Racers Place / Engine calculators
Throttle body sizing calculator
Size the throttle body for your engine’s airflow demand. Compare your selected diameter with the target air velocity and review airflow and power estimates.
Engine details
Enter absolute pressure: 1 bar represents a naturally aspirated engine in this model. For example, 1 bar of boost is approximately 2 bar absolute at an ambient pressure of 1 bar.
Throttle body details
Results
Instructions and definitions
Enter engine displacement, peak-power RPM and VE (e.g. 0.95). Select throttle position and number of throttles. Set target velocity, Cd and the diameter to compare. Results update automatically. The diameter is per throttle; Q_th is the combined flow through all throttles.
Original calculator velocity comparison: below 60 m/s = low, above 100 m/s = high. Otherwise, a difference of up to 5 m/s from the target is considered close. These thresholds are indicative.
Model assumptions
The original four-stroke engine calculation model is retained. Before the compressor, volume flow is multiplied by the numerical pressure value (reference pressure: 1 bar); after the compressor, engine volume flow is used. Temperature, pressure losses, throttle shaft geometry and pulsating flow are not modelled. The power estimate uses an adjustable CFM/HP factor and the original empirical compression-ratio correction. It is not measured engine power.
Formulas and units
- V = L / 1000 (m³)
- Q_cyl = V × RPM / 120 × VE (m³/s)
- Q_th = Q_cyl (After the compressor); Q_th = p × Q_cyl (Before the compressor); p = MAP / 1 bar
- Q_per = Q_th / N
- A_target = Q_per / (Cd × v_target)
- D = √(4 × A_target / π) × 1000 (mm)
- v = Q_per / (Cd × π × (D_selected / 1000)² / 4)
- CFM = p × Q_cyl × 60 / 0.0283168
- k = min(1.2, max(0.8, 1 + (CR − 10) × 0.025))
- HP ≈ CFM / (CFM/HP) × k