The compressor wheel spins at up to 200,000 RPM while flinging air outward at near-supersonic tip speeds. What it's made of determines how much boost you can run, how fast it spools, and whether it survives a single overspeed event.

Cast Aluminum (C355 alloy) is the standard. Sand-cast or investment-cast from an aluminum-silicon-magnesium alloy, then heat-treated to T6 temper. Cheap to produce, light (low rotational inertia = fast spool), and good for tip speeds up to about 1,700 ft/sec. The downside: cast structure has porosity and grain boundaries that become crack initiation sites under cyclic loading. Most stock turbos use cast aluminum because it survives the warranty period at OEM boost levels (15-20 psi).

Billet Aluminum (2618 or 7075 alloy) is CNC-machined from a forged aluminum bar. The forged grain structure is dense and aligned, eliminating porosity. This raises the burst speed by roughly 15-20% over cast, allowing tip speeds around 2,000 ft/sec. Billet wheels also let designers carve more aggressive blade geometry — extended tips, optimized backsweep, and razor-thin trailing edges that casting can't reliably produce. The classic example: Garrett's GTX series uses billet wheels to hit 76-80% peak efficiency where cast GT wheels topped out at 74%.

Titanium shows up on the turbine side more often than the compressor side, but a few extreme builds use titanium compressor wheels. Titanium-aluminum (TiAl) alloys have a strength-to-weight ratio about 40% better than aluminum, allowing higher tip speeds (2,300+ ft/sec) without flying apart. The catch: titanium is roughly 1.6x denser than aluminum, so the rotational inertia goes up, slowing spool. Cost is also brutal — a titanium compressor wheel can run 5-10x the price of billet aluminum.

Quick tip speed calculation: Tip speed (ft/sec) = (π × wheel diameter in inches × RPM) / 720. A 76mm (2.99") wheel at 150,000 RPM = (3.14159 × 2.99 × 150,000) / 720 ≈ 1,957 ft/sec. That's already past safe territory for cast aluminum — billet required.

Real-world example: The Precision 6870 Gen2 uses a billet 2618 aluminum wheel rated for 1,000+ HP. Owners running ethanol at 40+ psi routinely push these to 180,000 RPM. A cast equivalent would shed blades catastrophically — the failure mode is usually a single blade root cracking, then the unbalanced wheel destroys the housing in milliseconds.

Rule of thumb: if you're targeting more than ~600 wheel-hp from a single turbo, you want billet. Above ~1,200 hp on a small-frame turbo where RPM stays sky-high, titanium starts making sense despite the spool penalty.