Rocker arm geometry revisited - Chevelle Tech
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post #1 of 136 (permalink) Unread Feb 6th, 20, 9:49 PM Thread Starter
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Bill
 
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Location: Kirksville Mo.
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After having valve guide issues with my Brodix heads, I decided to go over my rocker arm geometry. I was running a older set of Comp Cams stainless rockers. The exhaust guides were way loose so after getting my heads back, the first thing I did was recheck the geometry using my Comp rockers and the Foxwell video. The attached picture is as narrow as I could get the sweep. The sweep measured about .090 and was off center by quite a bit.
https://i.imgur.com/H6zF3kp.jpg
Just as an experiment I shortened the pushrod to center up the sweep and that made the sweep way wide, around .120. So at that point I proved that .090 was as good as it gets with these Comp rockers. I was not going to leave it like that so I ordered a set of the AFR rockers from Summit. These are the ones that Straub and Foxwell designed. I had some time to mess with it tonight so I set up the exhaust using the mid lift method in the video. The sweep narrowed up to about .053 and the sweep also moved closer to center. Attached is a picture with the new rockers.
https://i.imgur.com/8cR1Wk5.jpg
So the new rockers fixed the exhaust issue. I also proved that .090 was as good as it gets using the mid lift method with the Comp rockers. My opinion is after using the mid lift method you should also check sweep because obviously some rockers will not give a good sweep even using mid lift, you need to check sweep to be sure. This weekend I will do the intake with both the Comp and AFR rockers and post pictures of that.


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540 BB
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post #2 of 136 (permalink) Unread Feb 7th, 20, 12:41 AM
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What's the measured distance, center to center, between the trunnion and the roller for the comp vs afr rocker?
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-Kevin
1965 Chevy Biscayne
<removed> 454cid, "peanut ports", Voodoo 60201
<coming soon> 460cid, Brodix RRO 270, 229/241 Hydraulic Roller
TH400, 3.36 Stock Rearend
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post #3 of 136 (permalink) Unread Feb 7th, 20, 5:27 AM
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Mike
 
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Re: Rocker arm geometry revisited

I chased this too, ended up using a .050 off set crower rocker, longer studs.

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post #4 of 136 (permalink) Unread Feb 7th, 20, 8:39 AM
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Quote:
Originally Posted by 65cayne View Post
What's the measured distance, center to center, between the trunnion and the roller for the comp vs afr rocker?
Still curious here. I surmise, in order to narrow the contact pattern, the length of the rocker was increased (on both sides to maintain 1.7 ratio) to decrease the arc.
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-Kevin
1965 Chevy Biscayne
<removed> 454cid, "peanut ports", Voodoo 60201
<coming soon> 460cid, Brodix RRO 270, 229/241 Hydraulic Roller
TH400, 3.36 Stock Rearend
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post #5 of 136 (permalink) Unread Feb 7th, 20, 9:30 AM
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Scott Foxwell
 
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Re: Rocker arm geometry revisited

Quote:
Originally Posted by 65cayne View Post
Still curious here. I surmise, in order to narrow the contact pattern, the length of the rocker was increased (on both sides to maintain 1.7 ratio) to decrease the arc.
The rockers I designed were shortened.

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Straub Technologies

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post #6 of 136 (permalink) Unread Feb 7th, 20, 9:30 AM Thread Starter
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Bill
 
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Re: Rocker arm geometry revisited

Quote:
Originally Posted by 65cayne View Post
Still curious here. I surmise, in order to narrow the contact pattern, the length of the rocker was increased (on both sides to maintain 1.7 ratio) to decrease the arc.
I can try to measure them but don't know that I would be able to tell much with just a dial caliper. I think they are backset around .060 but not sure. The intake and exhaust rockers are different, must be the backset but not sure. I do know some company's offer backset rockers and different fulcrums but I didn't want to fumble around experimenting for weeks so I ordered these. The intake on my heads was pretty good with the Comp rockers at about .060. I will do the same test with the intake this weekend.


