Turbo Sizing Calculator
Turbocharger selection from engine displacement and power target.
About this calculator
This calculator estimates the compressor airflow, pressure ratio, and rough turbo frame size needed to reach a target horsepower on a given engine. It computes airflow demand two independent ways and recommends sizing for whichever is higher: the engine's own volumetric airflow demand at your target boost and peak RPM (using the standard four-stroke relationship that each cylinder draws one intake charge per two crankshaft revolutions), and a simplified horsepower-based estimate using the common rule of thumb that gasoline engines need roughly 1 lb/min of airflow per 9-10 horsepower -- that mass-flow figure is then converted to standard CFM using standard sea-level air density so both estimates are on the same basis before comparing them.
Pressure ratio comes directly from boost pressure relative to standard atmospheric pressure (14.7 PSI), and compressor outlet temperature applies the adiabatic compression relationship -- temperature rises with pressure ratio raised to the power 0.283 (the (k-1)/k exponent for air) and is reduced by compressor efficiency, which is why a less efficient compressor produces more heat for the same boost level. The turbo- class and turbine A/R figures are rough industry-standard sizing buckets, not manufacturer-specific part recommendations -- actual turbo selection also depends on exhaust manifold design, desired spool characteristics, intended use (street versus race), and specific compressor and turbine maps that this simplified calculator does not model.
Inputs
Results
Pressure ratio
1.95
Required airflow (CFM)
1,220.9
How to Use This Calculator
- Enter Engine displacement (ci), Target HP, and Boost pressure (PSI).
- Set Peak RPM, Ambient temp (°F), and Compressor efficiency (%).
- Review Pressure ratio and Required airflow (CFM).
- Use Engine demand (CFM) and HP-based flow (CFM) to inform your decision.
How the result changes with Boost pressure (PSI)
| Boost pressure (PSI) | Pressure ratio | Required airflow (CFM) |
|---|---|---|
| 7 | 1.48 | 923.2 |
| 11 | 1.75 | 1,093.3 |
| 21 | 2.43 | 1,518.7 |
| 35 | 3.38 | 2,114.3 |
What each input means
- Engine displacement (ci)
- Engine displacement in cubic inches (e.g. 350 ci = 5.7L). Multiply liters × 61.02 to convert.
- Target HP
- Desired wheel horsepower at peak boost.
- Boost pressure (PSI)
- Target boost pressure in PSI gauge (typical street: 8-15, race: 15-30+).
- Peak RPM
- Engine RPM at which peak boost and power occur.
- Ambient temp (°F)
- Ambient air temperature in degrees Fahrenheit.
- Compressor efficiency (%)
- Compressor adiabatic efficiency (typical peak: 70-78%).
What each result means
- Pressure ratio
- Compressor pressure ratio = (boost + 14.7) / 14.7.
- Required airflow (CFM)
- Recommended compressor map flow rate in cubic feet per minute.
- Engine demand (CFM)
- Airflow the engine needs at given RPM and boost.
- HP-based flow (CFM)
- Airflow needed purely based on target HP.
- Compressor outlet (°F)
- Air temperature leaving the compressor before intercooler.
- Temp rise (°F)
- Temperature increase across the compressor.
- Displacement (L)
- Engine displacement converted to liters.
- Suggested turbine A/R
- Approximate turbine housing A/R ratio for your displacement.
How this is calculated
Worked example, using the default values
- Identify Input Parameters6 parametersEngine displacement (ci) = 350, Target HP = 500, Boost pressure (PSI) = 14, Peak RPM = 6500, Ambient temp (°F) = 75, Compressor efficiency (%) = 72 = 6 input(s) provided
- Calculate Pressure ratioPressure ratio = (boostPsi + 14.7) / 14.71.952 = 1.952
- Calculate Required airflowRequired airflow = max(engineAirflowCfm, targetAirflowCfm)1220.9 = 1220.9
- Calculate Engine demandEngine demand = (displacement / 2) * (peakRpm / 1728) * ve * pressureRatio1220.9 = 1220.9
- Calculate HP-based flowHP-based flow = (targetHp / 9.5) / 0.0765688 = 688
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the calculator compute airflow two different ways?
Engine demand (from displacement, RPM, and boost) tells you how much air the engine can physically ingest at your target boost and RPM, while the HP-based estimate tells you how much air is typically needed to produce your target horsepower using a standard gasoline airflow-to-power ratio. The calculator recommends sizing the compressor for whichever figure is higher, since undersizing against either one would leave the turbo unable to support your actual target.
Why does a less efficient compressor produce more heat for the same boost?
Compressing air always adds heat, and the adiabatic compression formula divides the ideal temperature rise by the compressor's efficiency -- a perfectly efficient compressor would add only the theoretical minimum heat for a given pressure ratio, while a less efficient one wastes more of its work as extra heat rather than pure pressure rise. That is why lower compressor efficiency directly increases both compressor outlet temperature and the resulting temperature rise figure.
Why does raising boost pressure increase compressor outlet temperature?
Pressure ratio -- boost pressure relative to atmospheric pressure -- is the direct driver of the adiabatic compression temperature rise formula: outlet temperature scales with pressure ratio raised to the 0.283 exponent. A higher boost target means a higher pressure ratio, which mathematically produces a higher compressor outlet temperature before any intercooling is applied, all else being equal.
Are the turbo class and A/R recommendations specific to any turbo brand?
No -- they are rough industry-standard sizing buckets based on typical compressor wheel inducer diameter ranges and common turbine housing sizing conventions, not recommendations tied to a specific manufacturer's part numbers. Real turbo selection should cross-reference the manufacturer's actual compressor and turbine maps for your specific target airflow, pressure ratio, and desired spool characteristics.
Does this calculator account for exhaust manifold design or spool characteristics?
No -- it estimates required compressor airflow and pressure ratio from engine and target-power inputs only, and does not model exhaust manifold design, turbine housing flow characteristics, or how quickly a given turbo will spool on your specific engine. Two turbos with similar compressor-side airflow capacity can spool very differently depending on turbine sizing and exhaust design, which this tool does not evaluate.
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