You've heard about compressor wheel tip clearance — that razor-thin gap between the blade tips and the shroud. The turbine side has the same problem, but the physics are nastier. The exhaust side runs at 900°C+, the housing grows, the wheel grows, and the bearings let the shaft float. Get the clearance wrong and you either lose efficiency to leakage or grenade the wheel against the housing on a cold start.

Why clearance exists at all: The turbine wheel spins inside the turbine housing with a small radial gap between the blade tips (exducer) and the contour wall. Exhaust gas wants to expand through the blades to do work. Any gas that sneaks around the blade tip instead of through the passage is lost energy — it bypasses the wheel and contributes nothing to spinning the shaft. This is called tip leakage.

The thermal expansion problem: On cold start, the housing and wheel are at ambient. Under full load, the wheel sees 950°C and the housing sees maybe 750°C (it's losing heat to the manifold and atmosphere). The Inconel wheel grows more than the cast iron housing in the radial direction. Designers must size the cold clearance so that hot clearance is small but never zero.

Rule of thumb: Turbine tip clearance is typically 0.3–0.6% of wheel exducer diameter. For a 50mm turbine wheel, that's 0.15–0.30mm cold radial gap. Each 0.1mm of excess clearance costs roughly 1–1.5% turbine efficiency — which translates directly to slower spool and lost boost response.

Real-world example: Borg-Warner's EFR series uses a tightly controlled gamma-titanium-aluminide turbine wheel with reduced thermal growth compared to Inconel. Because the wheel grows less when hot, the designers can run a tighter cold clearance — about 0.2mm on a 55mm wheel — without risking a rub during heat soak. That tighter gap is part of why EFR turbos spool 200–400 RPM earlier than equivalent Inconel-wheel competitors.

Failure mode: If the shaft bearings wear and the wheel develops radial play, the blade tips can contact the housing. You'll see witness marks — circumferential rub patterns on the housing contour and tipped/folded blade edges. Once a blade tip is bent even 0.1mm, that wheel is scrap; the imbalance will destroy the bearings within hours.

Inspection tip: With the turbo cold, push the shaft sideways and rotate it. You should feel zero contact between blade tips and housing. Any scraping means worn journal bearings or a damaged wheel — pull it apart before it lets go on boost.