Valves get the bump business from pushrods and rocker arms. Factoryrocker arms typically have a 1.6:1 ratio, which means the rocker armmultiplies the cam-lobe lift as something called valve lift. Valve liftis greater than cam-lobe lift.
Being Eccentric
The camshaft consists of lobes that convert rotary motion (round andround it goes) into linear (back and forth) motion. The cam lobe emergesfrom a base circle, which is the shaft's base diameter andcircumference. The base circle of a camshaft is also known as the heel,where there is no lift whatsoever. As the camshaft turns, the liftersfollow rises in the cam lobes. The maximum amount of this rise is calledlift. The cam transfers this lift, via the pushrod, to the rocker arm.The rocker arm not only transfers lift to the valve stem, it alsomultiplies lift. If our cam lobe has a lift of 0.300 inch and we have a1.6:1 rocker-arm ratio, there will be 0.480-inch lift at the valve stem.With most Ford small-block and big-block
V-8s, we typically see rocker-arm ratios of 1.6:1. The aftermarketbrings us greater ratios, like 1.7:1, which give us even greater valvelift.
When the cam lobe reaches its greatest lift, it's called the nose,located at the peak of the cam lobe. Areas leading up to the nose arecalled ramps. When we're choosing a cam-lobe profile, not only are weconcerned with peak lift, but the shape of the ramps. The rampsdetermine how quickly the valves open and close. We can ramp quickly upto peak lift, or we can ramp more slowly up to peak. If we ramp quickly,this can give valves and springs all kinds of grief. Ramping up quicklyis hard on valves and springs, even though it can be beneficial toperformance.
Cam Chatter
* Lift is the maximum amount that a cam lobe will open the valve.
* Duration is how long the valves will stay open from unseat toseat in degrees of camshaft rotation. Duration begins at .004 inch ofcamshaft lift, when the lifter begins to ride the ramp to peak lift,coming off of the base circle.
* Duration at 50 means duration, beginning at .050 inch of liftat the cam lobe. This is the industry standard for duration on each camlobe.
* Lobe Separation or Centerline is the amount of time betweenintake and exhaust valve action. This is in degrees, typically between102 and 114. For street engines, we want lobe separation above 112degrees for a smooth idle. Anytime lobe separation goes below 108degrees, idle quality suffers.
* Overlap is the period where the exhaust valve is still open,yet closing, and the intake valve is opening for intake stroke. Overlapallows incoming air/fuel to push spent exhaust gasses out. Thiscontributes greatly to the power picture. Valve overlap allows for goodcylinder scavenging between power cycles.
* The Ramp is the ascending or descending part of the cam lobethat leads to either the base circle or the peak.
* Flank is the ascending or descending portion of the cam lobepast the base circle nearest the area of peak lift.
* The Base Circle is the base portion of the camshaft where nolift takes place.
* The Heel is the bottom of the cam lobe where there is no lift.
* Intake Centerline is the position of the camshaft as it relatesto valve-timing events. If the intake centerline is 114 degrees, forexample, the intake valve reaches maximum lift at 114 degrees ofcamshaft rotation.
* Exhaust Centerline means exactly the same thing as intakecenterline, with the exhaust valve reaching maximum lift at a givennumber of degrees of camshaft rotation.
* Adjustable Valve Timing is when we have an adjustable camshaftsprocket that allows us to advance or retard valve timing to increase ordecrease torque. The objective is to make the most torque possible.Advance valve timing, and you increase torque; retard valve timing, andyou decrease torque.
* Dual-Pattern Camshafts employ two valve-timing patterns, onefor the intake side and one for the exhaust side.