The compressor wheel is the business end of the cold side — it's what actually pumps air into your engine. Two numbers define its character: trim and A/R. Understanding them separates guys who pick turbos from catalog blurbs from guys who actually match a turbo to a build.

Trim is a geometric ratio describing how aggressive the wheel is:

Trim = (inducer² / exducer²) × 100

The inducer is the smaller diameter where air enters (the eye of the wheel). The exducer is the larger diameter at the wheel's outer edge where air exits. A Garrett GTX3582R Gen II has a 61mm inducer and 82mm exducer: (61² / 82²) × 100 = 55 trim. Higher trim means a larger inducer relative to the exducer, which flows more air for a given wheel diameter — but spools slower because there's more mass to accelerate.

A/R (area-to-radius ratio) on the compressor housing controls how air enters the wheel. Smaller A/R = faster spool, lower top-end flow. Larger A/R = lazier spool, more top-end. On the cold side, A/R changes are subtle compared to hot-side A/R, which is where most tuning happens.

Blade design matters too. Modern wheels use extended-tip or MFS (machined-from-solid) billet construction with 6+6 blade configurations — six full blades and six splitter blades. The splitters fit between full blades partway down the wheel, increasing flow at high RPM without choking the inducer at low flow. Cast aluminum wheels are cheaper but flex at high boost; billet wheels handle 30+ psi without blade deflection.

Real-world example: Two builders both want 600 hp on a 2.0L. Builder A picks a Garrett G25-660 (54mm inducer, 67mm exducer, 65 trim) — small, snappy, full boost by 3,500 RPM, runs out of breath at 7,000. Builder B picks a G30-770 (58mm inducer, 76mm exducer, 58 trim) — laggier, full boost by 4,500, pulls hard to 8,000. Same horsepower target, completely different driving character.

Rule of thumb: Trim under 50 = response-biased; 50–60 = balanced; over 60 = flow-biased and laggy. For street cars on small engines, stay 52–58 trim. For drag cars where spool doesn't matter, 65+ trim lets you make huge top-end power from a relatively small wheel diameter.

Surge line position on the compressor map shifts with trim too — higher-trim wheels surge earlier at low flow, which is why response-biased builds tolerate aggressive blow-through plumbing while peaky high-trim setups demand careful boost ramp tuning.