![]() Note that the graph shows a 1.72 ratio rocker, but the action is similar between various ratios. This also can be seen on the attached graph. As a 1.65 ratio rocker is 10% higher ratio then a 1.50, the lift provided by the 1.65 rocker will be 10% greater with all cams. This can be changed by installing different ratio rockers. However, the valve lift is normally specified with standard 1.5 ratio rocker arms. This of course cannot be changed and will remain the same regardless of the rocker arm ratio. The cam is rated at some duration at.050 lifter/tappet rise. This becomes very obvious by reviewing the graph. By making the rocker some greater ratio, such as the standard 1.5, the lift of the lobe is multiplied by 1.5, the opening and closing rates are much faster, and the area under the overall 'curve' is much greater. If we used rocker arms that had a 1 to 1 ratio, the valve would follow the exact opening and closing as the cam lobe - same peak lift, same opening and closing rate, and same limited valve open area. As the obtainable lift is limited, the lift is multiplied by the rocker arm ratio to bring it up to the desired point(s). So, in cam design, as more lift is designed in, the base circle becomes smaller in diameter. Obviously, the peak lift of the lobe cannot be higher then the outer diameter of the cam bearings - otherwise, the cam could not be installed. The base circle must be kept large enough to not degrade the strength of the cam, while allowing for enough lift (in conjunction with the rocker arm) to meet all design goals. The cam lobe lift is dictated by the dimension from the base circle of the lobe to the lobe centerline, or peak of the lobe. The basic valve train consists of a cam, or cam lobe, some sort of cam follower, such as a lifter, a push rod, a rocker arm, and the valve (with the associated retainer/keepers, and valve springs). W hat are the overall effects of higher ratio rockers? Do they add stress to the engine?Īre they safe to use? Do they add power? What precautions should be considered before installing them? The following is a summary addressing these questions. ![]()
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