Issue with Stamped Steel Rockers in BB Chevy

[Jeff Goss]

Isn't that a function of duration? They way I understand it, you want the valve to open and shut as quickly as possible. Ideally, you would want the cam "ramp" to increase as steep as possible, then drop as quickly on the back side. This could cause the inherent "bounce" starting at the lifter and reverbrating through the valvetrain. If the spring couldn't keep up, you would get periods of increased lash as the valve tried to keep up. These harmonics cause the "beating" of the valvetrain.

I'm not the smartest tool in the shed, but it seems like when we started to vary duration in Stock, these problems started. I know Chevrolet motors aren't the only ones affected.

Jeff

[SSDiv6]

Originally Posted by Jeff Goss

Isn't that a function of duration? They way I understand it, you want the valve to open and shut as quickly as possible. Ideally, you would want the cam "ramp" to increase as steep as possible, then drop as quickly on the back side. This could cause the inherent "bounce" starting at the lifter and reverbrating through the valvetrain. If the spring couldn't keep up, you would get periods of increased lash as the valve tried to keep up. These harmonics cause the "beating" of the valvetrain.

I'm not the smartest tool in the shed, but it seems like when we started to vary duration in Stock, these problems started. I know Chevrolet motors aren't the only ones affected.

Jeff

Not duration, it is the lobe shape and ramp design. There are too many schools of philosophy among cam grinders on what does work and what does not work. I have heard the story of "opening and closing the valve as quickly as possible", which happens to be a tale. You design a camshaft for efficient filling and exhaust based on the particular engine specifications. A stocker cam for a big block Chevy will not work the same on a Mopar big block. One mistake made by racers is believing that all they need is big cylinder flow numbers. Big flow numbers do not mean anything if you do not have the velocity; a good cam is designed with parameters based on 275 ~ 300fps numbers.

[Jesse Knapp]

SS Div. 6. I, too, thought duration was the culpret. When you allow the valve to stay open longer it must close rapidly or there could be clearance problems. Doesn't excessive duration cause some of these problems stockers are having? I do know if your ramp design allows for quick closing, the spring pressure has to be increased or that valve will bounce. Wouldn't you say all these things are the root of the rocker arm problems? And then the roller rockers and girdles will solve everything until they remove the stock lift rule then the Jesel system will be necessary. Where does it stop? Your posts are excellent, by the way.

[SSDiv6]

Originally Posted by Jesse Knapp

SS Div. 6. I, too, thought duration was the culpret. When you allow the valve to stay open longer it must close rapidly or there could be clearance problems. Doesn't excessive duration cause some of these problems stockers are having? I do know if your ramp design allows for quick closing, the spring pressure has to be increased or that valve will bounce. Wouldn't you say all these things are the root of the rocker arm problems? And then the roller rockers and girdles will solve everything until they remove the stock lift rule then the Jesel system will be necessary. Where does it stop? Your posts are excellent, by the way.

Jesse, one problem I have seen with many engines is cams with the wrong duration. Dwell on the following statement: If you keep the cylinder filled for too long, how efficiently can you evacuate the cylinder during the exhaust event? What happens to the remaining exhaust? Think about it...you have a stalled port that happens to have an effect on the valve loading. It exuberates the valve train dynamics, especially at high RPM's.

It is also a function of efficient cylinder filling: you have to take in to account the cylinder head flow, velocity, stroke, rod length, valve size, piston, etc...for your camshaft design. The area under the curve will determine how good your cam performs. One of the biggest problems with the wrong duration and incorrect camshaft events is reversion. I have lost count on how many engines I have seen with this problem. Signs of reversion is a sooty intake manifold plenum and exhaust. Also, when your engine does not respond to jetting or fuel curve changes, most of the times you have a reversion problem.

[Jesse Knapp]

Originally Posted by SSDiv6

Jesse, one problem I have seen with many engines is cams with the wrong duration. Dwell on the following statement: If you keep the cylinder filled for too long, how efficiently can you evacuate the cylinder during the exhaust event? What happens to the remaining exhaust? Think about it...you have a stalled port that happens to have an effect on the valve loading. It exuberates the valve train dynamics, especially at high RPM's.

It is also a function of efficient cylinder filling: you have to take in to account the cylinder head flow, velocity, stroke, rod length, valve size, piston, etc...for your camshaft design. The area under the curve will determine how good your cam performs. One of the biggest problems with the wrong duration and incorrect camshaft events is reversion. I have lost count on how many engines I have seen with this problem. Signs of reversion is a sooty intake manifold plenum and exhaust. Also, when your engine does not respond to jetting or fuel curve changes, most of the times you have a reversion problem.

