maximum output with the best combination of low-RPM response and sustained high-load airflow, the compound turbo kit in S472/S488 or S475/S488 configuration is the top-of-range option - 550 to 700 HP with Jake brake preserved.
Will a turbo upgrade on my ISX15 improve fuel economy?
In most cases yes, particularly on trucks that are currently running a degraded or undersized turbo. Improved airflow allows more complete combustion, which extracts more mechanical work from each combustion event and reduces the fuel that exits as unburned heat through the exhaust. The improvement is most measurable on loaded highway runs and mountain routes. Pairing the turbo upgrade with a properly calibrated tune maximizes the fuel economy benefit.
Does a Cummins ISX15 turbo upgrade require tuning?
For a direct HE400VG replacement at stock or near-stock power levels, tuning is generally not required. For the S400 conversion and compound kit on deleted trucks, tuning is strongly recommended to optimize fueling, boost targets, and drive pressure parameters for the upgraded turbo configuration. Running a significantly larger or compound turbo system without calibration leaves performance on the table and may affect drivability. A Cummins tuner with experience on ISX15 deleted applications can provide calibration specific to the Diesel Power Source® kit configuration.
What is the HE400VG turbo on a Cummins ISX15?
The HE400VG is the Holset variable geometry turbocharger used as the factory turbo on ISX15 and X15 engines. It uses an electronic actuator to control movable vanes in the turbine housing - the same VGT mechanism used on the 6.7L Cummins in pickup trucks, scaled for the larger 14.9-liter engine. The actuator adjusts vane position based on engine load and speed, allowing the turbo to spool quickly at lower RPM and flow high volumes at higher RPM. Over time and high mileage the actuator and vane assembly are the components most likely to require service or replacement.
How does the ISX15 compound kit compare to compound kits for Cummins pickup trucks?
The ISX15 compound kit uses the same engineering principles as the Diesel Power Source® pickup compound kits - two matched turbos in series, application-specific inter-stage piping, complete kit with all installation hardware - scaled for the 14.9-liter ISX15 engine and its commercial truck duty cycle. The ISX kit uses S400-series T6-flange turbos rather than the S300/S400 combinations used on pickup applications, reflecting the higher exhaust flow of the larger engine. The compound technology, matched-system approach, and BorgWarner turbo hardware are consistent across both pickup and Class 8 platforms.
Can I upgrade my ISX15 turbo without replacing the exhaust manifold?
For a direct HE400VG replacement, yes - the factory manifold can be reused if it is in good condition. For S400 conversion and compound kit installations, a new manifold is required because the T6 flange on the S400-series turbos does not match the factory manifold's turbo mounting flange. The S400 conversion kit includes the T6 exhaust manifold and the compound kit includes the DPS high-flow manifold as part of the complete package. Even for HE400VG replacements, if the factory manifold shows cracks or seal joint leaks, replacing it at the same time is more efficient than returning to the same job later.
Browse the full ISX15 and X15 turbo upgrade lineup at Diesel Power Source® ISX15 Performance, or contact the team directly with your ESN and power goals for a recommendation specific to your truck.
