Free tools
Port velocity
The air velocity in the port determines both filling and where the power lands. Too high a velocity chokes the flow, too low gives weak filling. Enter the flow in CFM and the port's cross-sectional area, and you get the velocity in ft/s and m/s.
All tools
Air velocity in the port
177,0 m/s
Average air velocity in the port. The guide value for a well-filling port is often around 250-300 ft/s (75-90 m/s) at the power peak. Too high a velocity chokes the flow, too low gives weak filling.
How the calculation works
The air velocity is the flow divided by the area. The factor 2,4 rolls the conversion from cubic feet per minute to per second together with the change between square inches and square feet into a single constant. We take your cross-sectional area in square millimetres and convert it to square inches (1 square inch is 645,16 square millimetres) before dividing.
The velocity describes how fast the air must move through the given cross section to handle the flow. A smaller cross section gives higher velocity, which favours filling lower down but can choke the top end. When porting, the cross section is balanced against the target, since biggest is not the same as best.
Example
A flow of 600 CFM through a port with a 1600 mm² cross section (2,48 in²) gives an air velocity of about 581 ft/s, that is roughly 177 m/s.
Guide values for port velocity at the power peak
| Velocity | Interpretation |
|---|---|
| Under 200 ft/s (60 m/s) | Low, the port may be too large for the filling |
| 250-300 ft/s (75-90 m/s) | Common target for a well-filling port |
| Over 350 ft/s (107 m/s) | High, the port may choke the flow at the top end |
Guide values at the power peak. The optimal window varies with engine type, cam and rpm.
More tools
Related services
Building for real? We take care of the job.
Common questions about port velocity
Need help with your engine build?
Describe your project and we will get back to you within 24 hours. Quote after inspection.