Size a throttle body from horsepower or engine airflow, or estimate bore capacity. Compare CFM, bore count, area, margin, and diameter units.
Formula
The calculator uses two core relationships depending on the mode you pick.
Airflow demand from engine specs:
CFM = (CID × RPM × VE) ÷ 3456
where CID = displacement in cubic inches, RPM = peak engine speed, VE = volumetric efficiency (decimal).
Throttle body bore from airflow:
D = √(CFM ÷ 0.17)
where D = throttle body diameter in mm, CFM = required airflow.
Airflow-to-horsepower conversion:
HP = CFM ÷ k
where k ≈ 1.6 (street NA), 1.4 (built NA), 1.08 (~7 psi), 0.88 (~12 psi), 0.75 (~17 psi).
Interpretation
The result tells you the minimum single-bore throttle body diameter that will not choke your engine at peak demand. Undersizing restricts top-end power; oversizing hurts throttle response and low-RPM driveability. Match the bore to your intake manifold flange — a TB larger than the runner entry creates a step that disrupts flow.
- Under 55 mm: small 4-cylinders or very mild builds.
- 55–75 mm: stock to mild V6/V8, turbo-4.
- 75–92 mm: built NA V8 or moderate boost.
- 92–105 mm: high-HP NA or high-boost setups.
- Over 105 mm: extreme — usually twin TBs or custom intakes.
Quick Reference Table
| TB Diameter | Approx CFM | NA Street HP (k=1.6) | NA Race HP (k=1.4) | Boosted HP (k=0.88) |
|---|---|---|---|---|
| 58 mm | 572 | 358 | 408 | 650 |
| 65 mm | 718 | 449 | 513 | 816 |
| 75 mm | 956 | 598 | 683 | 1,087 |
| 85 mm | 1,228 | 768 | 877 | 1,396 |
| 92 mm | 1,439 | 899 | 1,028 | 1,635 |
| 102 mm | 1,769 | 1,106 | 1,264 | 2,011 |
FAQ
Does a bigger throttle body always add horsepower?
Only if the current one is actually restrictive at your peak airflow demand. If your TB is already sized correctly for the engine, a larger one adds no power and can soften throttle response.
What volumetric efficiency (VE) should I use?
Use 75–80% for stock engines, 85–90% for mild performance builds, 95–100% for race heads and cams, and over 100% for boosted engines (scaled by manifold pressure ratio). The calculator’s dropdown gives typical values for each case.
Should I measure the TB at the bore or the blade?
Use the bore diameter — the inside measurement of the housing where the blade sits. Manufacturers spec throttle bodies by bore, and that’s what the formula assumes.
Can I use one big throttle body instead of twin TBs?
For very high airflow (over ~1,500 CFM), twin throttle bodies or an oval/rectangular TB usually flow better than one oversized round bore because they match intake plenum geometry more cleanly. Above roughly 105 mm single-bore, check that your intake can actually use the extra flow.
