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Valve/cylinder ratio
The ratio between valve area and cylinder area determines how fast the air must move to fill the cylinder. Enter the bore, stroke, rpm and valve data, and you get the mean velocity through the valves and the valve area as a share of the bore.
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Mean velocity through the valves
60,6 m/s
Mean velocity through the intake valves (not the peak velocity, which is higher). The valve/bore percentage is the combined valve area divided by the piston area. Reckon 130-160 m/s as a common ceiling for the mean velocity on a street engine.
How the calculation works
The air filling the cylinder must pass through the valve area. By continuity (area times velocity is constant) the mean velocity through the valves equals the mean piston speed times the ratio between piston area and total valve area. Larger valves or more valves lower the velocity, higher rpm raises it. This is an average, the peak velocity mid-lift is considerably higher.
Valve area as a share of the bore is the classic valve/cylinder measure, that is combined intake valve area divided by the piston area. A higher value means more valve per cylinder and thus lower air velocity. If the mean velocity gets too high the valve chokes the top end, and then larger valves, more valves or better porting is the way forward.
Example
86 mm bore, 86 mm stroke, 7000 rpm and two intake valves of 35 mm give a mean velocity of about 60,6 m/s (199 ft/s), with a valve area of 33 percent of the bore.
Guide values for mean velocity through the valves
| Application | Mean velocity |
|---|---|
| Street engine | under 130 m/s |
| Sporty street and track | 130-150 m/s |
| Race, high-rpm | 150-160+ m/s |
Guide values for mean velocity, not peak velocity. Real flow and velocity distribution depend on the port shape and are confirmed on the flow bench.
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