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Intake runner length
A tuned intake runner uses the pressure wave in the channel to pack the cylinder extra at a chosen rpm. Enter the rpm you want to peak at, the intake closing and the air temperature, and you get a tuned runner length in mm, plus the wave speed and the time the wave has.
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Tuned runner length
386 mm
The runner is tuned so the pressure wave has time to bounce back in time. A longer runner gives the torque peak at lower rpm, a shorter runner moves the peak up the range.
How the calculation works
When the intake valve opens a pressure wave is sent up the runner, bounces off the open end and comes back. If you tune the length so the wave is back just before the valve closes, the cylinder is packed extra. The time available depends on the rpm and how late the intake valve closes. The wave speed is the speed of sound in the intake air, which rises with temperature, which is why the air temperature is part of the calculation.
Because the length is worked out from wave speed times time, warmer intake air (a faster wave) means a longer tuned runner, and a higher rpm a shorter one. A longer runner places the torque peak lower in the range and a shorter one moves it up. Use the figure as a starting point and then tune against the dyno, since real port geometry and the plenum have an effect.
Example
7500 rpm, intake closing 45 degrees after bottom dead centre and 20 °C air give a wave speed of about 1085 ft/s (331 m/s), an available time of around 7,0 ms and a tuned runner length of roughly 386 mm (15,2 inches).
Runner length and character
| Longer runner | Shorter runner |
|---|---|
| Torque peak lower in the range | Torque peak higher in the range |
| Better on the street and low rpm | Better at high rpm and racing |
| The wave covers a longer distance | The wave turns faster |
Direction, not exact values. Plenum volume, port size and cam affect the real outcome.
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Common questions about intake runner length
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