Hardened Pushrods
Banned
Joined: Oct 2002
Posts: 6,518
One more example from Crane Cams:
That example has a LOT less pressure than a LT4 cam spring!
Bret
Originally Posted by Crane Cames
"For quite some time, rocker arm “weight” and its effect on “valve float” has been the subject of much debate among interested enthusiasts. Unfortunately, many of the people making posts on the subject get “caught up in their underwear” because they don’t understand the difference between the terms “weight,” “mass,” and “moment of inertia.” This misunderstanding has resulted in a great deal of misinformation being posted as fact on various web forums. A very elementary explanation of what really happens follows.
“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 (like a ski jumper landing on the slope of a hill); 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 our tests at Crane, we 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# on the seat and 245# open!
Many people on website forums 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!)"
“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 (like a ski jumper landing on the slope of a hill); 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 our tests at Crane, we 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# on the seat and 245# open!
Many people on website forums 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!)"
Bret
Registered User
Joined: Feb 2000
Posts: 473
From: Lexington Park, Maryland, USA
I've read almost every word of every post on this thread and I think that SS-RRR still does not understand the difference between "stiff" and "hard".
Sir, it was quite obvious from the very 1st couple of posts on this thread that a stiff pushrod is being suggested by Bret and many others. You mentioned you had success with a "hard" pushrod from Lingenfelter and then found out that that pushrod was indeed "stiff" because it was made of an alloy chromolly.
Do you understand now? The pushrod should be stiff. Bret made a very good post about that and he also explained hardness. Even though I'm older than Bret I still value his knowledge and experience. A stock hardened pushrod is what is being recommended against. Not a hardened chromolly "stiff".
Karl Ellwein
Ellwein Engines
Sir, it was quite obvious from the very 1st couple of posts on this thread that a stiff pushrod is being suggested by Bret and many others. You mentioned you had success with a "hard" pushrod from Lingenfelter and then found out that that pushrod was indeed "stiff" because it was made of an alloy chromolly.
Do you understand now? The pushrod should be stiff. Bret made a very good post about that and he also explained hardness. Even though I'm older than Bret I still value his knowledge and experience. A stock hardened pushrod is what is being recommended against. Not a hardened chromolly "stiff".
Karl Ellwein
Ellwein Engines
Registered User
Joined: Oct 2002
Posts: 2,931
From: Upstate NY
As Karl pointed out, not so in pushrods. Here bigger IS also better.
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