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Intake Port Mach Index Calculator | |
Where ever the smallest cross-sectional area is located then it will eventually, and ultimately be the "Choke" point. If that Choke-area is at the pushrod, then it will set the Mach number the Port/Engine will be choked at. If the Choke area is the Curtain area, then it will set the Mach number.
The Mach Index is a mathematical expression of the speed, in this case, of the inlet velocity, relative to the speed of sound. Volumetric efficiency will start to fall off sharply past six tenths the speed of sound (Mach .6) so the engine designer will not want to exceed this .6 mach value at peak rpms. In as much as the speed of sound is temperature dependent and the exhaust valve has different requirements, these calculations do not apply to the exhaust valve.
To learn more about Mach Index or other theories try checking out these books on Amazon.com: Scientific Design of Exhaust & Intake Systems (Engineering and Performance) : and Internal Combustion Engine in Theory and Practice:
Intake Valve Closing Versus Mach Index
At high Mach Index Values the cylinder pressure may be well below inlet pressure at Bottom Dead Center. In this case, delaying the intake valve closing allows more flow into the cylinder up to the time when the inlet and cylinder pressures reach equality. Thus, the higher the value of the Mach Index, the later the intake valve should be closed to increase, or maintain volumetric efficiency.
Exhaust Valve Considerations
When the exhaust valve opens there are much higher pressures involved than was the case on the inlet event. The speed of sound is temperature dependent so don't use the above calculator for exhaust valve sizing. Your exhaust valve flow capacity should be in the range of 65 to 80 percent of the inlet valve capacity. Increasing the exhaust valve capacity beyond this will not give performance increases and will in fact hurt your power. As the intake valve size goes up, the exhaust size has to be adjusted accordingly.