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540 BB
10 to 4 comp.
Brodix Race Rite BB-2 Plus heads
Edelbrock RPM Air Gap
Quick Fuel 950 CFM Carb
Lunati Voodoo HR 231/239 @ .050, 600/600, 110 LSA
Johnson 2116 BBR lifters
Hooker Super Comp 2" primary headers
Tremec TKO 600 .82 overdrive
3.73 Posi
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post #7 of 136 (permalink) Unread Feb 7th, 20, 9:36 AM
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Scott Foxwell
 
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Re: Rocker arm geometry revisited

Quote:
Originally Posted by William Hennke View Post
After having valve guide issues with my Brodix heads, I decided to go over my rocker arm geometry. I was running a older set of Comp Cams stainless rockers. The exhaust guides were way loose so after getting my heads back, the first thing I did was recheck the geometry using my Comp rockers and the Foxwell video. The attached picture is as narrow as I could get the sweep. The sweep measured about .090 and was off center by quite a bit.
https://i.imgur.com/H6zF3kp.jpg
Just as an experiment I shortened the pushrod to center up the sweep and that made the sweep way wide, around .120. So at that point I proved that .090 was as good as it gets with these Comp rockers. I was not going to leave it like that so I ordered a set of the AFR rockers from Summit. These are the ones that Straub and Foxwell designed. I had some time to mess with it tonight so I set up the exhaust using the mid lift method in the video. The sweep narrowed up to about .053 and the sweep also moved closer to center. Attached is a picture with the new rockers.
https://i.imgur.com/8cR1Wk5.jpg
So the new rockers fixed the exhaust issue. I also proved that .090 was as good as it gets using the mid lift method with the Comp rockers. My opinion is after using the mid lift method you should also check sweep because obviously some rockers will not give a good sweep even using mid lift, you need to check sweep to be sure. This weekend I will do the intake with both the Comp and AFR rockers and post pictures of that.
Great follow up William. One thing to point out and maybe we're saying the same thing; 90* geometry will give you minimum sweep. If you set the rockers up for minimum sweep, you will end up with 90* geometry. They are one and the same and that's as good as it gets with any rocker. What I don't understand is the sweep you were getting with the Comp rocker. You should have been able to achieve that same ~.050" sweep with that rocker, just no where near the center of the valve. However, be that as it may, I'm glad you were able to get to the bottom of the guide wear and geometry issue.

FOXWELL MOTORSPORTS
Straub Technologies

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post #8 of 136 (permalink) Unread Feb 7th, 20, 9:40 AM
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Scott Foxwell
 
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Re: Rocker arm geometry revisited

Quote:
Originally Posted by William Hennke View Post
I can try to measure them but don't know that I would be able to tell much with just a dial caliper. I think they are backset around .060 but not sure. The intake and exhaust rockers are different, must be the backset but not sure. I do know some company's offer backset rockers and different fulcrums but I didn't want to fumble around experimenting for weeks so I ordered these. The intake on my heads was pretty good with the Comp rockers at about .060. I will do the same test with the intake this weekend.
The rockers aren't backset. I shortened the fulcrum length (distance from the center of the trunnion to the center of the roller tip) and in turn, did the same for the pushrod side in order to maintain the same ratio. I started with a standard set of Sharo BB Chev rockers and ended up shortening the intakes about .06" and the ex about .03" but this was specific for the AFR heads. It's just coincidence (but not a surprise) that they will work on other heads. I alaso re-configured the pushrod side of the rocker to get the geometry right there, as well.
One thing to note; you need to check the pushrod to guide plate clearance in the "crotch" of the push rod slot. The pushrod cup is closer to the trunnion with these rockers and you may have to do some clearancing there.
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post #9 of 136 (permalink) Unread Feb 7th, 20, 9:50 AM
 
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Re: Rocker arm geometry revisited

