Wrong and right valvetrain geometry
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From: Forked River NJ
Wrong and right valvetrain geometry
I am building a mild engine for David. It is a 350 with a mild comp cams and aftermarket aluminum heads. David had purchased a set of stock style comp cams rocker arms. Went to I stall them and found this
Obviously this is not going to work. With 1.47 springs the rocker was hitting the valve spring. I also had measured before and had to get .100 longer pushrods. This was not going to work. I bought a set of comp cams roller rockers which I have used on a lot of mild builds before and these are bullit proof for anything but solid roller cams. This morning I mocked up one and found this.
This is way better. Plenty of clearance for the rocker to valve spring. Also if you look closely the center of the roller tip is a little off center towards the pivot. As the lobe pushes the valve open the roller will roll across the valve tip. At full lift you want it a little over the center of the valve away from the pivot. So it is rolling across the center of the valve tip. This should always be checked and if it is not correct you will get valve guide wear quickly. You must run the correct size pushrod. Always check this. My "tip" of the day. Pun intended!
Obviously this is not going to work. With 1.47 springs the rocker was hitting the valve spring. I also had measured before and had to get .100 longer pushrods. This was not going to work. I bought a set of comp cams roller rockers which I have used on a lot of mild builds before and these are bullit proof for anything but solid roller cams. This morning I mocked up one and found this.
This is way better. Plenty of clearance for the rocker to valve spring. Also if you look closely the center of the roller tip is a little off center towards the pivot. As the lobe pushes the valve open the roller will roll across the valve tip. At full lift you want it a little over the center of the valve away from the pivot. So it is rolling across the center of the valve tip. This should always be checked and if it is not correct you will get valve guide wear quickly. You must run the correct size pushrod. Always check this. My "tip" of the day. Pun intended!
Drifting
Joined: Nov 2002
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From: Loveland, CO
Let the discussion begin.
I just went through trying to determine correct pushrod length (valvehead sweep pattern), rocker arm geometry, etc., on my engine and finally gave up. Too many opinions. I just split the difference.
Ralph
I just went through trying to determine correct pushrod length (valvehead sweep pattern), rocker arm geometry, etc., on my engine and finally gave up. Too many opinions. I just split the difference.
Ralph
Le Mans Master
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From: RI, Now Franklin/Nashville TN
I just did this procedure as well with a roller cam, comp cams roller tipped 1.52 rockers, and measuring the correct pushrod to use. I followed comp cams recommendation on how to pick and measure the correct pushrod for a roller cam. My result was exactly what Gordon is showing above…on the base circle of the cam lobe, the roller tip is slightly off center of the valve tip towards the intake manifold, at mid lift the roller ball is basically centered on the valve tip, and at maximum lift, the roller ball is slightly off center on the valve tip towards the exhaust manifold side. The roller ball should move in an arc over the valve tip from base circle lift (zero lift), mid lift and max lift. Everything looks great almost 1 year later-I checked.
The confusion really started when someone essentially stated that comp cams instructions were wrong (hard to believe but that was what was stated) and their "new technique" for measuring the pushrod was correct…did not buy it for one second. If the comp cams instructions were incorrect, all of us would heard about it long ago. Even if you employ some logic here, it makes sense that that the roller tipped ball has to move in an arc over the center of the tip of the valve..this is NOT rocket science.
The confusion really started when someone essentially stated that comp cams instructions were wrong (hard to believe but that was what was stated) and their "new technique" for measuring the pushrod was correct…did not buy it for one second. If the comp cams instructions were incorrect, all of us would heard about it long ago. Even if you employ some logic here, it makes sense that that the roller tipped ball has to move in an arc over the center of the tip of the valve..this is NOT rocket science.
Last edited by jb78L-82; Apr 17, 2015 at 01:00 PM.
