Enter your target horsepower, engine specs, or a throttle body diameter to size the right throttle body for your build or estimate the horsepower a given bore can support.
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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.
