|08-11-2012 06:50 AM|
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.
|08-10-2012 06:34 PM|
|08-10-2012 09:12 AM|
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.
|08-09-2012 11:43 PM|
|Leoman||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.|
|08-09-2012 08:08 PM|
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)....
|08-09-2012 11:35 AM|
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.
|12-24-2011 02:10 PM|
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.
|12-24-2011 12:06 PM|
|12-24-2011 07:55 AM|
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.
|12-24-2011 05:19 AM|
|12-23-2011 02:47 PM|
I have 17* KC Brodix heads and Scorpion 1.6 rockers with a 8mm valve tip. I tried the Mid Lift method. As I increased the pushrod length toward proper Mid lift dimensions the roller tip moved further outboard on the valve tip. When I arrived at the proper dimensions my sweep decreased but I was at the outboard 1/3 of the valve. This did not look good in my opinion. when I used the middle valve tip method the pushrod was much shorter, sat better on the stud shank was dead center on the valve but I'll admit had a larger sweep.
I believe the problem is the relationship between the stud axis and the valve stem axis (or the length between the roller tip centerline and the fulcrum centerline). If I had either of these distances changed, the former increased or the latter decreased, I would be able to make Mid Lift work.
I also checked my lift at the valve, and I showed full movement.
For my application of .550 lift hydraulic flat tappet, I should be good. I'll let you know how it wears over time.
|11-13-2011 07:20 PM|
Here is what happened to me using a dissasembled/cleaned of oil/reassembled/and cup travel measured stock lifter, and the running valve springs.
With the lifter bottomed out and all clearance removed, I started with the roller tip contact patch well on the inboard side of the stem, approximate to where CompCams says to put it. Upon rotating through two complete crank cycles measured max lift was nowhere near the cams measured .328 x 1.6 , but closer to .515. Each time I extended the pushrod length and recycled the crank, the lift increased and the roller tips location moved outward. I finally arrived at .525 lift and found the 90* locations right where Miller says they should be. I am however uncomfortable with where the roller tip ended up, wich leaves the witness marks outboard at approx .060 from the stems edge, and not much stud threads for the lock nuts. Now I only have one piston/one pushrod in right now so I doubt my needle torque wrench will show small ft.lb differences, but I could definitely "feel" the crank requiring less effort to turn this method vs. old method.
Now I'm still trying to figure out the lesser of two evils, pushrod to valvestem parrallelism, or roller tip to valvestem equidistance. Feel free to laugh at my artistic skills!
|11-12-2011 11:59 AM|
I would question why the engine ran better on the dyno w/o a vacuum advance- especially at peak, where the vac. adv. is supposed to be out of the picture.
|11-12-2011 10:58 AM|
Vacuum Advance Eliminator, Lock-Out Plate
My engine chassis dynoed better with out vac advance hooked, so I decided to run with out for now. I installed the vac advance eliminator that came with the MSD dizzy. Afterward just for kicks I put the timing light on it. It use to be set 22 static with 14 mech. all in 36. Is it normal for the eliminator to change the timing cause now I have 32 static and 40 something at all in at 2500 or so. I know in order to set the elminator in you have to move the rod hooked to the reluctor to set it down in place but i didn't think about it advancing the timing. Does this sound right?
|11-12-2011 10:08 AM|
of coarse not. The mid lift theroy is based on a 180* line drawn the folcrum center throught the roller tip center. Thats the way i set mine up and it works perfectly. i have the least amount of movement across the stem. my witness marks are .030 wide. Further more, at mid lift my pushrod is at 90* to the rocker as well.(are yours). Im as efficient as i can possibly be.
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