TECH TALK!!

If the port is turbulent at the mid point, backs up, stops flowing or goes into air speed choke the engine will "see" that situation far earlier and far more pronounced in the running engine because the air speeds are much higher. You may see the port stop flowing at .700 lift on the flow bench but in the running engine where peak air speeds can be as high as 150" the choke hits much sooner and cylinder fill is decreased substantially. Its called the MEAN INLET MACH NUMBER. When the MIMN reaches .55 the engine VE stops increasing or decreases at a faster rate. You spike the velocity with turbulence or an air speed choke and the engine stops increase air flow relative to engine speed. People always talk about air speed chokes but a turbulence choke will do the same thing just not as bad. If your turbulent and try to flow more air through turbulent area guess what happens to the air flow, it gets even more turbulent! The more air the engine demands, the more turbulent the flow becomes and the worst the choking situation gets. When a head is growing in flow, it assures us the port is happy, therefore the cylinder fill is sufficient. When it's sufficient, at any lift, it will make more power.

The more you turn the engine the more CSA you need. You enlarge the CSA to keep the same air speed.. Put a smaller engine on the same size heads and you must twist it higher in order to achieve the same air flow through the heads in order to get them back in the range where they are efficient ( air speed range). If the head is flowing well, your CSA would be larger. If it's flowing poorly, your CSA would be smaller. Everything is based off of valve size and how much it is flowing.

A venturi is designed to accelerate the air to a given velocity over a very short span and recover the energy at a given rate so as to not waste or use as little of the energy at hand to accomplish the task. Pressure recovery dynamics in the divergent region of the venturi are critical to the amount of air that can pass through it. That's why the bowl, throat, seat and chamber are so critical in a cylinder head. The bowl is the high pressure ( convergent ) region of the venturi and the throat is the venturi itself while the chamber acts like the ( divergent ) region to recover the energy.

FORMULAS

Velocity FPS = ( Test Press ^ .5 ) * 66.2

Port Velocity = ( CFM * 2.4 ) / Cross-Sectional Area

FPS = ( CFM * 2.4 ) / CA

FPS = ( CFM / CA ) * 2.4

CFM = FPS * CA * .41666667

CFM = ( FPS * CA ) / 2.4

CA = ( CFM / FPS ) * 2.4

CA = ( CFM * 2.4 ) / FPS