Calculator Inputs
Example Data Table
| Engine | RPM | VE | Boost | Throttle | Estimated Use |
|---|---|---|---|---|---|
| 2.0 L | 6500 | 90% | 0 psi | 60 mm single | Balanced street build |
| 3.0 L | 6000 | 92% | 8 psi | 70 mm single | Boosted utility engine |
| 4.8 L | 5600 | 88% | 0 psi | 75 mm single | Large displacement swap |
| 1.6 L | 7200 | 95% | 0 psi | 45 mm x 4 | Individual throttle body setup |
Formula Used
Airflow in liters per minute:
Airflow = Engine liters × 1000 × RPM × VE × Boost ratio ÷ 2
Boost ratio:
Boost ratio = (Boost psi + 14.7) ÷ 14.7
Throttle bore area:
Area = π × (Diameter ÷ 2)² × Number of throttle bodies
Air velocity:
Velocity = Airflow in m³/s ÷ Total throttle area in m²
Pressure drop estimate:
Pressure drop = Air density × Velocity² ÷ (2 × Discharge coefficient²)
Suggested diameter:
Diameter = √((4 × Required area) ÷ (π × Throttle body count))
How to Use This Calculator
Enter the engine displacement in liters. Add the planned maximum RPM. Use realistic volumetric efficiency for the engine build. A mild engine may use 80 to 90 percent. A strong naturally aspirated engine may use more.
Add boost pressure if the engine uses forced induction. Keep it at zero for naturally aspirated setups. Enter the current or planned throttle diameter. Choose the number of throttle bodies.
Use target velocity to control sizing behavior. Lower velocity favors less restriction. Higher velocity can improve response but may create pressure loss. Press the calculate button. Review the result block above the form.
MegaSquirt Throttle Body Planning Guide
Why Throttle Sizing Matters
A throttle body is more than a round opening. It controls air speed, engine response, manifold pressure behavior, and tuning stability. In a MegaSquirt project, these details matter because the controller reacts to sensor changes. A throttle that is too small can limit airflow near peak power. A throttle that is too large can make low speed control sharp and difficult. Good sizing helps the engine breathe without making drivability harsh.
Airflow and Engine Demand
The calculator estimates airflow from displacement, RPM, volumetric efficiency, and boost. Four stroke engines pull air once every two crank rotations. This is why the formula divides by two. Boost increases the mass of air entering the engine. Higher volumetric efficiency also increases demand. These values help create a practical airflow estimate before fabrication.
Bore Area and Velocity
Bore area is calculated from the throttle diameter. Multiple throttle bodies add more total area. Air velocity is then found by dividing airflow by area. This value is useful during construction planning. Very high velocity can suggest a possible restriction. Very low velocity can mean the throttle is oversized for the target engine behavior.
Pressure Drop Review
The pressure drop result is an estimate. It uses air density, velocity, and discharge coefficient. A smooth throttle body usually flows better than a rough or poorly blended part. Sharp edges, weld beads, and sudden transitions can increase losses. Use the result as a planning guide, not as a final dyno number.
Fabrication Notes
Construction projects often involve custom manifolds, adapters, brackets, and linkages. Keep the throttle shaft aligned. Avoid sudden diameter changes near the plate. Place sensors where they receive stable readings. For MegaSquirt tuning, clean throttle position signals and steady manifold pressure are important. Test the final assembly for leaks before tuning.
Final Sizing Advice
Choose a throttle body that supports peak airflow while keeping good part throttle control. The suggested diameter includes a safety factor. This helps cover real world losses. Compare the result with available parts. Then consider packaging, cable movement, idle control, and service access. A practical part is often better than the largest part available.
FAQs
1. What does this calculator estimate?
It estimates airflow, bore area, air velocity, pressure drop, and suggested throttle diameter for MegaSquirt engine projects.
2. Can I use it for naturally aspirated engines?
Yes. Set boost pressure to zero. Then enter displacement, RPM, volumetric efficiency, and throttle size.
3. Can I use it for boosted builds?
Yes. Add the planned boost pressure. The calculator adjusts airflow demand using a pressure ratio.
4. What is a good target velocity?
Many builds work well around 70 to 100 m/s. Lower values reduce restriction. Higher values may sharpen response.
5. Why does throttle count matter?
More throttle bodies increase total area. This lowers velocity for the same airflow and changes response behavior.
6. Is pressure drop exact?
No. It is an estimate. Real pressure loss depends on shape, plate angle, inlet design, and surface finish.
7. What discharge coefficient should I use?
A value from 0.75 to 0.90 is common. Smooth parts use higher values. Rough parts use lower values.
8. Should I choose the largest throttle body?
No. Oversized parts can make control sensitive. Choose a size that supports airflow and keeps smooth drivability.