4BT Compound Turbo Kit for 3.9L Cummins Swaps | Round Port (16V) | 1999 and Newer
The Diesel Power Source® compound turbo kit for the Cummins 4BT 3.9L engine (round port, 16V, 1999 and newer) is a complete twin turbo system that pairs a quick-spooling DPS HE351CW primary turbo with a high-flow S366 atmospheric turbo, delivering 200 to 500 HP, 22-27% more horsepower than a single turbo setup, and dramatically lower EGTs under load. This kit includes the manifold, both turbos, all piping, oil drain lines, downpipe, gaskets, clamps, and hoses for a complete installation.
This is the same compound turbo system as the rectangular port 4BT compound kit, with a round port exhaust manifold designed for 1999 and newer 4BT engines with 4 valves per cylinder (16V). The turbo system, piping, and all other components are identical. The only difference is the manifold port shape to match your cylinder head.
Round Port (16V) 4BT Engines
The 1999 and newer Cummins 4BT engines use a cylinder head with 4 valves per cylinder (16V total) and round exhaust ports. These engines are found in later-production industrial equipment, commercial vehicles, marine applications, and some medium-duty truck platforms. The round port head design provides improved flow characteristics compared to the earlier 8V rectangular port heads, and this manifold is cast specifically to match those round port dimensions.
If you are unsure which port shape your engine has, look at the exhaust side of the cylinder head where the manifold bolts on. Round ports indicate a 16V head (1999 and newer). Rectangular ports indicate an 8V head (1998 and older), and you need the rectangular port compound kit instead.
How Compound Turbos Work on a 4BT
A compound (twin turbo) system uses two turbochargers working in series. The smaller HE351CW primary turbo spools first at low RPM, delivering immediate throttle response on the 4BT's compact 3.9L displacement. As RPM and load increase, the larger S366 atmospheric turbo takes over the majority of the compression work, providing sustained airflow that a single turbo cannot deliver without lag on a 3.9L engine.
The two-stage compression delivers intake air that is over 100°F cooler than a single turbo at the same boost pressure, with boost-to-drive pressure ratios approaching 1:1. This means lower EGTs, more complete combustion, and more usable power across the entire RPM range.
Specifications
| Specification | Value |
|---|---|
| Primary Turbo | DPS HE351CW (quick spool) |
| Atmospheric Turbo | S366 (high flow) |
| HP Range | 200 to 500 HP |
| HP Increase | 22-27% over single turbo |
| Manifold | Single-piece ductile iron, T3 flange |
| Port Type | Round port (16V, 4 valves per cylinder) |
| Engine Fitment | Cummins 4BT 3.9L, 1999 and newer |
| CARB Status | Not CARB certified |
What's Included
- DPS HE351CW Primary Turbo (quick-spooling high-pressure turbo)
- S366 Atmospheric Turbo (high-flow low-pressure turbo)
- Single-Piece Exhaust Manifold with T3 flange (round port, ductile iron)
- Complete charge piping
- Oil drain lines
- 4" exhaust downpipe
- Gaskets
- Clamps and hoses
- All mounting hardware
Important: This is an aftermarket performance system designed for custom and swapped applications. Fabrication, tuning, and supporting modifications may be required depending on your specific build and engine bay configuration. Contact Diesel Power Source® with photos of your engine bay if you need fitment confirmation.
Key Features
- Complete Compound Kit — Both turbos, manifold, piping, downpipe, oil lines, and all hardware in one box
- 22-27% HP Increase — Over single turbo setups at the same fueling level
- 200 to 500 HP — Sized for the full 4BT power range from stock to built
- Lower EGTs Under Load — Two-stage compression runs dramatically cooler than any single turbo
- Faster Spool Than Single Turbo — HE351CW lights up fast on 3.9L displacement
- Round Port Manifold — Cast specifically for 16V (4 valves per cylinder) cylinder heads
- Proven DPS Compound Architecture — Same design philosophy used on thousands of 5.9L and 6.7L Cummins kits
- Compact Packaging — Designed for the tight engine bays common in 4BT applications
Fitment
This compound turbo kit fits Cummins 4BT 3.9L engines with round exhaust ports (16V, 4 valves per cylinder), 1999 and newer. These engines are found in later-production industrial, commercial, marine, and medium-duty vehicle applications.
This kit is designed for aftermarket and swapped applications. Fitment depends on engine placement, chassis configuration, and available clearances. Some fabrication may be required depending on your specific build.
Have a 1998 or older 4BT with rectangular exhaust ports (8V)? See the 4BT Compound Turbo Kit for Rectangular Port (8V).
Looking for a 4BT manifold only? See the 4BT Exhaust Manifold (Round Port).
Related 4BT Cummins Products
Frequently Asked Questions
Look at the exhaust side of the cylinder head where the manifold bolts on. If the exhaust ports are round, you have a 16V engine (4 valves per cylinder, 1999 and newer) and need this kit. If the ports are rectangular (wider than tall), you have an 8V engine (2 valves per cylinder, 1998 and older) and need the rectangular port compound kit.
Yes. The turbo system (HE351CW primary + S366 atmospheric), piping, downpipe, and all other components are identical between both kits. The only difference is the exhaust manifold, which is cast with round ports to match the 16V cylinder head on 1999 and newer 4BT engines.
The round port (16V) 4BT engines are found in 1999 and newer applications including later-production industrial equipment, commercial vehicles, marine applications, and some medium-duty truck platforms. These engines use a cylinder head with 4 valves per cylinder for improved flow characteristics.
This compound system supports 200 to 500 HP depending on fueling and supporting modifications. The compound kit provides a 22-27% horsepower increase over a single turbo setup at the same fueling level.
