Calculator Inputs
Formula Used
Engine airflow: CFM = CID × RPM × VE × pressure ratio ÷ 3456
Power airflow: CFM = horsepower × 1.5
Air density: ρ = P ÷ R T
Pressure drop area: A = Q ÷ Cd √(2ΔP ÷ ρ)
Velocity area: A = Q ÷ V
Diameter: D = √(4A ÷ π)
The calculator uses the larger area from pressure drop and velocity checks. It then adds the selected safety margin.
How to Use This Calculator
- Enter engine displacement in cubic centimeters.
- Enter the highest rpm used for sizing.
- Add estimated volumetric efficiency.
- Enter boost pressure if the engine is boosted.
- Add target horsepower or a known manual airflow.
- Set pressure drop, discharge coefficient, and velocity limit.
- Choose the sizing basis, then press Calculate.
- Use CSV or PDF buttons to save the result.
Example Data Table
| Engine | RPM | VE | Boost | Target Power | Suggested Range |
|---|---|---|---|---|---|
| 1600 cc street engine | 6200 | 88% | 0 psig | 130 hp | 48 to 55 mm |
| 2000 cc tuned engine | 7000 | 95% | 0 psig | 240 hp | 65 to 72 mm |
| 2500 cc boosted engine | 6800 | 92% | 12 psig | 420 hp | 78 to 90 mm |
| 5700 cc performance engine | 6000 | 90% | 0 psig | 430 hp | 85 to 95 mm |
Why Throttle Body Size Matters
A throttle body controls how much air enters the engine. Its diameter affects flow, response, and pressure loss. A small bore can restrict top end power. A very large bore can soften low speed control. Good sizing balances peak airflow with stable velocity.
The physics is simple. Air needs enough area to pass through the blade opening. That area depends on volume flow, discharge coefficient, and the allowed pressure drop. It also depends on the velocity limit you choose. This calculator checks both limits. It then recommends the larger diameter, because the larger value is safer for flow.
Useful Inputs
Engine displacement, speed, and volumetric efficiency estimate engine demand. Boost pressure raises the absolute pressure ratio. Target power gives another airflow estimate. Manual airflow lets you test a known dyno or flow bench value. Air temperature changes density. Warmer air is less dense, so it can need more volume flow for the same mass flow.
Pressure drop is important. A lower pressure drop needs a larger throttle. A higher pressure drop allows a smaller bore, but it may reduce manifold pressure. Discharge coefficient represents blade, shaft, and edge losses. A clean entry may flow better than a sharp or blocked design.
Reading the Result
The result includes engine based airflow, power based airflow, and selected design airflow. It also shows area by pressure drop and area by velocity. The final diameter includes your safety margin. That margin helps cover sensor error, port mismatch, filter restriction, and future tuning changes.
Use the answer as a design guide. Actual engines have bends, plenums, couplers, and throttle angle effects. These details change real flow. Racing engines may accept higher velocity. Street engines often need smoother control. Match the size to the complete intake system.
Practical Notes
Do not choose a bore only because it looks large. Oversizing can make tuning harder near idle and part throttle. Undersizing can limit power near redline. Compare the calculated size with available throttle bodies. Pick the nearest larger common size when performance is the goal. Pick the nearest practical size when drivability matters most. Always verify clearance, linkage sweep, and sensor range. Check idle hardware before buying parts or machining adapters first.
FAQs
What is a throttle body size calculator?
It estimates throttle bore diameter from airflow, pressure drop, velocity, and engine demand. It helps compare practical sizes before buying or machining parts.
Is a larger throttle body always better?
No. A larger bore can support more airflow, but it can reduce fine control at low throttle openings. The best size balances peak power and drivability.
What discharge coefficient should I use?
A typical throttle body may use 0.65 to 0.85. Smooth entries and cleaner shapes use higher values. Conservative estimates use lower values.
Why does pressure drop matter?
Pressure drop shows how much restriction the throttle creates. Lower allowed drop needs more area. Higher allowed drop permits a smaller diameter.
What velocity limit is reasonable?
Many street estimates use 70 to 100 m/s. Racing setups may allow more. Lower values usually favor smoother flow and less restriction.
Can I use this for boosted engines?
Yes. Enter boost pressure to adjust pressure ratio and density. Real boosted layouts vary, so confirm the result with intake data when possible.
Why include horsepower airflow?
Horsepower gives a second demand estimate. It is useful when the engine airflow estimate is uncertain or the engine has unusual efficiency.
Should I pick the exact calculated size?
Usually, choose the nearest available size. For performance builds, choose the nearest larger size. For street control, avoid unnecessary oversizing.