S300 Turbo for Duramax: LBZ, LMM, LML and L5P

An S300 swap on a Duramax is a bigger job than the same swap on a Cummins, and it is worth being clear about that before you start ordering parts. The reason is mounting position: the Duramax carries its turbo in the valley of the V8, under the intake, rather than hanging off an accessible exhaust manifold.

This guide covers what that actually means, how to size by generation, and when a drop-in replacement is the better answer than a conversion.

Why the Duramax Is Different

On an inline six, the turbo sits on the side of the engine with room around it. On a Duramax it sits in the valley, fed by up-pipes from both banks, packaged under the intake and surrounded by everything else in the engine bay.

Three consequences:

  1. You are buying a kit, not a turbo. A conversion needs a dedicated mounting pedestal or manifold arrangement, up-pipes, charge piping and a downpipe. Sourcing those piecemeal rarely goes well.
  2. Drain routing is tighter. Less room to establish the near-vertical drop the drain needs, and more in the way between the turbo and the pan.
  3. Heat management matters more. A turbo in the valley under the intake is in a confined, hot space.

None of this makes the swap unreasonable — it is done regularly. It does mean the planning has to be done up front.

Sizing by Generation

LB7 and LLY (2001–2005)

Earlier Duramax generations respond well to modest 300-frame sizing. An S363 or S366 is the usual range on a conversion. The LLY in particular benefits from the airflow given its known heat behaviour.

LBZ and LMM (2006–2010)

The LBZ is the generation most often built, and the S366 is the common landing point. LMM follows similar sizing with emissions hardware to work around.

LML (2011–2016)

The LML carries more emissions equipment and a different turbo arrangement. Conversions are done but the packaging work is greater. S366 through S372 is the usual range.

L5P and L5D (2017–present)

The L5P's factory turbo is more capable than earlier generations, which changes the calculation. Many L5P owners are better served by a drop-in upgrade that keeps the factory packaging than by a full S300 conversion.

Drop-In or Conversion?

This is the honest first question on a Duramax, and for a lot of owners the answer is not the S300.

A Drop-In Makes Sense When

  • You want more power without changing the manifold, up-pipes and downpipe
  • The truck is a daily driver or tow rig and you value keeping factory packaging
  • You are on an L5P, where the factory turbo has real headroom

See the Duramax drop-in turbo range.

An S300 Conversion Makes Sense When

  • You have passed what a drop-in supports
  • The build is already apart, or a full kit is planned
  • You want the flexibility of interchangeable turbine housings and wheel options
  • You are building toward compounds

Fitment Hardware for a Duramax S300 Conversion

  • Mounting kit — pedestal or manifold arrangement appropriate to your generation, plus up-pipes
  • 4.25" marmon turbine discharge flange and clamp — 3", 3.5" or 4". Downpipe routing out of the valley is a fabrication job on most conversions.
  • Compressor discharge v-band sized to your charge pipe — see v-band assemblies
  • Oil drain flange at 50mm (1.97") bolt centre. In the confined valley position, drain length is a real decision — the 100mm (3.94") option exists for exactly this kind of packaging.
  • External wastegate and dump tube
  • Turbo blanket — worth more here than on most platforms given the confined mounting

Also see the Felder Kustom Fabrication diesel range for kit-level Duramax hardware.

Drain Routing in the Valley

This is the part of a Duramax conversion most likely to cause trouble later, because the valley position works against everything a drain wants.

  • Get the standoff you need. A longer drain moves the hose connection out from under the intake and past whatever is crowding the turbo before the line has to turn down. This is what the 65mm (2.56") and 100mm (3.94") lengths are for.
  • Protect the drain from heat. A hose routed against a hot surface in a confined space cooks the oil in it.
  • Confirm the sump entry point. Above oil level, always. This gets overlooked when the routing is complicated.
  • Do not accept a low spot to make the routing easier. A dip in the line traps oil and creates a column the centre housing has to push through. If the routing will not work without a low spot, reclock the turbo.

Full routing rules and a leak diagnosis table: S300 Turbo Oil Drain Guide.

Compound Setups on a Duramax

In a Duramax compound arrangement the S300 typically serves as the high-pressure charger, with a larger atmospheric turbo mounted outboard where there is room. Packaging is the constraint rather than turbo selection — there is not space in the valley for two turbos, so the atmospheric charger goes elsewhere and the plumbing between them becomes the project.

Size the S300 smaller than you would as a single. See the S400 Turbo for Duramax guide for the atmospheric side.

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Planning a conversion? Send your build details through the Turbo Spec Form before you buy hardware.

Related Guides

Frequently Asked Questions

Is an S300 swap harder on a Duramax than a Cummins?

Yes. The Duramax mounts its turbo in the valley of the V8 under the intake rather than on an accessible exhaust manifold, so a conversion requires a dedicated mounting arrangement, up-pipes and a fabricated downpipe. Plan for a full kit rather than just a turbo.

Should I do an S300 conversion or a drop-in turbo?

A drop-in makes sense if you want more power while keeping factory packaging, particularly on an L5P where the factory turbo has real headroom. A conversion makes sense once you have passed what a drop-in supports, or if you are building toward compounds.

What size S300 suits an LBZ Duramax?

The S366 is the common landing point on LBZ builds. Earlier LB7 and LLY generations tend toward the S363 to S366 range.

What drain length should I use on a Duramax conversion?

Usually a longer one. The valley position is confined, and the 65mm (2.56") or 100mm (3.94") lengths exist to move the hose connection out past the intake and surrounding hardware before the line turns down toward the pan.

Can I run compounds on a Duramax with an S300?

Yes, typically with the S300 as the high-pressure charger and a larger atmospheric turbo mounted outboard where there is room. Packaging rather than turbo selection is the constraint, since there is not space in the valley for two turbos.

Power ratings on this page are quoted in crank horsepower (HP). Figures previously published in wheel horsepower (WHP) have been restated using our in-house tested conversion — the hardware and its capability are unchanged.