I am familiar with reversion and the loss of power it causes. So, too much duration can allow unburned fuel into the exhaust and exhaust gases into and contaminating the fresh fuel charge, or reversion. Since all heads flow differently and velocity is determined by piston seal I believe, two identical engines can or would require different cam duration since stroke, rod length, valves and pistons are identical. I thought reversion was caused not as much by length of time a valve is open, but by when it opens and closes. My apology for drifting from the original thread title. It's gotten interesting for me.

[SSDiv6]

Originally Posted by Jesse Knapp

I am familiar with reversion and the loss of power it causes. So, too much duration can allow unburned fuel into the exhaust and exhaust gases into and contaminating the fresh fuel charge, or reversion. Since all heads flow differently and velocity is determined by piston seal I believe, two identical engines can or would require different cam duration since stroke, rod length, valves and pistons are identical. I thought reversion was caused not as much by length of time a valve is open, but by when it opens and closes. My apology for drifting from the original thread title. It's gotten interesting for me.

Do not worry about the drifting from the subject...it is all related. Yes, you got it...that is the reason why I shy away from using the same grind for the same application, unless my analysis shows that I am close enough to run the same grind.

[Chris Hill]

Jason,

It does not matter if the valve is hollow or solid, just the mass of the valve. But you can not ignore the stiffness of the valve itself. Some of the cup guys have tried very light intake valves, specifically very light heads of the valve. They ran into issues where the valve started to bounce off the seat due to the valve itself not being stiff enough when the valve hits the seat. You have to look at the entire valvetrain stiffness, including the cam bearing journals, bending stiffness of the camshaft between journals, valve stiffness, and valve seat stiffness. Generally, the least stiff member of the valvetrain is the rockerarm. And you only include roughly 1/3 of the mass of the rocker in your effective valvetrain mass. Some of your cup teams are using steel rockers instead of aluminum, as this drives the total valvetrain stiffness up without driving up the valvetrain effective mass.

Jeff Lee,

The beehive does help with decreased mass, but it also helps the valvespring harmonics issue. A beehive spring does not have constant stiffness and thus not a constant natural frequency. If a system does not have a constant natural frequency, harmonics are not as much of an issue. That is a very big reason nitrogen air springs are used in F1 and very high end motorcycle racing engines. Yes, air springs are light, but the very large decrease in valvespring harmonics is a big plus as well.

SSDiv6,

You may want to contact SAE about your proposed book, as I know they are looking for more valvetrain information. But then you would reveal yourself  A very old book exists by Michael Turkish that is the main valvetrain book I’ve read, plus internal research from my previous company. I know SAE recently published a book in the past couple of years about valvetrains, but I have not had an opportunity to view it yet.

On the poor angle issues on the small block MoPars and others, I agree the angularity will increase the force ratio in the valvetrain. For instance the rocker ratio may be 1.5, but the actual force applied between the pushrod and lifter cup may be 1.7~1.8. In my opinion, the increase in force ratio will increase the loading on the rocker arm, but not enough to cause rockerarm failure with out allot of lash and the subsequent impact loading. If you don’t believe me, take your peak valvetrain kinematic acceleration (generally during opening at approximately ¼ to 1/3 lift) and look at the stress on the rocker arm. You will find a pretty large safety factor for stress in the rockerarm. The only way to break it is impact stress.

Warren Johnson wrote a great article in the Dragster about this years ago, I'll try to find it and post it.

Chris, Hollow valves that are filled with sodium. It acts like a dead blow hammer when it hits the seat. Use something in the hollowed out area. Use your imagination and then get back with me.

[JRyan]

SSDiv6,

I think you mean EXACERBATE, which is "to increase severity or violence", as opposed to EXUBERATE, which is "to make things happy". Also, I've never seen "overfilling a cylinder" as a problem. If that were the case, supercharging wouldn't work.

Jerry

[SSDiv6]

Originally Posted by JRyan

SSDiv6,

I think you mean EXACERBATE, which is "to increase severity or violence", as opposed to EXUBERATE, which is "to make things happy". Also, I've never seen "overfilling a cylinder" as a problem. If that were the case, supercharging wouldn't work.

Jerry

Jerry, thank you for the correction...hard to type after a 15 hour work day!!! In regards to over filling a cylinder as a problem, superchargers and turbos are RPM controlled. Also, they have means to control the excess of pressure by relieving the pressure when they reach certain levels. A normally aspirated engine does not have those means, therefore you use the cam timing events to do so. I spent enough time with the Series I and Series II GM Supercharged engines and saw too many engines blow up when guys changed the settings on the boost control selenoid. Camshafts for supercharged and turbo engines have timing events completely different than the current Stocker grinds, especially in the overlap numbers to alleviate reversion.