On the compressor side, air enters the small inducer and exits the larger exducer — the wheel acts as a centrifugal pump, accelerating air outward. The turbine side does the opposite. Hot exhaust enters the large outer diameter (the turbine inducer, also called the major diameter) and exits axially through the smaller exducer (or minor diameter) at the wheel's center. This is a radial inflow turbine — gas spirals inward, giving up energy to the blades as it loses radius and velocity.

Why inflow? Because exhaust gas arrives via the volute already spinning tangentially at high velocity. Letting it spiral inward converts that tangential momentum into shaft torque efficiently — Euler's turbomachinery equation says work extracted equals the change in r × Vθ (radius times tangential velocity). A larger inducer radius means more leverage on the shaft for the same gas velocity.

The two diameters do different jobs:

• Inducer (major) diameter: Sets how much exhaust energy the wheel can capture. Bigger inducer = more flow capacity, more top-end power, but more rotational inertia and slower spool.
• Exducer (minor) diameter: Controls discharge velocity into the downpipe. A smaller exducer increases exit velocity (more energy extracted) but raises backpressure. Bigger exducer reduces backpressure and helps high-RPM breathing.

Turbine trim uses the same formula as compressor trim: Trim = (exducer² / inducer²) × 100. A Garrett GTX3582R turbine wheel might have a 68 mm inducer and 62 mm exducer, giving a trim of (62²/68²) × 100 ≈ 83. Higher trim = more exducer area relative to inducer = better high-RPM flow but less low-end response.

Real-world example: The Mitsubishi Evo IX's TD05HR-16G6 turbo runs a relatively small turbine inducer (~52 mm) for fast 4G63 spool below 3,500 RPM. Swap to a larger 18G turbine wheel with a 56 mm inducer and you'll gain 40+ peak horsepower but lose 500 RPM of spool — the classic top-end-vs-response tradeoff lives in those two diameters.

Rule of thumb: Match turbine inducer diameter to about 90–95% of compressor exducer diameter for balanced sizing. If your compressor exducer is 60 mm, target a turbine inducer around 54–57 mm. Bigger than that creates a "lazy" hot side that can't extract enough energy to spin the cold side at low loads.

Blade design at the inducer matters too: leading edges are typically radial (no backsweep) because exhaust gas density and temperature make backswept inducers prone to cracking from thermal cycling and high stress concentrations at the blade root.