The hub-to-tip ratio is one of the most overlooked numbers in compressor wheel design. It's the ratio of the inducer hub diameter (the solid center boss the blades attach to) divided by the inducer tip diameter (the outer edge of the blade at the inlet). Typical values run between 0.30 and 0.45, and that small range hides massive performance consequences.

Why does it matter? The inducer face is where air enters the wheel. Any area blocked by the hub is area that cannot flow air. The effective inlet flow area is:

• A_flow = (π/4) × (D_tip² − D_hub²)

So a wheel with a 60mm inducer tip and a 24mm hub (ratio 0.40) has roughly 2,374 mm² of inlet area. Shrink the hub to 18mm (ratio 0.30) and area jumps to 2,573 mm² — an 8% gain in flow area without changing tip diameter. That's free top-end flow.

So why not just make the hub tiny? Three reasons:

• Shaft stiffness: The hub must clamp around the turbo shaft. Smaller hub means smaller shaft, which means lower critical speed and more vibration risk at 200,000+ RPM.
• Blade root stress: Centrifugal loads on the blades concentrate at the root. A small-diameter hub means less material to react those loads — blades start failing in fatigue.
• Bore stress: The hub bore sees extreme hoop stress from interference fit and rotational loads. Billet 2618 aluminum wheels can run smaller hubs than cast 354 aluminum because of higher fatigue strength.

Real-world example: Compare Garrett's GTX3582R Gen I (cast wheel, ~0.42 hub-to-tip) to the G35-900 (billet wheel, ~0.34 hub-to-tip) with the same 68mm inducer. The G35 flows roughly 15% more air at the same tip speed, and most of that gain comes from reduced hub blockage. This is why billet wheels dominate modern high-output applications — the material lets engineers push the hub smaller.

Rule of thumb: For every 0.05 reduction in hub-to-tip ratio, expect roughly 5–8% more choke flow, assuming blade design stays compatible. But also expect the wheel to cost 2–3× more because it must be billet machined rather than cast.

The hub-to-tip ratio also affects surge margin. A smaller hub means the blade leading edge sees a more uniform velocity profile across its span, which delays surge onset at low flow. That's a double win — more top-end flow and wider operating range.