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Doubt about current rocker setup

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Old 01-22-2013, 07:38 AM
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Originally Posted by Danspeed1
Brian, I have read many of your posts and have based most of the work I have done so far off of them. Its great that you were able to post in this thread. They only thing I can't seem to wrap my mind around is how the rocker movement on the valve tips just sounds so destructive, yet everyone continues to agree the stock rockers are the best. Doesn't make much sense.

I know I am beating a dead horse with this one, however I have one last question. In addition to your threads, I have been reading a lot of threads that vettenuts has posted. My last question; if shimming the rockers is bad because it creates more wear, then what is the point of even checking the wipe pattern if you shouldn't adjust it anyway. My previous questions were all based off the posts vettenuts has made in regard to checking wipe pattern and trying to get the stock rocker arms to have a centered wipe pattern. Well if you shouldn't shim them, then how do you achieve this?

Sorry for all the questions, but I am really interested in making sure I have all my bases covered. I am also the type of person who wants to understand everything I am doing and the reasons why. I do appreciate your patience and education on this subject.

DG
The reason so many people like the stock rockers is because they're the lesser of two evils, and they cost less money. Roller rockers need increased open spring pressure to control the heavier mass, which then requires better than standard 5/16" .080" wall pushrods. The entire valve train ends up heavier with no real benefit. A heavier valve train requires more energy to move it, so there are parasitic losses also. A lot of people have also had roller rockers fail, and the stock rocker bodies never fail.

Checking the wipe pattern makes a lot of sense when it comes to roller rockers, if you can narrow the wipe pattern that's a good thing. Applying the same thought to stock rockers seemed like a good idea, but few realized that shimming up stock rockers for a more narrow wipe pattern was a recipe for disaster. The reason I'm so familiar with this scenario is because when I worked for TFS we did the same thing. When the valve angle was rolled over to 13.5 degrees naturally the thing to do to "optimize" the geometry was to decrease the width of the wipe pattern. The heads were released and within 6 months we got a pair of heads back with less than 10,000 miles on them and the valve tips were destroyed. I was talking to my friend Mike Tomaszewski at T/A Performance about this same time, and he told me they had 2 different geometry's on their heads depending on whether a guy was running stock rockers or roller rockers. He also told me about how a stock rocker must ride on the valve tip the same way a rocking chair rocked on the floor. I had attended a seminar in 1995 hosted by Ken Sperry of GM and he had talked about half arc and full arc geometry and then it all clicked. Talk about having egg on your face... TFS then did months of durability testing, sometimes running engines 24 hours a day for days on end. By early 2006 the TFS heads went from running 7.700" long pushrods to running 7.500" long pushrods, the trunnion height was reduced that much. The 7.700" version was basically optimized for roller rockers, and the current 7.500" was optimized for stock rockers. Although the heads continued to be made with bronze guides, which will never last with stock rockers, so roller rockers were still required.

Fast forward a few years after that, someone shimmed up stock rockers and gained power on a dyno, and the rocker shimming craze was on. The reason shimming stock rockers increases power is because you actually increase the rocker ratio and therefore valve lift. When a stock rocker is shimmed the point where the rocker scroll contacts the valve is moved away from the trunnion, this increases rocker ratio. Obviously at the expense of long term durability. If you have a track only drag car, you can shim stock rockers to gain power and will probably never see any damage.

If you check stock GM wipe pattern with all things stock the wipe pattern is centered, although it's very wide. The only two things that affect rocker geometry with bolt down stock rockers is trunnion height and valve tip height. If you don't shim your rockers or mill the rocker pad then that's fixed. Changing the trunnion bearings does not affect the trunnion height. The valve tip can get higher when the head has a valve job performed and the contact patch should be checked to ensure the rocker scroll is actually contacting the valve tip rather than the edge of the valve. After heads have been run you can actually pull the rockers off and inspect the scroll and tell a lot about your geometry. If you see the round edge of the valve etched into the inside portion of the rocker scroll then you know the valve tip is too high and the edge of the valve is contacting the scroll. I've seen high mileage stock engines with this round edge etched in the rocker scroll. Although some of that may have been caused from the rocker scroll radius wearing away.

I hope this helps.
Old 01-22-2013, 02:34 PM
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That helped greatly. I can finally wrap my head around whats going on here, and more importantly put all the other posts I have read to rest. It makes sense now that there has been an evolution to the understanding of how the stock rockers ride on the valve tip and that was what was leading to all the misinformation I was finding online. Thank you so much for taking the time to write all that. I think that has to be one of the best written explanations I have ever read on the net.

I am going to run the stock rockers with the trunion upgrade for a season and they I will take pictures of the results and post them online in the fall. I usually do about 10K-15K a year.

Thanks again,

DG



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