Estimate the turbo airflow you need in lb/min and CFM, plus boost and compressor pressure ratio, from engine size, RPM and target horsepower.
Turbo Size Formula
Estimate actual airflow and a pressure operating point for a four-stroke engine. Select a turbo using the exact manufacturer compressor map and its corrected-flow conventions. The core power-to-airflow relation is:
The retained legacy diameter formula below is an unvalidated heuristic, shown only in calculation details. It does not select a turbo, wheel design or model:
The boost mode now accounts for manifold temperature and is consistent with the target-power mode. With pressure in psi, swept volume in ft³/min and temperature in °R:
Swept flow = displacement × RPM / 2 × VE Actual lb/min = MAP × swept flow × 144 / (53.35 × temperature) PR = (MAP + outlet loss) / (ambient pressure − inlet loss)
Variables:
- Wa is the required compressor airflow in pounds of air per minute (lb/min)
- HP is the target horsepower at the crank
- AFR is the air-fuel mass ratio. Presets 11.5, 7.9 and 20 are illustrative gasoline, E85 and diesel assumptions; enter an override for the engine operating point. These are not tuning targets.
- BSFC is brake specific fuel consumption in lb fuel per hp-hour. Presets 0.50, 0.63 and 0.38 are editable examples, not universal fuel constants.
- D is the output of an unvalidated legacy diameter proxy in millimeters; the relationship has no verified manufacturer basis.
- PR is the pressure ratio across the compressor
- Boost is manifold gauge pressure. Ambient pressure is absolute and editable; blank uses 14.7 psia. Inlet and outlet pressure losses start at zero.
- RPM is engine speed and VE is volumetric efficiency as a fraction in the formula (the field accepts percent). Displacement is converted to ft³, and °R = °F + 459.67. The air gas constant 53.35 is an engineering approximation.
The three modes estimate airflow and manifold pressure from crank power, actual airflow and compressor PR from boost, or illustrative power from actual airflow. Corrected flow is not calculated: use the selected map’s inlet and reference conditions before plotting. CFM equivalent assumes 0.0765 lb/ft³ and is neither manifold flow nor a universal standard-air convention. See Garrett’s calculation guide and BorgWarner’s compressor-map guide.
Legacy Diameter Heuristic and Fuel Assumptions
This table demonstrates only the unvalidated legacy equation Wa = 0.0224D² and the gasoline assumptions AFR 11.5, BSFC 0.50. These are calculated proxy values, not manufacturer flow ratings or turbo recommendations. Named turbo matches have been removed because diameter alone cannot establish capability.
| Proxy diameter | Equation-only airflow | Illustrative gasoline crank power |
|---|---|---|
| 38 mm | 32.3456 lb/min | 337.519304 hp |
| 42 mm | 39.5136 lb/min | 412.315826 hp |
| 46 mm | 47.3984 lb/min | 494.592 hp |
| 49 mm | 53.7824 lb/min | 561.207652 hp |
| 54 mm | 65.3184 lb/min | 681.583304 hp |
| 58 mm | 75.3536 lb/min | 786.298435 hp |
| 62 mm | 86.1056 lb/min | 898.493217 hp |
| 67 mm | 100.5536 lb/min | 1,049.254957 hp |
| 76 mm | 129.3824 lb/min | 1,350.077217 hp |
The coefficient 0.0224 is retained only to explain the old heuristic. It has not been validated against manufacturer maps. It must not be used to infer turbo frame, efficiency, surge, choke or safe shaft speed. The next table contains illustrative editable fuel assumptions; its airflow-per-100-hp values are rounded to one decimal.
| Fuel | AFR under boost | BSFC (lb/hp-hr) | Airflow per 100 hp |
|---|---|---|---|
| Gasoline | 11.5:1 | 0.50 | 9.6 lb/min |
| E85 | 7.9:1 | 0.63 | 8.3 lb/min |
| Diesel | 20:1 | 0.38 | 12.7 lb/min |
Example Problems
Example 1: 400 crank hp on a 2.0 L gasoline engine. At AFR 11.5 and BSFC 0.50, actual airflow is 38.333333 lb/min, or 501.089325 CFM equivalent at the assumed density. At 6,800 RPM, 95% VE, 90°F manifold temperature and 14.7 psia ambient pressure with zero losses, calculated manifold gauge pressure is 19.518654 psi and compressor PR is 2.3278. This is an operating-point estimate, not a wheel or turbo model recommendation.
Example 2: power from actual airflow. At 48 actual lb/min, the gasoline assumptions give 500.869565 crank hp. The retained E85 assumptions give 578.661844 hp. These differences follow the assumed AFR and BSFC; they do not guarantee output or prove that a compressor can supply this flow at the required PR.
Frequently Asked Questions
What does a quoted compressor inducer diameter mean? It describes a wheel inlet diameter, but wheel design, trim and operating pressure also affect capability. Diameter alone does not identify a map or a compatible turbine and housing. The legacy diameter proxy in details is not a verified sizing relationship.
Should I select the next diameter up? No universal size-up rule follows from this calculation. Compare multiple operating points on the exact manufacturer map, including surge, choke, efficiency and speed limits, and assess turbine matching and response. A maximum-flow figure or a single RPM point cannot predict spool.
Why can an actual build need different boost? Temperature, VE, pressure losses and fuel-consumption assumptions alter the operating point. Manifold gauge pressure and compressor PR are distinct; lower ambient pressure raises PR at a fixed gauge boost. A model result at or below zero gauge boost is not proof of naturally aspirated capability.
