BBC Roker Arms

[Stockerchevy]

Originally Posted by Reed Granrt

I hope you all don't mind my offering an opinion on this subject. In this case, lets speak of stud mounted roller rockers since they are the easiest to check. All rockers can be check similarly to this but relatively the same way. Rocker ratio is pure physics. Its ratio is built in when it was originally built. Placement of all pivot points will determine its true ratio. When it is installed, its true ratio can only be obtained at one time and that is when all pivot points are lined up and perpendicular to the valve. Anything where these pivot points are above or below this line will result in a ratio that is less than its true ratio and is a result of traveling thru a circular sweep. You must also understand that two (2) places is going thru that circular sweep; the valve end AND the push rod end. So now lets understand that both ends are going thru a sweep that results in loss of true ratio. To obtain the largest area under the curve, proper rocker geometry must be obtained. NOTHING you do with the push rod will make the ratio larger than when the rocker was built. This is why I tell people to make themselves a fixture to check their rocker ratio. Do not start on this journey unless you know where you started from. If you have a rocker that is larger (or shorter) than stock and unless you contort the pushrod to get your specs in, you have a difficult task ahead. If you want correct rocker geometry then you DO NOT use a pushrod to correct lift. If you swap camshaft to see if one is better than the other and you do not set proper geometry on both, how do you know that one is better (or worse) than the other. You should be comparing apples to apples and not apples to oranges. In setting rocker geometry, it does not matter where the rocker rides on the tip of the valve although you better hope it is on the valve tip some place. This will always give you the shortest sweep across the tip and it will give you the shortest amount of motion of the pushrod. Yes the pushrod. As the rocker sweeps across the valve tip, the pushrod will move in and out away from the camshaft. You also want that motion to be the shortest distance possible. If you miss the geometry, you will affect (lessen) duration, rocker acceleration and deceleration and will ultimately change the load on the rocker, pushrod, and cam and lifters, as well as the rocker stud in the head. All loads will be greater. Yes you can use pushrod length to alter acceleration and lift but it will be at the expense of loss of geometry. Your choice. Im just talking forces. In my humble opinion, obtaining proper geometry will result in getting the "best" travel you can get from circle motion and linear travel. There is no way you can speak of proper geometry when you are saying that you are adjusting push rod length to get proper lift. You either do one or the other; proper geometry or correct lift. If you are adjusting puhrods to get lift, you have decided that is more important than geometry and doing that will give you more speed or et. My question is; how do I know which I need; more area under the curve or less. In my opinion, making pushrod length to obtain different lifts should be a tuning mechanism and not a result. Let me git off this horse fore I fall. Mama said shut up and quit.
reed

I know engine builders discuss the rocker 90 degree at 1/2 lift while others discuss the 1/3! All I know is that a lot of these lobes are accelerating pretty quickly making it tough on valve train survival. It seems to me that the 1/3 just accelerates the valve too quick if I understand it theory! Thoughts on which you prefer?

I'm not willing to give up durability for 5 horse but look forward to added performance with some of the assymmetrical lobes that allow the valve being set down more gentally. In summation, all these things that add up make building power so intriguing!

Jim

[Reed Granrt]

Originally Posted by Stockerchevy

I know engine builders discuss the rocker 90 degree at 1/2 lift while others discuss the 1/3! All I know is that a lot of these lobes are accelerating pretty quickly making it tough on valve train survival. It seems to me that the 1/3 just accelerates the valve too quick if I understand it theory! Thoughts on which you prefer?

I'm not willing to give up durability for 5 horse but look forward to added performance with some of the assymmetrical lobes that allow the valve being set down more gentally. In summation, all these things that add up make building power so intriguing!

Jim

Jim
I have been in discussions of the 1/3--2/3 and 1/4--3/4. Both have merit, but I look at forces and or less forces. Both of the above can indeed give you accelerated travel of the components. You can do the same with the grind of the cam. With any method other than 1/2 theory, you will have more sweep travel across the valve and wasted motion with the pushrod and end travel of the rocker tip. If you want to know which you prefer, set the push rod up both ways and measure the forces for turning thru-out the cycle. But do not forget, that the rocker system can only be measured in a static environment. Once you start going beyond a certain amount of acceleration, dynamic occurrences will began to take over. More deflection(camshaft, rockers) , more breakage, having to add more spring pressure to keep it in control (Shubeck lifter breakage) and numerous other things that we can talk of. Without sophisticated equipment (spin tron) , you will not know what those occurrences are. One thing that I have found is that once a part starts to deflect, it will continue to try to go further out of its natural form until you remove that force. Problem with that is that when it tries to return to its natural state, it might try to harmonic or vibrate. Now there is some things I like to vibrate but not with valves, springs, rocker arms and push rods. Man, watch a push rod try to go back straight when traveling at 8000 RPM's. Oh crap I got a go Mama set my time is up and git back to work. Damn she sure is hard on a man.
reed

[Stockerchevy]

Talking of valve train, what are your thoughts on the ISKY chrome silicone springs like the 8005-Plus springs etc......? Harmonics, how do you figure which is right? I know TREND displays harmonic freq with their push rods.

Jim

[Stockerchevy]

Thanks guys for the responses. Great info, and thanks for taking the time to share!

Jim