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Throttle body size
Too large a throttle body gives sluggish response, too small a one chokes the top end. Enter the target power and the throttle shaft diameter, and you get the bore at two air velocities: 200 ft/s for torque and 150 ft/s for power.
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Torque (200 ft/s)
87 mm
Throttle body diameter at two air velocities: 200 ft/s gives the fastest throttle response and torque, 150 ft/s gives more at the top end. The diameter accounts for the throttle shaft's blockage. The air demand is based on the rule of thumb of 1,5 CFM per horsepower.
How the calculation works
The air demand is estimated with the rule of thumb of 1,5 CFM per horsepower. The demand is converted to a required open area at a chosen air velocity, and then the throttle shaft's blocked surface is added so the real diameter leaves enough free opening. The factor 2,4 is the same lumped constant used for port velocity.
Two velocities give two recommendations. 200 ft/s keeps the velocity up for fast throttle response and strong torque, while 150 ft/s gives a larger bore that lets more through at the top end but responds more slowly. A larger throttle body is not automatically better, it must match the intake, displacement and how the car is to be driven.
Example
500 hp with a 10 mm throttle shaft gives an air demand of 750 CFM, which points to about 87 mm bore at 200 ft/s and about 100 mm at 150 ft/s.
Air velocity and character
| Goal | Air velocity | Power |
|---|---|---|
| Torque and response | 200 ft/s | Smaller bore, fast throttle response |
| Top-end power | 150 ft/s | Larger bore, more at the top end |
Also consider the intake's plenum volume. A larger throttle body gives a better top end but slower response.
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Common questions about throttle body size
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