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Intake velocity
When the air passes from one cross section to another, the velocity changes inversely with the area. Enter the area and velocity at the first point and the area at the second point, and we work out the velocity there using the continuity equation.
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Velocity at point 2
54,7 m/s
The velocity is inversely proportional to the area. Guide values: minimum cross section 260-300 ft/s (79-91 m/s), port window 240-280 ft/s, runner 200-250 ft/s.
How the calculation works
The continuity equation says the flow is constant, that is area times velocity is the same through the whole channel. If the cross section grows the velocity falls, if it shrinks it rises. So the velocity at point 2 equals area 1 times velocity 1, divided by area 2.
The air velocity is central to how well the intake fills the cylinder. Too low a velocity gives sluggish filling, too high gives losses and turbulence. Guide values usually sit around 260 to 300 ft/s at the minimum cross section and lower further out towards the plenum. This is an idealised calculation; the real flow is measured on the flow bench.
Example
With area 1 at 1290 mm², velocity 1 at 82 m/s and area 2 at 1935 mm², the velocity at point 2 is about 54,7 m/s, that is roughly two thirds since the area is 1,5 times larger.
Guide values for air velocity in the intake
| Point in the intake | Air velocity |
|---|---|
| Minimum cross section (MCSA) | 260-300 ft/s (79-91 m/s) |
| Port window | 240-280 ft/s (73-85 m/s) |
| Runner | 200-250 ft/s (61-76 m/s) |
| Plenum inlet | 150-200 ft/s (46-61 m/s) |
| Throttle body | 300-350 ft/s (91-107 m/s) |
Guiding range. Smooth area transitions reduce turbulence and losses.
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