4BT Compound Turbo Kit for 3.9L Cummins Swaps | Rectangular Port (8V) | 1998 and Older
The Diesel Power Source® compound turbo kit for the Cummins 4BT 3.9L engine (rectangular port, 8V, 1998 and older) 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.
Built using the same proven compound architecture that Diesel Power Source® uses on thousands of 5.9L and 6.7L Cummins pickup truck kits, this system is engineered specifically for the 4BT platform. The HE351CW lights up fast at low RPM for instant throttle response, while the S366 carries the top-end airflow that single turbos on a 3.9L engine simply cannot match. The result is a 4BT that spools quicker, runs cooler, and makes more power across the entire RPM range.
Why the 4BT Responds So Well to Compound Turbos
The 4BT is essentially two-thirds of the legendary 6BT (5.9L Cummins). It shares the same forged-steel connecting rods, durable pistons, and head design as its 6-cylinder sibling, which means it is highly receptive to turbocharging. But the 3.9L displacement creates a specific challenge for single turbo setups: a turbo small enough to spool quickly on 3.9 liters will choke at higher RPM, and a turbo large enough to flow well on top will be lazy and laggy off the line.
Compound turbos eliminate that trade-off entirely. The HE351CW primary turbo is sized to spool fast on the 4BT's smaller displacement, delivering immediate boost response that makes the engine feel alive from idle. As RPM and load increase, the S366 atmospheric turbo takes over the majority of the compression work, providing sustained airflow that a single turbo cannot deliver without unacceptable lag. The two turbos share the workload so each operates in its most efficient range, producing boost air that is over 100°F cooler than a single turbo at the same pressure and boost-to-drive pressure ratios approaching 1:1.
Who Builds with a 4BT?
The Cummins 4BT has become the go-to diesel engine for custom builds and conversions because of its compact size, mechanical simplicity, and legendary Cummins durability. At roughly 30 inches long and 750 pounds, it fits where a 6BT cannot. Common 4BT swap and conversion applications include:
- Jeep Wrangler and Cherokee — The most popular 4BT swap platform. The compact engine fits in the Jeep engine bay and delivers diesel torque for trail crawling, overlanding, and daily driving.
- Classic Truck Builds — Ford F-series, Chevy C/K, Dodge D-series, and other vintage trucks where a compact diesel conversion is the goal.
- Off-Road and Overland Rigs — Broncos, Blazers, Land Cruisers, and purpose-built trail machines where fuel economy and low-RPM torque matter more than peak horsepower.
- Step Vans and Delivery Vehicles — Commercial vehicles where the 4BT was originally found and where reliability and fuel economy are critical.
- Marine Applications — Boats where the 4BT's compact footprint and mechanical injection are ideal.
- Industrial and Stationary Power — Generators, pumps, and equipment where the 4BT provides reliable mechanical diesel power.
The compound turbo kit takes these builds to the next level. Whether you are building a Jeep Cummins swap that needs to tow a trailer, an overland rig that needs reliable power at altitude, or a work truck that needs to run cool under sustained load, this compound system delivers the airflow and thermal management that single turbo 4BT builds cannot match.
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 the immediate throttle response that makes the 4BT feel responsive from idle. As RPM and load increase, the larger S366 atmospheric turbo takes over the majority of the compression work, providing sustained airflow for higher power applications without the lag that large single turbos create on a 3.9L displacement 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. For 4BT builds that tow, work under sustained load, or operate at altitude where EGT management is critical, compound turbos provide a level of thermal efficiency that no single turbo can match.
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 | Rectangular port (8V, 2 valves per cylinder) |
| Engine Fitment | Cummins 4BT 3.9L, 1998 and older |
| 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 (rectangular 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. Every 4BT swap is unique. 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
- Smoother Power Delivery — Usable boost from low RPM through the top end
- Proven DPS Compound Architecture — Same design philosophy used on thousands of 5.9L and 6.7L Cummins kits
- No ECM or Programming Required — Mechanical 4BT engines respond directly to increased airflow
- Compact Packaging — Designed for the tight engine bays common in 4BT swap applications
Rectangular Port vs Round Port: Which Kit Do You Need?
