S400 Turbo for Duramax: LBZ, LMM, LML & L5P Kits and Sizing
An S400 turbo on a Duramax is a bigger undertaking than the same swap on a Cummins, and it is worth understanding why before you start buying parts. The Duramax mounts its turbo in the valley between the heads, fed by up-pipes from both banks. Converting to a big single means relocating or replacing that whole arrangement.
Done properly it works extremely well — the 6.6L has the displacement to spool a large frame and a bottom end that tolerates serious power. This guide covers sizing by generation and the fitment hardware a conversion needs.
Why Duramax S400 Swaps Are Different
- Valley mounting. The factory turbo sits between the heads, not on an accessible inline manifold. Space is tight and heat is concentrated.
- Up-pipes. Exhaust arrives from both banks and has to be merged into the turbine inlet. Most conversions replace both up-pipes.
- Kits, not parts. Because of the above, most Duramax S400 conversions use a dedicated kit with its own manifold, up-pipes, and mounting. Plan for a kit rather than assembling from scratch.
- Drain routing. Getting a gravity drain out of the valley and down to the pan with a clean angle is one of the fiddlier parts of the job.
S400 Sizing by Duramax Generation
LB7 (2001–2004)
The earliest Duramax, and the one most likely to need injector attention before anything else. Once fuelling is sorted the LB7 responds well to a big single.
- Street/strip: S467 or S472 on T4
LLY, LBZ, LMM (2004.5–2010)
The LBZ in particular has a strong reputation among Duramax builders — good bottom end, good fuel system, and the most popular generation for S400 conversions.
- Street/strip: S472 or S476 on T4
- Competition: S480 and up on T6
LML (2011–2016)
More emissions equipment to work around, and the conversion is correspondingly more involved. Fuel system is capable.
- Performance builds: S476 through S480
L5P (2017–present)
The L5P has the strongest factory fuel system of any Duramax and responds very well to airflow. It is also the generation where a bolt-in VGT upgrade is a genuinely competitive alternative to a big single for street use.
- Performance builds: S476 through S488 on T6
- Street and towing: consider a drop-in upgrade instead — see 6.6L L5P / L5D Duramax Drop-In Turbo Upgrades
Quick Reference
| Generation | Typical Size | Flange | Note |
|---|---|---|---|
| LB7 | S467–S472 | T4 | Injectors first |
| LLY / LBZ / LMM | S472–S476 | T4 | LBZ is the popular platform |
| LML | S476–S480 | T4 or T6 | More emissions to work around |
| L5P | S476–S488 | T6 | Drop-in VGT is a real alternative |
Shop S400-Frame Turbos
- TNT467 67MM T4
- TNT472 72MM T4
- TNT476 76MM T4
- TNT480 80mm T6
- TNT488 88mm T6
- All TNT4R T4 · All TNT4R T6
Prefer to keep the factory location? See the TNT Duramax drop-in turbos.
Fitment Hardware
Turbine Discharge
- T6 turbo → 5.75" marmon downpipe flange with clamp
- T4 turbo → 4.62" half marmon downpipe flange with clamp
Compressor Discharge
Compressor outlet discharge flange with clamp sized to your charge piping.
Oil Drain
50.8mm (2.00") bolt centre, 12AN port. Valley-mounted conversions often need a longer drain to get the hose connection clear of surrounding structure and establish a workable angle down to the pan.
- 35mm (1.38") Billet 12AN drain flange
- 65mm (2.56") Billet 12AN drain flange
- 100mm (3.94") Billet 12AN drain flange
Boost Control and Heat
- External wastegate and dump tube
- Turbo blanket — valley heat is significant on a Duramax
- Gaskets and o-rings
Single or Compound?
Compounds are common on Duramax for the same reason they are on Cummins: they preserve drivability. An S400 as the atmospheric charger over a stock-frame or S300 high-pressure turbo gives you low-end response plus a high ceiling.
On a valley-mounted V8 the packaging is harder than on an inline six, so compound Duramax setups almost always come from a purpose-built kit.
Related Guides
- S400 Turbo Guide: Sizes, Specs, Flanges and Fitment
- What Size S400 Turbo Do I Need?
- S400 Turbo Flange Guide
- S400 Turbo Oil Drain Guide
- S400 Turbo for Cummins · S400 Turbo for Power Stroke
- 6.6L L5P / L5D Duramax Drop-In Turbo Upgrades
- What Is the L5P / L5D Duramax Turbo?
Planning a Duramax build? Send the details through the Turbo Spec Form.
Frequently Asked Questions
What size S400 turbo is best for an L5P Duramax?
Performance L5P builds typically run an S476 through S488 on a T6 flange, since the L5P has the strongest factory fuel system of any Duramax generation. For street and towing use, a drop-in variable geometry upgrade is often the better choice.
Do I need a kit for a Duramax S400 swap?
In almost all cases, yes. The Duramax mounts its turbo in the valley between the heads and feeds it with up-pipes from both banks, so a conversion needs a dedicated manifold and mounting arrangement rather than a turbo alone.
Which Duramax generation is best for an S400 conversion?
The LBZ is the most popular platform among Duramax builders, thanks to its bottom end and fuel system. LLY and LMM conversions are also common. Earlier LB7 trucks usually need injector work first.
What oil drain does an S400 need on a Duramax?
A 50.8mm (2.00") bolt centre flange with a 12AN port. Because the turbo sits in the valley, many conversions need a longer drain — 65mm or 100mm — to clear surrounding structure and get a workable angle down to the pan.
Is an S400 better than a drop-in turbo on an L5P?
It depends on the truck's job. An S400 makes more top-end power, while a drop-in variable geometry upgrade preserves the factory location, exhaust brake, and towing manners. Dedicated performance builds favour the S400; trucks that tow usually do not.
Can you run compounds with an S400 on a Duramax?
Yes, with an S400 as the atmospheric charger over a smaller high-pressure turbo. Packaging is harder than on an inline six, so compound Duramax setups almost always come from a purpose-built kit.