valve float info
Apr 28, 2006 | 08:58 PM
#1
valve float info
Common Misconception of Valve Float
Source: Crane Cams
“Valve float” is a common term for a situation best described as “valve train separation.” This occurs due to inertia load imparted into the valve train by the action of the cam lobe against the follower. Flex in the valve train (the majority of which is located in the pushrod) is the prime contributor to valve train separation. The initial loads imparted into the pushrod cause it to bend (somewhat like a pole vaulter’s pole) and then return to a straight configuration. This unloads a sharp energy pulse to the rocker arm, which transfers it into the valve/valve spring assembly. This often results in “valve lofting,” which causes the valve to operate in a different path than that described by the lobe profile. At the same time, the lifter without any load against it can also be launched off the opening ramp of the lobe, and then, as load is re-established, either strike the nose of the lobe and eventually damage it; land on the closing ramp; or land on the base circle with significant and often damaging impact. If “lofting” can be controlled (by design or good fortune and the lifter lands gently on the closing ramp), it adds to area under the curve and more power. If it is uncontrolled (which happens the vast majority of the time), it can be damaging to valve train components and will compromise performance. Most of the time, power flattens out or is lost when “valve train separation” occurs. Again, the biggest culprit in causing this situation is the flex of the pushrod. In tests conducted by Crane Cams, they claim to have found 12-hp in a 350 Chevy with a 204/214 @ .050" cam (.420"/.443" valve lift) just by going from a .065" wall pushrod to a .080" wall pushrod, and the springs were only 110 lbs. on the seat and 245 lbs. open.
Many people tend to think that the “weight” of the rocker arm is the cause of valve float. If the rocker is rigid and properly designed, it should contribute very little to valve float. Weight in this case is not the prime issue, but rather the “moment of inertia” of the rocker design. “Moment of inertia” is the affect of where the mass of the rocker arm is located relative to its center of rotation. One rocker can be much heavier than another and still have a smaller moment of inertia because of where its mass is located; so weighing rockers to determine their affect of valve float is really not effective at all. (FYI: “mass” is a measure of a body’s inertia; while “weight” is the affect of gravity on “mass.” “Moment of inertia” is unaffected by weight, but is affected by where “mass” is located relative to the center of rotation).
Source: Crane Cams
“Valve float” is a common term for a situation best described as “valve train separation.” This occurs due to inertia load imparted into the valve train by the action of the cam lobe against the follower. Flex in the valve train (the majority of which is located in the pushrod) is the prime contributor to valve train separation. The initial loads imparted into the pushrod cause it to bend (somewhat like a pole vaulter’s pole) and then return to a straight configuration. This unloads a sharp energy pulse to the rocker arm, which transfers it into the valve/valve spring assembly. This often results in “valve lofting,” which causes the valve to operate in a different path than that described by the lobe profile. At the same time, the lifter without any load against it can also be launched off the opening ramp of the lobe, and then, as load is re-established, either strike the nose of the lobe and eventually damage it; land on the closing ramp; or land on the base circle with significant and often damaging impact. If “lofting” can be controlled (by design or good fortune and the lifter lands gently on the closing ramp), it adds to area under the curve and more power. If it is uncontrolled (which happens the vast majority of the time), it can be damaging to valve train components and will compromise performance. Most of the time, power flattens out or is lost when “valve train separation” occurs. Again, the biggest culprit in causing this situation is the flex of the pushrod. In tests conducted by Crane Cams, they claim to have found 12-hp in a 350 Chevy with a 204/214 @ .050" cam (.420"/.443" valve lift) just by going from a .065" wall pushrod to a .080" wall pushrod, and the springs were only 110 lbs. on the seat and 245 lbs. open.
Many people tend to think that the “weight” of the rocker arm is the cause of valve float. If the rocker is rigid and properly designed, it should contribute very little to valve float. Weight in this case is not the prime issue, but rather the “moment of inertia” of the rocker design. “Moment of inertia” is the affect of where the mass of the rocker arm is located relative to its center of rotation. One rocker can be much heavier than another and still have a smaller moment of inertia because of where its mass is located; so weighing rockers to determine their affect of valve float is really not effective at all. (FYI: “mass” is a measure of a body’s inertia; while “weight” is the affect of gravity on “mass.” “Moment of inertia” is unaffected by weight, but is affected by where “mass” is located relative to the center of rotation).
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