Issue with Stamped Steel Rockers in BB Chevy

[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.