How turbo compressor sizing works
Picking a turbocharger comes down to matching the compressor's airflow capacity to how much air your engine actually needs to breathe at the RPM where you want boost. Undersize the compressor and it runs out of flow, choking the engine and stalling power gains. Oversize it and the turbo spools late, lagging badly before it makes useful boost. This calculator uses the standard airflow-demand method used by engine builders and turbo manufacturers to estimate the flow figure you need before comparing compressor maps.
Step 1: Engine airflow demand (CFM)
A four-stroke engine only draws air on every other revolution, so its volumetric airflow demand is CFM = (CID × RPM × VE) / 3456, where CID is displacement in cubic inches (liters × 61.0237), RPM is the engine speed you care about (usually redline or your target boost-onset speed), and VE is volumetric efficiency as a percentage. This is the same equation used to size carburetors and throttle bodies — a turbo simply needs to deliver this airflow at pressure instead of at atmospheric pressure alone.
Step 2: Convert to compressor mass flow (lb/min)
Compressor maps are plotted in pounds of air per minute, not CFM, because mass flow accounts for the fact that boosted air is denser than atmospheric air. The conversion multiplies CFM by the standard sea-level air density (about 0.0763 lb/ft³) and by the pressure ratio created by boost: lb/min = CFM × 0.0763 × (boost psi + 14.7) / 14.7. The pressure ratio itself — (boost + atmospheric) ÷ atmospheric — is also the figure you plot against mass flow to read a compressor efficiency map.
Step 3: Estimate power potential
Turbo builders commonly use the rule of thumb that 1 lb/min of airflow supports about 10 horsepower on pump gasoline. That comes directly from the fuel-flow relationship: horsepower × BSFC (brake specific fuel consumption, ~0.50 lb/hp-hr for a boosted gasoline engine) × air-fuel ratio (~12:1 under boost) ÷ 60 minutes gives the fuel and air an engine burns per minute, which rearranges to horsepower ≈ lb/min × 10. It is an approximation, not a guarantee — actual output depends on the specific engine, fuel, intercooling, and tune.
Choosing volumetric efficiency
VE is the hardest input to know exactly without a dyno. Stock engines typically run 80-90% VE near their power peak; well-developed heads, cams, and intake/exhaust work can push naturally aspirated VE to 95-105%; and an engine already under boost can show VE above 100% because the turbo itself is helping fill the cylinder. When in doubt, 85% is a reasonable, slightly conservative starting point for a street engine.
Reading the result
Once you have a target lb/min figure, compare it to a compressor's flow map at your expected pressure ratio: the point should sit inside the map's efficiency islands (commonly 70%+ efficiency), not out past the surge line on the left or the choke line on the right. A turbo whose map comfortably covers your lb/min and pressure ratio at good efficiency will spool reasonably and support the horsepower this calculator estimates.