Drifting
Joined: Jul 2013
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From: Tri-Cities TN
This is posted by an Engine builder over on Speed Talk. His credentials include 2X winner of the Mahle Clevite Engine Builder of the Year. Now in R&D his engines have won multiple Truck, Busch/Nationwide/Infinity, and Cup.
by Warp Speed » Fri Apr 17, 2015 3:16 am
^^^^^^^^^^^^ 8)
Bottom line is, the correct geometry for the rocker is a mid lift approach, which always gives the narrowest sweep pattern at the valve. How narrow this sweep can be is a function of body length vs lift.
Pushrod side geometry is pretty much left up to the rocker manufacture. I have seen people make gains by deviating from the mid lift strategy, but that is purely pointing to an incorrect cam, and the changing of the events at the valve that resulted...............
This is assuming a roller tip rocker is used. Requirements may get a little different when using a pad tip depending on design but.........
Warp Speed
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In a PERFECT world it would be centered but we don't live in a perfect world so throw out being "Centered". You want the the narrowest sweep on the tip of the valve. It doesn't have to be centered. Aftermarket rockers, longer valves, stud location on head all affect this.
Most will fine they have to short a pushrod.
by Warp Speed » Fri Apr 17, 2015 3:16 am
^^^^^^^^^^^^ 8)
Bottom line is, the correct geometry for the rocker is a mid lift approach, which always gives the narrowest sweep pattern at the valve. How narrow this sweep can be is a function of body length vs lift.
Pushrod side geometry is pretty much left up to the rocker manufacture. I have seen people make gains by deviating from the mid lift strategy, but that is purely pointing to an incorrect cam, and the changing of the events at the valve that resulted...............
This is assuming a roller tip rocker is used. Requirements may get a little different when using a pad tip depending on design but.........
Warp Speed
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In a PERFECT world it would be centered but we don't live in a perfect world so throw out being "Centered". You want the the narrowest sweep on the tip of the valve. It doesn't have to be centered. Aftermarket rockers, longer valves, stud location on head all affect this.
Most will fine they have to short a pushrod.
Drifting
Joined: Jul 2013
Posts: 1,299
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From: Tri-Cities TN
I just did this procedure as well with a roller cam, comp cams roller tipped 1.52 rockers, and measuring the correct pushrod to use. I followed comp cams recommendation on how to pick and measure the correct pushrod for a roller cam. My result was exactly what Gordon is showing above…on the base circle of the cam lobe, the roller tip is slightly off center of the valve tip towards the intake manifold, at mid lift the roller ball is basically centered on the valve tip, and at maximum lift, the roller ball is slightly off center on the valve tip towards the exhaust manifold side. The roller ball should move in an arc over the valve tip from base circle lift (zero lift), mid lift and max lift. Everything looks great almost 1 year later-I checked.
The confusion really started when someone essentially stated that comp cams instructions were wrong (hard to believe but that was what was stated) and their "new technique" for measuring the pushrod was correct…did not buy it for one second. If the comp cams instructions were incorrect, all of us would heard about it long ago. Even if you employ some logic here, it makes sense that that the roller tipped ball has to move in an arc over the center of the tip of the valve..this is NOT rocket science.
The confusion really started when someone essentially stated that comp cams instructions were wrong (hard to believe but that was what was stated) and their "new technique" for measuring the pushrod was correct…did not buy it for one second. If the comp cams instructions were incorrect, all of us would heard about it long ago. Even if you employ some logic here, it makes sense that that the roller tipped ball has to move in an arc over the center of the tip of the valve..this is NOT rocket science.
For every head to have a perfectly centered sweep pattern you would have to design a rocker for every head and every application that it is going to be put in. IE the same head used with std length valve then used with .200" long valves. It would take 2 rockers. So the COMPROMISE is, you want the minimum sweep and it doesn't have to be centered. If you compromise minimum sweep over being centered with more sweep then you have lost lift, change the cam profile that the valve sees, and increased valve guide wear.
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From: Hermosa
Even if you employ some logic here, it makes sense that that the roller tipped ball has to move in an arc over the center of the tip of the valve..this is NOT rocket science.
