Posted on: - 2/9/2026

Turbo Sizing 101 (The Foundation)

Turbo sizing isn’t about one number. It’s about how multiple components work together.

At a high level, turbo sizing includes:

  • Frame size (S300, S400, etc.)
  • Compressor wheel size (the intake side)
  • Turbine wheel & housing size (the exhaust side)
  • A/R ratio (housing efficiency and response)

When someone says “It has an S300,” they’re referring to the turbo family, not the exact turbo.

What Do S300 and S400 Actually Mean?

S300 and S400 are frame size classifications, most commonly associated with the BorgWarner S-series turbochargers.

Think of frame size like engine blocks:

  • Same family
  • Different displacements
  • Different use cases

S300 Frame Overview

  • Medium-frame turbo
  • Faster spool
  • Broad operating range
  • Excellent street and towing manners

S400 Frame Overview

  • Large-frame turbo
  • Much higher airflow potential
  • Designed for high horsepower
  • Requires more RPM and fueling to be efficient

Compressor Wheel Size (Why S363 ≠ S467)

Compressor size is usually listed in millimeters and tells you how much air the turbo can move.

Examples:

  • S363 = 63mm compressor wheel in an S300 frame
  • S366 = 66mm compressor wheel in an S300 frame
  • S467 = 67mm compressor wheel in an S400 frame
  • S475 = 75mm compressor wheel in an S400 frame

Even though a 66mm and 67mm sound close, the frame size difference changes everything about how the turbo behaves.

Turbine Housing & A/R Explained (Street vs Top-End)

The turbine housing controls how exhaust energy spins the turbo.

  • Smaller A/R (ex: .91, 1.00)
    • Faster spool
    • Better low-RPM response
    • Higher exhaust backpressure
  • Larger A/R (ex: 1.10, 1.32)
    • Slower spool
    • Better top-end efficiency
    • Lower backpressure at high RPM

This is why two “same size” turbos can drive completely differently.

S300 vs S400: Real-World Comparison

Spool Characteristics

S300

  • Spools quickly
  • Strong midrange
  • Ideal for daily driving and towing

S400

  • Slower spool
  • Comes alive at higher RPM
  • Best suited for aggressive street, race, or compound setups

Horsepower Potential (Generalized)

S300

  • ~500–800 HP depending on size and setup

S400

  • ~700–1,200 HP depending on size and setup

Power is always limited by fuel, engine strength, and tuning, not just turbo size.

Street & Towing Use

S300

  • Excellent throttle response
  • Lower EGTs when towing
  • More forgiving on stock or lightly built engines

S400

  • Not ideal for heavy towing as a single turbo
  • Better suited for performance-focused builds
  • Shines when paired with a smaller turbo in compounds

Why S300 Over S400 (and Vice Versa)?

Choose an S300 if:

  • You daily drive your truck
  • You tow regularly
  • You want quick response and drivability
  • Your horsepower goals are realistic for the street

Choose an S400 if:

  • You’re chasing big horsepower numbers
  • Your engine and fuel system are built
  • You care more about top-end than low-RPM response
  • You’re running compound turbos (S300 over S400 is extremely common)

Common Misunderstanding: Bigger Isn’t Always Better

A turbo that’s too large:

  • Feels lazy
  • Makes more heat
  • Is harder to tune
  • Can actually make less usable power

Matching turbo size to how the truck is used matters more than peak horsepower claims.

The Big Picture

Turbo sizing is about balance.

  • S300 turbos prioritize response, drivability, and versatility
  • S400 turbos prioritize airflow, efficiency at high RPM, and maximum power

Neither is “better” on its own. The right choice depends entirely on:

  • Intended use
  • Engine setup
  • Fuel system
  • Driving style

If you’re unsure where you fall, start with how you use the truck, not the biggest turbo you’ve seen on social media.

Adam_Blog
By: Adam