Valve train geo. need opinions

Originally Posted by retyler

What you say here is contradictory if you read your first sentence. If you look at the pictures I posted you will see that movement on the X axis is at a very minimum and there is very little movement across the stem and I should be getting maximum travel on the Y axis. I'll try to put it in simpler terms. With the valve train set up and the motor running the rocker only contacts the valve stem in one small area as depicted in the photo. It does not make a full sweep across the valve stem. All I want to do is move that contact point to the center of the stem. You do not need to sweep across the valve. When you do that you are putting angled stress on the valve. Think about it as taking your finger and pushing straight down on the stem. There would be no side movement. That is pretty much what I am doing except the pressure point is a little off from center. Forget the sweep altogether because it is very minimal in my case.

You understand this correctly. I am sure BLKFRD had a mistatement.

I don't subscribe to mid lift theory.... it doesn't seem to work too well on engines I have tried it on, but have nodoubt it will work "properly" on some layouts. I can see the theory behind it has merit behind the idea but in practical terms it has some holes in it.
When I was messing with around with the idea, as the valve left the mid lift position and headed back to the seat, things worked not too bad... but when going from mid lift position to to full lift, the tip kept rolling to the intake side too much for my likeing. It seemed to shorten the ratio going back to the seat and lengthen it towards full lift. I put this down to the fact that the fulcrums head stud and valve are at different angles. Changing the pushrod length ofcourse changed matters for better (in my mind) geometry but then that threw the 90 degree rule by doing that.

I was going to draw a pic of what was happening to explain but then found this while looking for some drawing/s - MID-LIFT.COM
If you have alook at their pic, which is setup at mid-lift. On that example, if you draw an arc of the tips travel, centered at the rockers fulcrum point you may see what was happening to me. As the valve closes, roller the tip is getting "longer" and as it approaches max lift it (needs to) get "shorter".
The valve angle vs the stud angle promotes this, especially in my experiences as the studs and valve angles are converging on eachother. The opposite happens towards full lift, the valve/rocker stud angles are heading away from each other. Check the link out and imagine an arc drawn at the rockers roller tip as it goes thru the motions and the center of the arc being the rockers fulcrum point and you may see what I'm getting at...

Great Link, but the examples shown are for SBC. SBF has different angles for the cam/pushrod, and rocker tip/valve top. So be carefull about using that data to referance what is happening in your situation.

It would seem that Under Arc is better then Mid Lift Arc. Under Arc has a faster rate at the start of the valve un-seating. This should also aid how much power you loose in lift. When the valve is partial closed, the spring force is a lot less, so traveling faster does not put un-do stress on the valvetrain...less loss. As the spring pressure gets higher as the valve approaches full lift, the geometery changes so you have more leverage for the slower moving valve lift, taking advantage of the leverage, also decreases lift loss.

The faster you unseat the valve and get it traveling, the better power your going to make. More important then ensuring you are getting max lift from having a Mid-lift arc. (in my opinion)

Originally Posted by retyler

What you say here is contradictory if you read your first sentence. If you look at the pictures I posted you will see that movement on the X axis is at a very minimum and there is very little movement across the stem and I should be getting maximum travel on the Y axis. I'll try to put it in simpler terms. With the valve train set up and the motor running the rocker only contacts the valve stem in one small area as depicted in the photo. It does not make a full sweep across the valve stem. All I want to do is move that contact point to the center of the stem. You do not need to sweep across the valve. When you do that you are putting angled stress on the valve. Think about it as taking your finger and pushing straight down on the stem. There would be no side movement. That is pretty much what I am doing except the pressure point is a little off from center. Forget the sweep altogether because it is very minimal in my case.

Sorry...did not mean all the way across the tip. Instead sweep a small amount in one direction, stop, and then sweep back to the starting point at full lift. Overall, the patch will be small like you have now or smaller.

The angle of the rocker stud does not have any impact on the rocker tip movement. The pivot point it static, it doesn't move up or down the stud as the valve moves. "Mid-lift" geometry is based completely on the angle of the rocker to the valve.

The only affect the stud angle has is that it moves the entire rocker body closer to the valve with a longer pushrod, or further from the valve with a shorter pushrod. This is where the dimensions of the rocker itself have to be designed for mid-lift geometry. The distance from the trunion to the roller has to be exactly right such that at mid-lift, the rocker can sit at a 90 degree angle to the valve with the roller tip close to center of the valve stem and trunion of course located on the stud.

