A "clearance ramp" is an area of SLOOOOOW lift where the base circle transitions into the lifting portion of the lobe. For instance, if a cam has a .006" intake clearance specification, that amount of lift (times the rocker arm ratio) is ground into the lobe to allow all the parts of the valve train to get into solid contact with each other before the faster lift associated with actually opening the valve gets under way. It's easy to see if a degree wheel is mounted on the crankshaft and a dial indicator is positioned to monitor cam lobe or valve motion. The dial indicator will show a slow lift until the clearance is taken up, then the rate of lift will increase quickly as the valve is forced open. On the closing side of the lobe, the rate of closure will be very rapid until the valve ALMOST touches the seat, then it will slow down and drop it onto the seat gently- - - -then the clearance gets re-established.
Adjusting to the wrong clearance will result in battered valve seats, and/or incorrect timing of the opening/closing event of the valve. Since hydraulic lifters never actually have any clearance, the slow beginning and ending portions of the lobe are much shorter duration and of less "lift" than with a solid lifter cam. It's possible to run solid lifters on a hydraulic cam, but the clearance MUST be adjusted to approximately .003"-.004", and the valve action might be noisy unless the clearance is monitored and corrected very frequently.
Jerry