Quote:
Originally Posted by William Hennke View Post
After having valve guide issues with my Brodix heads, I decided to go over my rocker arm geometry. I was running a older set of Comp Cams stainless rockers. The exhaust guides were way loose so after getting my heads back, the first thing I did was recheck the geometry using my Comp rockers and the Foxwell video. The attached picture is as narrow as I could get the sweep. The sweep measured about .090 and was off center by quite a bit.
https://i.imgur.com/H6zF3kp.jpg
Just as an experiment I shortened the pushrod to center up the sweep and that made the sweep way wide, around .120. So at that point I proved that .090 was as good as it gets with these Comp rockers. I was not going to leave it like that so I ordered a set of the AFR rockers from Summit. These are the ones that Straub and Foxwell designed. I had some time to mess with it tonight so I set up the exhaust using the mid lift method in the video. The sweep narrowed up to about .053 and the sweep also moved closer to center. Attached is a picture with the new rockers.
https://i.imgur.com/8cR1Wk5.jpg
So the new rockers fixed the exhaust issue. I also proved that .090 was as good as it gets using the mid lift method with the Comp rockers. My opinion is after using the mid lift method you should also check sweep because obviously some rockers will not give a good sweep even using mid lift, you need to check sweep to be sure. This weekend I will do the intake with both the Comp and AFR rockers and post pictures of that.
Thank you for doing this.
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Chris Straub
Mfg Performance Parts

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Why is it we never have time or money to do it the right way in the beginning but we always have time and money to do it over again?
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post #10 of 136 (permalink) Unread Feb 7th, 20, 9:53 AM Thread Starter
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Bill
 
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Re: Rocker arm geometry revisited

Quote:
Originally Posted by steelcomp View Post
Great follow up William. One thing to point out and maybe we're saying the same thing; 90* geometry will give you minimum sweep. If you set the rockers up for minimum sweep, you will end up with 90* geometry. They are one and the same and that's as good as it gets with any rocker. What I don't understand is the sweep you were getting with the Comp rocker. You should have been able to achieve that same ~.050" sweep with that rocker, just no where near the center of the valve. However, be that as it may, I'm glad you were able to get to the bottom of the guide wear and geometry issue.
Yes, I do understand they are one and the same. Using the mid lift method in my opinion requires a bit of eye balling using the key stock (I used a bigger allen wrench) so I suppose I could be off a little (I don't think so but I am leaning over the front end of the car). I didn't go longer on the push rod and check it because I felt it was already too far off center. that might have narrowed up the sweep. So far looks like I will need a 8.850 exhaust push rod I added .050 for the hyd lifter preload.
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540 BB
10 to 4 comp.
Brodix Race Rite BB-2 Plus heads
Edelbrock RPM Air Gap
Quick Fuel 950 CFM Carb
Lunati Voodoo HR 231/239 @ .050, 600/600, 110 LSA
Johnson 2116 BBR lifters
Hooker Super Comp 2" primary headers
Tremec TKO 600 .82 overdrive
3.73 Posi
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post #11 of 136 (permalink) Unread Feb 7th, 20, 9:55 AM
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Quote:
The rockers aren't backset. I shortened the fulcrum length (distance from the center of the trunnion to the center of the roller tip) and in turn, did the same for the pushrod side in order to maintain the same ratio.
One thing to note; you need to check the pushrod to guide plate clearance in the "crotch" of the push rod slot. The pushrod cup is closer to the trunnion with these rockers and you may have to do some clearancing there.
I dont understand this Scott. Mathematically, a shorter fulcrum would have more degrees through a given arc to achieve the same lift. More degrees = more sweep across the valve tip.

Conversely, let's say your fulcrum was an exaggerated 12" for the same valve lift, there would be virtually zero sweep across the valve stem.

