August 17, 2004

Valves get the bump business from pushrods and rocker arms. Factory rocker arms typically have a 1.6:1 ratio, which means the rocker arm multiplies the cam-lobe lift as something called valve lift. Valve lift is greater than cam-lobe lift.

Being Eccentric
The camshaft consists of lobes that convert rotary motion (round and round it goes) into linear (back and forth) motion. The cam lobe emerges from a base circle, which is the shaft's base diameter and circumference. The base circle of a camshaft is also known as the heel, where there is no lift whatsoever. As the camshaft turns, the lifters follow rises in the cam lobes. The maximum amount of this rise is called lift. The cam transfers this lift, via the pushrod, to the rocker arm. The rocker arm not only transfers lift to the valve stem, it also multiplies lift. If our cam lobe has a lift of 0.300 inch and we have a 1.6:1 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 aftermarket brings us greater ratios, like 1.7:1, which give us even greater valve lift.

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 are called ramps. When we're choosing a cam-lobe profile, not only are we concerned with peak lift, but the shape of the ramps. The ramps determine how quickly the valves open and close. We can ramp quickly up to 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 quickly is hard on valves and springs, even though it can be beneficial to performance.

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 to seat in degrees of camshaft rotation. Duration begins at .004 inch of camshaft 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 lift at the cam lobe. This is the industry standard for duration on each cam lobe.
  • Lobe Separation or Centerline is the amount of time between intake and exhaust valve action. This is in degrees, typically between 102 and 114. For street engines, we want lobe separation above 112 degrees for a smooth idle. Anytime lobe separation goes below 108 degrees, 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. Overlap allows incoming air/fuel to push spent exhaust gasses out. This contributes greatly to the power picture. Valve overlap allows for good cylinder scavenging between power cycles.
  • The Ramp is the ascending or descending part of the cam lobe that leads to either the base circle or the peak.
  • Flank is the ascending or descending portion of the cam lobe past the base circle nearest the area of peak lift.
  • The Base Circle is the base portion of the camshaft where no lift 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 relates to valve-timing events. If the intake centerline is 114 degrees, for example, the intake valve reaches maximum lift at 114 degrees of camshaft rotation.
  • Exhaust Centerline means exactly the same thing as intake centerline, with the exhaust valve reaching maximum lift at a given number of degrees of camshaft rotation.
  • Adjustable Valve Timing is when we have an adjustable camshaft sprocket that allows us to advance or retard valve timing to increase or decrease torque. The objective is to make the most torque possible. Advance valve timing, and you increase torque; retard valve timing, and you decrease torque.
  • Dual-Pattern Camshafts employ two valve-timing patterns, one for the intake side and one for the exhaust side.

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