I also followed the "procedure" for valve geometry with the roller tip centered over the valve tip and paid the price for that mistake. My valve guides now have excessive wear from the forward and aft (intake side exhaust side) movement of the roller tip over the valve tip during operation.
Fortunately for me I caught it before it destroyed the valve guides, just not before it caused some excessive wear.
The roller tip ideally is going to push the valve tip straight down. However since it moves in an arc that is not possible. The best alternative is for it to move in the smallest possible arc. Be it over the center of the valve stem or not. Just as long as it's on the valve stem.
Using Straub's advice mine now move in a .045" sweep across the valve tip. With the centered on the valve tip method they moved .120" across the tip.
That back and forth motion of the rocker tip is what tries to move the valve stem back and forth in the valve guide. This causes the excessive wear. The less it moves back and forth the less force applied to the valve stem side load and thereby the valve guide. Makes sense to me.
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Le Mans Master
Joined: Apr 2011
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From: Hermosa
I guess I would say that comp cams, and others, method is not incorrect but it is not universally correct either.
It may work in stock type applications and others where you get lucky, but it does not work in all applications.
The method illustrated by Straub will work in all applications with the exception of where it runs off the valve stem tip. In that case then a different rocker manufacturer or offset rocker or different installed height may be required to correct the geometry.
It may work in stock type applications and others where you get lucky, but it does not work in all applications.
The method illustrated by Straub will work in all applications with the exception of where it runs off the valve stem tip. In that case then a different rocker manufacturer or offset rocker or different installed height may be required to correct the geometry.
Le Mans Master
Joined: Oct 2007
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From: RI, Now Franklin/Nashville TN
You are right, it's not rocket science, it's geometry.
I also followed the "procedure" for valve geometry with the roller tip centered over the valve tip and paid the price for that mistake. My valve guides now have excessive wear from the forward and aft (intake side exhaust side) movement of the roller tip over the valve tip during operation.
Fortunately for me I caught it before it destroyed the valve guides, just not before it caused some excessive wear.
The roller tip ideally is going to push the valve tip straight down. However since it moves in an arc that is not possible. The best alternative is for it to move in the smallest possible arc. Be it over the center of the valve stem or not. Just as long as it's on the valve stem.
Using Straub's advice mine now move in a .045" sweep across the valve tip. With the centered on the valve tip method they moved .120" across the tip.
That back and forth motion of the rocker tip is what tries to move the valve stem back and forth in the valve guide. This causes the excessive wear. The less it moves back and forth the less force applied to the valve stem side load and thereby the valve guide. Makes sense to me.
I also followed the "procedure" for valve geometry with the roller tip centered over the valve tip and paid the price for that mistake. My valve guides now have excessive wear from the forward and aft (intake side exhaust side) movement of the roller tip over the valve tip during operation.
Fortunately for me I caught it before it destroyed the valve guides, just not before it caused some excessive wear.
The roller tip ideally is going to push the valve tip straight down. However since it moves in an arc that is not possible. The best alternative is for it to move in the smallest possible arc. Be it over the center of the valve stem or not. Just as long as it's on the valve stem.
Using Straub's advice mine now move in a .045" sweep across the valve tip. With the centered on the valve tip method they moved .120" across the tip.
That back and forth motion of the rocker tip is what tries to move the valve stem back and forth in the valve guide. This causes the excessive wear. The less it moves back and forth the less force applied to the valve stem side load and thereby the valve guide. Makes sense to me.
Drifting
Joined: Jul 2013
Posts: 1,299
Likes: 93
From: Tri-Cities TN
I agree that that there might be better ways to measure the valve train geometry and the straub technique seems reasonable and most accurate. I would have to measure the total spread of the arch of the rocker ball on my valve tips but when I checked the valves recently, all was good from a wear perspective and arch pattern. I do know that the rocker at mid lift was at 90 degrees and the roller ball was in the center of the valve tip.