<edit> assume both fulcrums are 90 deg to valve at mid-lift

What am I missing?
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-Kevin
1965 Chevy Biscayne
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<coming soon> 460cid, Brodix RRO 270, 229/241 Hydraulic Roller
TH400, 3.36 Stock Rearend
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post #12 of 136 (permalink) Unread Feb 7th, 20, 10:10 AM
 
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Re: Rocker arm geometry revisited

Quote:
Originally Posted by 65cayne View Post
I dont understand this Scott. Mathematically, a shorter fulcrum would have more degrees through a given arc to achieve the same lift. More degrees = more sweep across the valve tip.

Conversely, let's say your fulcrum was an exaggerated 12" for the same valve lift, there would be virtually zero sweep across the valve stem.

<edit> assume both fulcrums are 90 deg to valve at mid-lift

What am I missing?
Longer valves and longer rocker arm stud on these 2 intersecting angles naturally shorten the distance. This distance is different on both intake and exhaust due to the fact the valves are different lengths. Hence the reason for needing 2 rocker arms, 1 for each.

Chris Straub
Mfg Performance Parts

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Why is it we never have time or money to do it the right way in the beginning but we always have time and money to do it over again?
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post #13 of 136 (permalink) Unread Feb 7th, 20, 11:03 AM
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Re: Rocker arm geometry revisited

Quote:
Originally Posted by 65cayne View Post
I dont understand this Scott. Mathematically, a shorter fulcrum would have more degrees through a given arc to achieve the same lift. More degrees = more sweep across the valve tip.

Conversely, let's say your fulcrum was an exaggerated 12" for the same valve lift, there would be virtually zero sweep across the valve stem.

<edit> assume both fulcrums are 90 deg to valve at mid-lift

What am I missing?
You're not missing anything. Theoretically and mathematically you are correct. If one was to accurately measure the sweep of a longer fulcrum vs a shorter fulcrum, the latter would be wider, all else being the same. The problem isn't in the actual sweep width if either rocker were to be optimized, the problem is, when you optimize the geometry of the longer rocker, it doesn't fit the application. It's too long. The pattern ends up way off center on the valve tip. The typical solution is to just shorten the pushrod, lower the pivot point, and roll the tip back till it's in the center of the valve. Problem then, is, you end up with .100" wide sweep, and that's what wipes out guides and creates other problems. It's putting the location of the roller tip ahead of proper geometry. Proper geometry will give you the narrowest sweep regardless of how wide that end sup being, and that's the goal. The sweep, as a function of rocker design, is what it is. We can't change that, we can just shoot for minimizing it through proper geometry. Hope that helps.
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Straub Technologies

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post #14 of 136 (permalink) Unread Feb 7th, 20, 11:08 AM
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Scott Foxwell
 
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Re: Rocker arm geometry revisited

Quote:
Originally Posted by William Hennke View Post
Yes, I do understand they are one and the same. Using the mid lift method in my opinion requires a bit of eye balling using the key stock (I used a bigger allen wrench) so I suppose I could be off a little (I don't think so but I am leaning over the front end of the car). I didn't go longer on the push rod and check it because I felt it was already too far off center. that might have narrowed up the sweep. So far looks like I will need a 8.850 exhaust push rod I added .050 for the hyd lifter preload.
Yes, the initial 90* relationship does take some discernment and the more discerning you are there, the better the results will be since from there, it's just math. Your .053" sweep with the new rocker is spot on so I'd say you were good with that one. Good job.
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Straub Technologies

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post #15 of 136 (permalink) Unread Feb 7th, 20, 11:49 AM
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Quote:
Originally Posted by cstraub View Post
Longer valves and longer rocker arm stud on these 2 intersecting angles naturally shorten the distance. This distance is different on both intake and exhaust due to the fact the valves are different lengths. Hence the reason for needing 2 rocker arms, 1 for each.
I get that the rocker stud and valve angle intersect as they get longer. What I dont get is how a shorter fulcrum decreases sweep across the valve for a given lift.

-Kevin
1965 Chevy Biscayne
<removed> 454cid, "peanut ports", Voodoo 60201
<coming soon> 460cid, Brodix RRO 270, 229/241 Hydraulic Roller
TH400, 3.36 Stock Rearend
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