The only 'flaw' I see to mid-lift geometry is that you would need a different rocker for a .500 lift cam than you would for a .600 lift cam, for different valve installed heights, etc... it is impossible to have a 'perfect' rocker that will be able to obtain mid-lift geometry on all cam/head/valve combos and have the contact pattern centered on the valve tip. You either have to compromise mid-lift geometry to get the contact pattern centered, or vice versa. That's why the mid-lift documentation states that the location of the contact patch is insignificant compared to the width of the contact patch in terms of valve guide wear... and I think I'd tend to agree, although I'm no expert on valvetrain stresses.

If the valve, rocker and push rod are all set to 90 degrees when the valve is at its half way point in the lift then the travel is going to be as small as it can possibly be. If you have more or less angle at the have-way point you will end up with more travel. This is the "proper" way to set the geometry.

Originally Posted by Motorhead

The angle of the rocker stud does not have any impact on the rocker tip movement. The pivot point it static, it doesn't move up or down the stud as the valve moves. "Mid-lift" geometry is based completely on the angle of the rocker to the valve.

The only affect the stud angle has is that it moves the entire rocker body closer to the valve with a longer pushrod, or further from the valve with a shorter pushrod. This is where the dimensions of the rocker itself have to be designed for mid-lift geometry. The distance from the trunion to the roller has to be exactly right such that at mid-lift, the rocker can sit at a 90 degree angle to the valve with the roller tip close to center of the valve stem and trunion of course located on the stud.

The only 'flaw' I see to mid-lift geometry is that you would need a different rocker for a .500 lift cam than you would for a .600 lift cam, for different valve installed heights, etc... it is impossible to have a 'perfect' rocker that will be able to obtain mid-lift geometry on all cam/head/valve combos and have the contact pattern centered on the valve tip. You either have to compromise mid-lift geometry to get the contact pattern centered, or vice versa. That's why the mid-lift documentation states that the location of the contact patch is insignificant compared to the width of the contact patch in terms of valve guide wear... and I think I'd tend to agree, although I'm no expert on valvetrain stresses.

This post is absolutely spot on.... and this is exactly what I found when trying the theory.
Though the stud angle does not really effect the situation, the valve angle in relation to the fulcrum point certainly does.... as does the roller tip height at "mid lift" ....relative to the fulcrum. Whether the tip is above, below or equal to the plane of the fulcrum, has a major effect on the arc it scribes and its ability to have uniform travel with the angled valve.
If the stud/valve/rockers were all 90 degrees to each other, it would work 100% of the time... but they're not.

Originally Posted by PaulS1950

If the valve, rocker and push rod are all set to 90 degrees when the valve is at its half way point in the lift then the travel is going to be as small as it can possibly be. If you have more or less angle at the have-way point you will end up with more travel. This is the "proper" way to set the geometry.

That's not entirely true. You get max lift when the rocker tip is at 'mid-lift' geometry. Anthing over or under mid-lift will give up movement on the Y axis to movement on the X axis. However, on the pushrod side of the rocker, the opposite is true. Since your Y movement is fixed (cam lift), placing the pushrod seat either above or below mid-lift will give more rotational movement of the rocker creating more lift, but with a movement that is more non-linear to the cam lobe.

Lower than mid-lift will give faster movement off-the-seat, but slower movement at peak lift, which could be considered desireable for more valve open time, but will also 'slam' the valve shut (this is exactly what Crane's "quick lift" technology is, a low pushrod seat in the rocker body). Above mid-lift will do the opposite, come off the seat slower, and move faster at max-lift, which sets the valve shut easier, but gives less 'area under the curve' for overall valve lift. BOTH of these will give more peak lift, but you are effectively changing the cam lobe profile, and is that something that you really want to do? You have to ask yourself, did the cam grinder design the lobe for optimal valve movement and valvetrain stability, or did they just grind something that looked like an egg and call it good. I think either way, WE are not a better judge than the cam grinder as to what the valve movement SHOULD be doing... the valve's movement should match the designed cam profile, which is what mid-lift accomplishes.

Hhhmm...back to the original question, I guess the only way to find out the effects on the setup of a shorter pushrod is to try one and see what happens to the geometry. Due to the nature of the design of the rocker, it may not change appreciably.
As you say, if some are worse than others, it tends to indicate something else is causing them not to be centered... be it rocker stud machining differences, valve installed height or whatever?

You get maximum lift when the rocker arm is at full travel not when it is at midlift.
I think I must be confused because your terminology doesn't make sense to me at all.
Maybe you should leave the "X" and "Y" out of it and define what you mean by "midlift"