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Runner volume
An intake runner is usually slightly tapered, and both the volume and how quickly it tapers affect filling and response. Enter the entry diameter, exit diameter and length, and we work out the runner volume as the mean area times the length, plus the included taper angle.
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Runner volume
209,0 cc
The volume is the mean area times the length (trapezoidal approximation). Street is often at 2-4 degrees of included angle, race 3-6 degrees.
How the calculation works
Each end of the runner has an area that is π times radius squared. Because the channel tapers we take the average of the two areas and multiply by the length. That gives a trapezoidal approximation of the volume, which we divide by 1000 to get cc from measurements in mm.
The included angle describes how steeply the runner tapers. It is worked out as twice the angle between the walls seen from the centre, that is 2 × atan of half the diameter difference divided by the length. A small angle keeps air velocity high, while a larger angle increases flow but lowers velocity. These are geometric guide values; the real flow is measured on the flow bench.
Example
A runner with a 44 mm entry diameter, 50 mm exit diameter and 120 mm length gives a mean area of about 1742 mm², a volume of 209 cc and an included angle of 2,86 degrees.
Guide values for included taper angle
| Application | Included angle |
|---|---|
| Street | 2-4 degrees |
| Race | 3-6 degrees |
| Larger angle | more flow, lower velocity |
| Smaller angle | higher velocity, less flow |
Guiding range. A longer runner lowers the rpm of the power peak, a shorter one raises it.
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Common questions about runner volume
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