What Size S300 Turbo Do I Need? Sizing by Horsepower Target

Picking an S300 size is a trade between airflow and response. Every size in the 300 frame shares the same centre section, the same drain geometry and the same mounting hardware, so the only thing changing as you step up is how much air the compressor moves and how long it takes to get there.

This guide walks through how to choose, starting from what the truck actually does rather than from a horsepower number on a forum post.

Start With the Job, Not the Number

The 300 frame exists because response matters. If peak power were the only consideration you would buy a 400-frame turbo and accept the spool penalty. So the first question is not "how much power do I want" — it is "where in the RPM range do I need it."

  • Tows, hauls, daily driven. You need boost at low RPM under load. Stay small. Oversizing here makes the truck worse in every situation you actually use it.
  • Street driven, occasionally raced. The middle of the frame. You can afford some spool for a meaningful gain up top.
  • Strip or dedicated race. The top of the frame — and at that point, honestly compare against a small S400.
  • Compound high-pressure charger. Different rules entirely. See the compound section below.

S300 Sizes and Their Power Ceilings

Size Inducer US Standard Max Power (crank) Best Suited To
S363 63mm 2.48" 940 HP First upgrade, towing, stock-appearing
S366 66mm 2.60" 1,090 HP All-around street and tow
S369 69mm 2.72" 1,120 HP Street/strip
S372 72mm 2.83" 1,140 HP Serious street/strip
S376 76mm 2.99" 1,210 HP Top-end single
S380 80mm 3.15" 1,300 HP Maximum airflow in the frame

Read that column as a ceiling, not a target. It is the maximum crank horsepower the compressor will support with the turbo working at the edge of its map — not the number we would build to, and not the point at which we would move you to a different frame. Our practical crossover to the 400 frame sits around 820 HP, well below the top of this table, because past that an S400 makes the same power with less strain and more headroom.

These figures also assume the rest of the build supports them. A turbo does not make power on its own — fuel system, head studs, tune and turbine housing all cap what you will actually see.

The Turbine Wheel Matters As Much As the Compressor

This is the part most sizing conversations skip. Two builders can order the same S372 and end up with turbos that drive completely differently, because the turbine wheel is a separate choice.

  • 76x68 — earliest response. Pairs with the smaller compressors for a genuinely quick-spooling combination.
  • 80x73 — the balanced default across most of the range.
  • 82x75 — most turbine flow, latest response. Available on the larger compressors where you need the exhaust side to keep up.

Going up a compressor size and down a turbine wheel is not the same as staying put. If you are trying to add top-end without losing your low-end, the turbine wheel is often the better lever.

Full breakdown: S300 Turbine Wheels and Housings Explained

Sizing for a Compound Setup

In a compound arrangement the S300 is the high-pressure charger, sitting under a larger atmospheric turbo. It is not breathing atmosphere — it is breathing air the bottom turbo has already compressed, which is denser. The same compressor flows meaningfully more mass in that position.

The practical result: size the S300 smaller than you would as a single. A builder who would run an S372 as a standalone often wants an S363 or S366 in the high-pressure position. Oversizing the top charger in a compound setup is the most common way people end up with a truck that is slower everywhere it matters.

Match the S300 to the manifold pressure it will actually see, not to the power target of the whole system.

When You Should Be Looking at an S400 Instead

Three honest signals that the 300 frame is the wrong choice:

  1. You are shopping the very top of the frame. If an S376 or S380 is the only thing big enough, a small S400 will usually make that power with less strain and more headroom.
  2. The truck does not need low-RPM response. A dedicated race application has no reason to pay the 300 frame's airflow ceiling for spool it does not use.
  3. You are already planning the manifold. If you have not bought the T4 manifold yet, the cost of choosing between frames is at its lowest right now. After the manifold and downpipe are done, switching frames gets expensive.

As a working number, around 820 HP is where we would move you across to the 400 frame. That is a selection threshold rather than a physical limit — individual 300-frame turbos are rated well above it, up to 1,300 crank on the largest — but buying to a ceiling instead of to a working range is how people end up with a turbo that is technically big enough and unpleasant to live with.

Full comparison: S300 vs S400 Turbo: Which Frame Is Right for Your Build?

What Else You Need to Order

Size selection is only half the job. Whatever S300 you land on, the fitment hardware is the same:

  • T4 manifold, open or divided to match your turbine housing
  • 4.25" marmon turbine discharge flange and clamp
  • Compressor discharge v-band sized to your charge pipe
  • Oil drain flange at 50mm (1.97") bolt centre, 10AN or 12AN, in a length that clears your install

See the S300 Turbo Flange Guide and the S300 Turbo Oil Drain Guide for the full picture.

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Still deciding? Send your build details through the Turbo Spec Form and we will spec it with you.

Related Guides

Frequently Asked Questions

What is the most popular S300 size?

The 66mm and 69mm compressors cover the widest range of builds — enough airflow to be a real upgrade over stock while keeping the low-RPM response that makes the 300 frame worth choosing in the first place.

Should I size up "just in case"?

No. On the 300 frame, oversizing costs you the response you bought the frame for. If you genuinely need the airflow, the honest answer is usually a 400-frame turbo rather than the largest 300.

Does the turbine wheel change what size compressor I should pick?

Yes. The turbine wheel controls how readily the exhaust side drives the compressor. A larger compressor with a matched larger turbine behaves very differently from the same compressor on a smaller turbine, so treat the two as one decision.

What size S300 should I run as a compound high-pressure charger?

Usually smaller than you would run as a single. The high-pressure turbo breathes air already compressed by the atmospheric charger, so the same compressor flows more mass in that position. Size it to the manifold pressure it will actually see.

How do I know if I need an S400 instead?

If you are shopping at the very top of the 300 frame, if the truck has no need for low-RPM response, or if you have not yet committed to a manifold, it is worth comparing directly against a small S400 before ordering. As a working number, around 820 HP is where we would move you across frames.

How much horsepower will an S300 support?

Maximum crank horsepower runs from 940 HP on an S363 to 1,300 HP on an S380. Treat those as compressor ceilings rather than build targets — they describe what the compressor will support at the edge of its map, not where the turbo is pleasant to run.

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.