S300 Turbo for Cummins: 5.9L and 6.7L

The Cummins is the S300's natural home. The 5.9L 12v, the 5.9L 24v and the 6.7L have all been running 300-frame turbos since the aftermarket existed, and the swap is better understood on this platform than on any other.

This guide covers sizing by generation, what the factory turbo does and does not leave behind, and the fitment hardware the swap actually needs.

Why the S300 Suits the Cummins

The inline six is a torque engine that spends its working life at low RPM. The 300 frame is the size class that builds boost there. That pairing is why the S300 became the default single-turbo upgrade on this platform rather than something larger.

It is also a straightforward mechanical fit: the factory turbo mounts on the exhaust side with reasonable access, so a T4 manifold swap is a well-trodden job rather than a fabrication project.

Sizing by Generation

5.9L 12v (1989–1998)

The mechanical 12v is the classic S300 host. With a P-pump and modest fuelling, an S363 or S366 covers most builds. The 12v's low-RPM torque means oversizing here is felt immediately — stay smaller than you think you need.

5.9L 24v (1998.5–2007)

Both the VP44 and common-rail 24v respond well to the same range. The S366 is the most common landing point. Common-rail trucks with fuelling upgrades can justify an S372.

6.7L (2007.5–present)

The 6.7L displaces more and can carry a larger turbo without the response penalty being as sharp. S366 through S372 is the common range, with S376 on builds that have the fuel to support it.

Note that many 6.7L owners are better served by a drop-in HE300VG replacement than by a full S300 swap — if you want more power without changing the manifold, exhaust brake and emissions hardware, start there.

What the Factory Turbo Leaves Behind

Nothing from the factory HX35, HX40, HE351 or HE300VG carries over to an S300 swap. Specifically:

  • The manifold does not fit. Factory manifolds are not T4. A T4 manifold, open or divided, is required.
  • The downpipe does not fit. The S300 uses a 4.25" marmon turbine discharge. A new downpipe flange and clamp is required.
  • The drain does not carry over. The S300 is mid frame, 50mm (1.97") bolt centre. Do not assume the factory drain hardware transfers.
  • Wastegate strategy changes. A T4 open or divided housing generally means an external wastegate and dump tube rather than an internal actuator.
  • On the 6.7L, the exhaust brake goes away. The VGT's braking function is not replicated by a fixed-geometry S300. If you tow in mountains, weigh that.

Fitment Hardware for a Cummins S300 Swap

  • T4 exhaust manifold — open or divided to match your turbine housing. This is the significant purchase in the swap.
  • 4.25" marmon turbine discharge flange and clamp — 3", 3.5" or 4"
  • Compressor discharge v-band sized to your charge pipe — see v-band assemblies
  • Oil drain flange at 50mm (1.97") bolt centre, 10AN or 12AN
  • Oil feed line and restrictor appropriate to the bearing
  • External wastegate and dump tube
  • Turbo blanket for underhood heat

Drain Routing on a Cummins

The inline six generally gives you a reasonable path from the turbo down the side of the block to the pan, which is part of why this swap is forgiving. Two things still catch people:

  1. Clearing the block and accessories. This is what the 65mm (2.56") and 100mm (3.94") drain lengths exist for — getting the hose connection out past whatever is in the way before the line turns down.
  2. Entering the pan above oil level. If the drain terminates below the oil in the pan, oil cannot leave the turbo. Confirm the fitting height rather than assuming.

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

Compound Setups on a Cummins

Compounds are common on this platform, and the S300 is normally the high-pressure charger — the smaller turbo mounted close to the manifold, fed by a larger atmospheric turbo.

Two practical points:

  • Size it smaller than you would as a single. The high-pressure turbo breathes already-compressed air, so the same compressor moves considerably more mass. An S363 or S366 is a common high-pressure choice under a 400-frame atmospheric turbo.
  • The drain is working harder. Compound high-pressure turbos see demanding conditions. 12AN is the safer choice here.

The atmospheric turbo above it is usually a 400-frame unit — see the S400 Turbo for Cummins guide.

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Related Guides

Frequently Asked Questions

What size S300 is best for a 5.9 Cummins?

An S363 or S366 covers most 5.9L builds. The 12v in particular is a low-RPM torque engine where oversizing is felt immediately, so staying smaller than you think you need is usually the right instinct.

Do I need a new manifold for an S300 swap on a Cummins?

Yes. Factory Cummins manifolds are not T4, so a T4 manifold — open or divided to match your turbine housing — is required. It is the significant purchase in the swap.

Will my factory downpipe work?

No. The S300 uses a 4.25" marmon turbine discharge, which requires a matching flange and clamp. Black Sheep Industries sells the flange and clamp together.

Does an S300 swap remove the exhaust brake on a 6.7L?

Yes. The factory VGT's braking function is not replicated by a fixed-geometry S300. If you tow in mountainous terrain, weigh that before committing — a drop-in HE300VG replacement may suit you better.

What size S300 should I use as a compound high-pressure turbo?

Smaller than you would run as a single. An S363 or S366 is a common high-pressure choice under a 400-frame atmospheric turbo, because the high-pressure turbo breathes already-compressed air and flows more mass in that position.

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.