HomemadeTurbo - DIY Turbo Forum

Duration refers to how long a valve is opened in relation to crankshaft rotation. This open valve time period is expressed in degrees of crankshaft rotation. So, a cam specification of 220 degrees duration simply means the cam holds the valve open for 220 degrees of crankshaft rotation. As strange as this may sounds, more duration can be helpful in high RPM engines but not low RPM engines. The extra degrees of open valve time in high RPM engines gives the air flow a little more time to get into (or out of) the cylinder in spite of the piston's stroke. However, at lower RPMs, more duration can cause less power because the valves will be open at the wrong time in relation to the piston's stroke up or down in the cylinder.
While duration refers to how long the valve is opened, cam lift is used to determine how wide the valve is opened. If the valves are not opened wide enough, they will cause a restriction for the air trying to enter or exit the cylinder. However, opening the valve past a certain point will not increase the flow to (or from) the cylinder. It is necessary to understand a large amount of science to understand how the flow is related to how wide the valve is opened and how this affects the engine's power. Too much isnt good but too little also isnt good. but like in most performance....you loose some here..but you gain some there. hope this helps.