Turbo Size Calculator
Determine engine airflow demand and pressure ratio to help select a turbocharger that supports your horsepower goals.
How to Use This Calculator
Enter engine displacement and RPM
Use liters or convert to liters. Pick the RPM where you expect peak power and boost.
Estimate volumetric efficiency
Stock engines range 85–95%. Purpose-built heads or cams may exceed 100%.
Set your target boost
Leave at zero to see naturally aspirated airflow. Add boost psi to compute pressure ratio and corrected flow.
Formula
CFM = (CID × RPM × VE) ÷ 3456
Corrected CFM: CFM × Pressure Ratio
Pressure Ratio: (Boost + 14.7) ÷ 14.7
Mass Flow: CFM × 0.0765 = lb/min
Example: 2.0 L (122 CID), 6500 RPM, 90% VE, 15 psi boost → CFM = 122 × 6500 × 0.9 ÷ 3456 = 206 CFM → Corrected CFM = 206 × 2.02 = 416 CFM → Mass flow ≈ 31.8 lb/min.
About the Turbo Size Calculator
Selecting a turbo involves matching compressor airflow and pressure ratio to your engine’s demand. This calculator offers a quick estimate before diving into compressor maps.
When to Use This Calculator
- Turbo upgrades: Compare airflow requirements to compressor maps when sizing new turbos.
- Engine swaps: Determine if your existing turbo can support a different displacement.
- Forced-induction planning: Evaluate airflow demand before investing in hardware.
- Educational labs: Demonstrate volumetric efficiency and pressure ratio concepts.
Why Use Our Calculator?
- ✅ Physics-based: Uses classic airflow equations to approximate engine demand.
- ✅ Boost aware: Calculates pressure ratio and corrected mass flow for compressor map matching.
- ✅ Quick and clear: Perfect starting point before detailed simulation.
- ✅ Mobile friendly: Use in the garage, classroom, or dyno cell.
Common Applications
Performance tuners: Gauge whether an existing turbo can support a power goal.
Engine builders: Cross-check airflow requirements during component selection.
Students: Study how RPM, displacement, and VE interact with boost.
Tips for Best Results
- Use measured VE or dyno data when available for accuracy.
- Consult compressor maps: keep corrected lb/min within the turbo’s efficiency island.
- Account for intercooler pressure drop and altitude when fine-tuning PR.
- Remember fueling and supporting mods must also meet the power target.
Frequently Asked Questions
How do I pick volumetric efficiency?
Stock engines often range 85–95% at peak power. High-performance heads or camshafts can exceed 100%. Use dyno data when possible.
Does this calculator recommend a specific turbo?
No. It provides airflow and pressure ratio targets. Compare those to compressor maps (CFM or lb/min) to select a turbo within the efficient range.
What about twin turbos?
Divide the corrected airflow between the number of turbos (e.g., twin setup halves the flow per turbo when they share equally).
Can I use this for diesel engines?
Yes. Adjust efficiency and boost values accordingly. Diesel engines often run higher boost and different AFR, which affects mass flow.