What Determines the Correct Net Valve Lift
(We are assuming the correct port and valve area requirements have been met and there are no other restrictions in the intake or exhaust tracks.)

Net intake valve lift can be determined using the required intake valve/port area and the effective valve circumference.  The effective valve area is calculated from the measured port flow CFM at the convergence valve lift.  The valve diameter is calced from the effective valve area, then the effective valve circumference is calced from the effective valve diameter.  The required intake valve area is divided into the effective valve circumference, giving us the net valve lift required to acheive an effective valve curtain area equal to the required intake valve/port area.  


The Parameters Involved


Net intake valve lift is affected by a few parameters with obvious relationships.  The bore is the first on our list, and has a squared relationship to the net valve lift.  As the bore increases, the cylinder volume increases by the square of the bore increase.  The net valve lift must also be increased by the square of the bore increase to accomodate the additional inlet charge required to fill the cylinder volume increase.  This affects the required intake valve/port area.


The stroke is second on our list, and has a direct relationship with the required net valve lift.  As the stroke increases, the cylinder volume increases by the same percentage.  The net valve lift must also be increased by the same percentage to accomodate the cylinder volume increase.  This affects the required intake valve/port area.


Obviously, the cylinder volume is the parameter that directly affects the required net valve lift.  In the supply and demand equation to calculate net valve lift, it is the initial part of the demand, which calculates the required intake valve/port area.


Mean piston velocity also has a direct affect on the net valve lift, and when introduced into the equation changes the cylinder volume into the rate of volume demand in CFM and also affects the required intake valve/port area.


Volumetric efficiency % is directly related to the net valve lift, and affects the required intake valve/port area.


The intake port flow CFM, measured at the convergence valve lift, is the supply portion of the equation.  It has an inverse, square root relationship to the net valve lift.  So, as the CFM increases, the net valve lift requirement decreases by the square root of the CFM increase.  This determines the effective port area.


The mean port velocity affects the supply portion of the equation, and also has an inverse, square root relationship to the net valve lift.  Increasing the mean port velocity will decrease the net valve lift requirement by the square root of the mean port velocity increase.  This is also a determining factor for the effective port area.


Try writing an equation for the net intake valve lift​ using the information given on this page.

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