2026-06-18
The hub-to-tip ratio on a turbine wheel is the ratio of the hub diameter (the solid center boss the blades attach to) divided by the tip diameter (the outer swept circle) at the exducer — the discharge side where exhaust exits axially. It looks like a throwaway number on a spec sheet, but it controls how much exhaust energy the wheel can actually extract before choking.
Why it matters: exhaust gas enters radially at the inducer and exits axially at the exducer. The annular flow area at the exducer equals π/4 × (tip² − hub²). A fat hub shrinks that annulus, restricting discharge flow and raising backpressure. A skinny hub opens flow area but leaves less material to anchor the blades against centrifugal load — and at 150,000 RPM with blade tips seeing 500 m/s, that material has a job.
Typical ranges:
Real-world example: The Garrett GTX3582R Gen II turbine wheel runs roughly 65mm tip / 28mm hub, giving a hub-to-tip ratio around 0.43. Compare to a Cummins HE351VE diesel turbine at roughly 60mm tip / 33mm hub — ratio near 0.55. The diesel wheel runs higher backpressure tolerance, lower EGTs, and lives in a torque-dominated rev band; the gas wheel needs to flow hard up to 8,000+ RPM and accepts a slightly more restrictive annulus to gain rotor stiffness.
Rule of thumb: For a target horsepower (gas), pick exducer tip diameter at roughly 1 mm per 12 HP, then size the hub for a ratio between 0.40 and 0.50. Below 0.38, blade root stress climbs sharply; above 0.55 on a gas turbine, you're choking flow at high RPM.
Engineers also watch the exducer trim (exducer² / inducer² × 100). Hub-to-tip and trim work together — a high-trim wheel with a fat hub looks flowy on paper but bottlenecks at the discharge annulus. Always check both numbers before believing the marketing.