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  #61 (permalink)  
Old 12-24-2011, 12:06 PM
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Quote:
Originally Posted by Cobra720
Quote: AP72 "Exessive scrub will kill your guides."
This is where I'm understanding it now. I am better off being off center and pushing straight on the axis of the valve stem, or close to it, then to have +.100 scrub on the center.
I'll have to order longer push rods than I expected.
Lou
Within reason. If your rocker is going off the valve tip though then you have something besides pushrod length wrong. Unless you can adjust the pivot of the rocker laterally (closer or further from the valve stem) then you're very likely to have your wipe pattern off center which is not best but okay as long as you have minimal scrub. To have the perfect layout you'll need to move the pivot point and adjust the pushrod length, but that's going a bit far for most people,

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  #62 (permalink)  
Old 12-24-2011, 02:10 PM
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FWIW different brands of rocker arms will often have different patterns across the valve tip even w/the same length p-rod. For "Smokey" fans, he talks about this in his Power Secrets book. Said basically if you didn't see what you wanted after adjusting the p-rod length on one type rocker, change the brand and see what the results w/them were.

Now that's not going to be an option for most guys, but it does show that there can be a difference in designs between the various makes.
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  #63 (permalink)  
Old 08-09-2012, 11:35 AM
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I know this is old stuff, and I didn't read every page so pardon me if I'm reposting known info, but this article in Circle Track is at variance with the well-known Comp Cams page.

Rocker Arm & Valve Train Geometry - Circle Track Magazine

It references both Miller and Smokey Yunick, and has some great drawings which show why a narrow roller track means more valve lift than a wide track.

I also emailed Comp on their diagram last year sometime, and never got a reply.

All this having been said, I can't imagine anybody who's, e.g., changing rocker arms, pulling the heads to change valve height (guess lash caps might work if you're lucky) just to get an extra .005 lift on a street motor. But the philosophy of a short roller track across the valve tip is correct, if real-world balanced against not having the pattern too far off-center. Note that the shorter the track, the closer you get to the ideal of having the exhaust-side limit of the roller track occur at mid-lift (not at full lift as Comp shows). Dead-centering really isn't necessary, nor IMO is having the lifter fully primed for checking unless you're racing and want every last .001 at any cost. Else, it's perfection versus the law of diminishing returns here.

Last edited by Leoman; 08-09-2012 at 11:47 AM.
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Old 08-09-2012, 08:08 PM
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Different brands of rocker arms...

I've tried several different roller rocker arms with the mid-lift method and there was a considerable difference. The first try was the steel Comp rockers- If I got the correct mid lift point on the valve end, I was way out on the pushrod side. If I corrected the pushrod side to the 90 degree mid lift position the valve end was way off. My conclusion was that the rocker did not have the correct angle between the pushrod end and the valve end (big block chev). I then tried a set of Jim Miller's rockers I was lucky enough to find. His rockers were correct on both ends, I was able to get 90 degrees at mid lift on both the valve end and the pushrod end! I did need to go way longer on both intake and exhaust pushrods, one more than the other. From memory I think it was around .200" longer, but every engine will vary. My point being that not all rockers are correctly manufactured, so it does pay to try another brand or style if you are not getting the results that Miller describes in his articles. Also, I had to install longer rocker arm studs to provide enough threads for the polylocks once I installed the longer pushrods. Miller explains in his article how roller rockers were incorrectly designed originally, and many have not been changed to this day. Too bad Miller rockers are not easy to come by, I understand that he only now offers a very high end "Pro" rocker on a special order basis (if at all)....
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  #65 (permalink)  
Old 08-09-2012, 11:43 PM
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Wow, you'd think somebody would jump on that, but I guess that would be admitting you made a mistake. So goes industry in any field.
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Old 08-10-2012, 09:12 AM
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BTW, it appears from a look at Miller's website (MID-LIFT.COM ENTRY PAGE) that the older PA series mid-lift rockers are available at $342/set. There's also a "G-tool" for $208. Online instructions are visible for both. Pretty interesting!

I also read somewhere that as this concept has been around for a long time and in fact was endorsed by Chevy racing some time back, GM since decided upon the "66% rule" (mid-lift would be the 50% rule by analogy) to minimize wear and tear at high spring loads during the last phases of valve opening. Don't know much about that.
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  #67 (permalink)  
Old 08-10-2012, 06:34 PM
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Quote:
Originally Posted by Grant4060 View Post
I've tried several different roller rocker arms with the mid-lift method and there was a considerable difference. The first try was the steel Comp rockers- If I got the correct mid lift point on the valve end, I was way out on the pushrod side. If I corrected the pushrod side to the 90 degree mid lift position the valve end was way off. My conclusion was that the rocker did not have the correct angle between the pushrod end and the valve end (big block chev). I then tried a set of Jim Miller's rockers I was lucky enough to find. His rockers were correct on both ends, I was able to get 90 degrees at mid lift on both the valve end and the pushrod end! I did need to go way longer on both intake and exhaust pushrods, one more than the other. From memory I think it was around .200" longer, but every engine will vary. My point being that not all rockers are correctly manufactured, so it does pay to try another brand or style if you are not getting the results that Miller describes in his articles. Also, I had to install longer rocker arm studs to provide enough threads for the polylocks once I installed the longer pushrods. Miller explains in his article how roller rockers were incorrectly designed originally, and many have not been changed to this day. Too bad Miller rockers are not easy to come by, I understand that he only now offers a very high end "Pro" rocker on a special order basis (if at all)....
Pester him he reads this site from time to time, he might just get back in the game to shut us up. And yes he is right and yes there are many rockers made with wrong geometry. Nice thing about factory coined rockers with their skidding ends is that you can't see these problems, the visability a roller tip rocker allows you to be come frightened by being able to see what's really going on and it often ain't pretty.

Bogie
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  #68 (permalink)  
Old 08-11-2012, 06:50 AM
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I went with the Miller method. It sounded logical and correct. I have a 351w with Brodix KC 195 17* heads and Scorpion 1.6 rockers, 5/16 valve stems. When I put the stock length push rods in I had a centered pattern about .125 wide. After I increased the push rods maybe .400 ( that I can remember) I narrowed the pattern to .065 and the contact point went out to the outboard side of the valve stem about .060 from running off the end. The push rod side angle almost mirrored the valve tip side. I also had to buy longer rocker studs with a longer shank to accommodate this setup ( They were chevy SB). I was really happy with this and everything works really well.
What would need to happen to fix the industry, even if the rockers had a standard geometery, the relationship of the rocker arm stud angles to the valve angles are all over the map on heads. There is 6* between a 23 * head and my 17* heads, I can't imagine how far outboard the witness mark would have moved with a head with alot more angle.
Anyway, I'm convinced that the miller method is the way to go.
Lou
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