4BT Cummins 8V Rectangular Port Exhaust Manifold Upgrade
A cracked or warped exhaust manifold on your 4BT 8V (Rectangular Port) Cummins can cost you power, drive up EGTs, and create annoying exhaust leaks. This high-flow replacement manifold is built to fix those issues and support higher performance, towing, and work use.
*This is not a Factory Fitment Replacement, This is an AFTERMARKET replacement manifold.
Why Upgrade Your 4BT 3.9L Cummins Manifold?
- Stock manifolds are known to warp and crack under heat and boost.
- Exhaust leaks before the turbo hurt spool-up, boost, and efficiency.
- High EGTs from poor flow can shorten engine and turbo life.
- Upgraded flow and durability support future performance mods.
- 40 degree angle T3 Flange
Key Features
- High-flow design: Improved runner and collector geometry to reduce backpressure and support better turbo response.
- Heavy-duty casting: Thick-wall material and reinforced structure to resist cracking and thermal fatigue.
- Rectangular port configuration: Matches common 4BT / 3.9L Cummins rectangular-port cylinder heads.
- Turbo performance: Helps the turbo spool quicker and hold boost more consistently under load.
- Great stock replacement: Works with factory-style turbo location while giving you room for future upgrades.
- Direct bolt-on: Uses factory-style mounting locations for an OEM-like installation.
Fitment & Applications
- Engine: Cummins 4BT / 3.9L (8-valve style head as applicable)
- Port shape: Rectangular exhaust ports
- Common uses:
- 4BT truck swaps and custom builds
- Industrial and equipment applications
- Work trucks and light commercial vehicles
Possible Replacement Part Numbers
Designed as an upgraded replacement for common OEM rectangular-port 4BT / 3.9L Cummins exhaust manifolds.
**Please check fitment carefully before purchase, verify against your original part number and application before ordering.
- 3901635 - Komatsu PC60-7, PC100-6, PC120-6, PC120-7
- 4934697 - Marine Equipment
- 4988420 - Outdoor Equipment
- 4932577 - Marine Equipment
- 3960056 - Marine Equipment
- Customers needing a stronger, longer-lasting replacement for a cracked or warped stock manifold.
- 4BT swaps looking for better flow and cooler EGTs without moving the turbo.
- Work and tow rigs that spend long hours under load and heat.
These manifolds are not intended as direct OEM replacements. They are designed for aftermarket builds, swaps, performance applications, and custom setups. Fitment will depend on your turbo configuration, chassis space, downpipe routing, and accessory clearance.
These manifolds are commonly used in 4BT swap platforms, including but not limited to: Jeep Wrangler YJ, TJ, and JL conversions Ford Bronco and Ranger swaps Square-body Chevy trucks Land Rovers and off-road builds Classic cars like Camaros and Mustangs Freightliner MT45 / MT55 step vans Grumman Olson UPS/FedEx trucks Mini school buses and shuttle buses Skid steers, trenchers, and forklifts Small boats and marine applications Military Humvees, trailers, and gensets
Yes. These manifolds are built with a stronger casting and improved design to help handle thermal cycles, boost pressure, and heavy-duty use better than many factory manifolds.
4 Reviews
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Does the job
Simple install and seems well built. Good for my project
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Solid casting
Installed on my 4BT K5 build. Manifold is thick and the flange was straight. Turbo mounted clean and the truck lights the turbo quick.
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Good manifold
Put this on a bread truck 4BT. Everything bolted up easy and the turbo placement worked great. Shipping was fast too.
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Perfect for my 4bt swap
Used this on a 4BT swapped into an old Chevy project. T3 flange lined up perfect with my turbo and the ports matched the head really well. Spool feels better than the old manifold.