| Feature | This Kit (Rectangular Port) | Round Port Kit |
|---|---|---|
| Port Shape | Rectangular | Round |
| Valves Per Cylinder | 2 (8V total) | 4 (16V total) |
| Engine Years | 1998 and older | 1999 and newer |
| Most Common In | Step vans, bread trucks, shuttle buses, Jeep swaps, classic truck swaps | Later industrial, marine, and commercial applications |
| Injection | Mechanical (VE or P7100) | Electronic |
| Swap Popularity | Most common 4BT swap engine | Less common in swap market |
Not sure which port shape your engine has? Look at the exhaust side of the cylinder head where the manifold bolts on. If the exhaust ports are rectangular (wider than tall), you have the 8V engine and need this kit. If the ports are round, you have the 16V engine and need the round port compound kit.
Fitment
This compound turbo kit fits Cummins 4BT 3.9L engines with rectangular exhaust ports (8V, 2 valves per cylinder), 1998 and older. This includes mechanically injected 4BT engines commonly found in step vans, bread trucks, shuttle buses, and used in Jeep, truck, and custom vehicle swaps.
This kit is designed for aftermarket and swapped applications. Fitment depends on engine placement, chassis configuration, and available clearances. Some fabrication may be required for intake, exhaust, and intercooler routing depending on your specific build.
Have a 1999 or newer 4BT with round exhaust ports (16V)? See the 4BT Compound Turbo Kit for Round Port (16V).
Looking for a 4BT manifold only? See the 4BT Exhaust Manifold (Rectangular Port).
Related 4BT Cummins Products
Frequently Asked Questions
The difference is the exhaust manifold. The rectangular port kit fits 1998 and older 4BT engines with 2 valves per cylinder (8V total). The round port kit fits 1999 and newer 4BT engines with 4 valves per cylinder (16V total). The turbo system is the same on both kits. Look at the exhaust side of your cylinder head to confirm your port shape before ordering.
The 4BT's 3.9L displacement creates a fundamental problem for single turbo setups. A turbo small enough to spool quickly on 3.9 liters will run out of airflow at higher RPM. A turbo large enough to flow well on top will be laggy off the line. Compound turbos solve this by using a small primary turbo (HE351CW) for fast spool and a large atmospheric turbo (S366) for top-end airflow. You get the best of both worlds with no compromise, plus lower EGTs and cooler intake air temperatures than any single turbo setup.
This kit fits all 1998 and older Cummins 4BT 3.9L engines with rectangular exhaust ports. It is designed for swap and custom applications, but every 4BT swap is unique in terms of engine placement and available clearances. Some fabrication may be required for intake, exhaust, and intercooler routing depending on your specific build. Contact Diesel Power Source® with photos of your engine bay for fitment confirmation.
This compound system supports 200 to 500 HP depending on fueling and supporting modifications. Stock 4BT internals are generally considered reliable to approximately 250-280 HP. Builds beyond that benefit from upgraded head studs, injectors, and fuel system modifications. The compound kit itself is not the limiting factor in the system.
No. The 1998 and older 4BT engines use mechanical fuel injection (VE pump or P7100). There is no ECM, no computer, and no programmer required. The mechanical injection system responds directly to the increased airflow from the compound turbo system.
Yes. This kit uses the same proven compound architecture that Diesel Power Source® has built thousands of for 5.9L and 6.7L Cummins pickup trucks. The turbo staging, airflow design, and compound principles are the same, engineered specifically for the 4BT's 3.9L displacement and compact packaging requirements.
The compound system itself is a complete kit with both turbos, manifold, piping, downpipe, and all hardware. However, because every 4BT swap is unique in terms of chassis, engine placement, and available clearances, some fabrication may be required for intake routing, exhaust routing, and intercooler connections depending on your specific build.
One of the key advantages of compound turbos on any diesel engine is improved combustion efficiency from cooler, denser charge air. Many 4BT owners report the same or better fuel economy with compounds compared to a single turbo because the engine does not have to work as hard to produce the same power. A properly set up 4BT swap typically achieves 17-25 MPG depending on the vehicle and application, and compound turbos help maintain that efficiency even under load.
