Changing Thinking.

For decades we have put smaller ring gaps on the second ring. In the mid 90's I had a customer that did destructive testing for the OEM's and for some of the Cup teams. He told me back then....open up the second ring gap...its worth power. Then he told me the story...

In development of the LS platform GM was going to thinner metric rings. One reason was to put the top ring as high as it could to reduce the area in between the piston, ring and cylinder wall. This was to reduce emissions and meet the new government stds.

In testing the top ring experienced flutter. This caused loss of HP and loss of ring tension. One engineered theorized if the 2nd ring's gap was opened up it would reduce the pressure on the backside of the top ring. This was tested and not only did it eliminate ring flutter it made HP. This finding was leaked to the top Cup teams and since the early 90's cup engines have had larger ring gaps on the second ring.

Fast forward almost 20 years later....piston mfg started changing instructions in their piston kits stating to open up the 2nd ring gap. Wait a minute....we been doing this for almost a 100 years this way...we can't be wrong in our thinking and doing.

Professional engine builders knew this for years. In the 90's I would tell customers to do this. Some went on with how they did it, some tried it and said thank you. The ones that said thank you didn't tell anyone....

I would invite anyone to do the sweep method on 1 bank the 90 degree way stated in the video on the other side and drive it for 5K miles. YOu will see first hand what works.

 

Smokey Yunick has this to say about the roller rocker to valve stem interface;
"The sweep of this point-of-contact (roller to stem) is the most critical aspect of the rockerarm geometry"
He also goes on to say;
"When the valvetrain geometry is correct the point-to-contact should never move closer than about 0.20" to the edge of the stem butt."
So he is in a rond-about-way confirming that it does not need to be centered. His book was published in 1983. Guess the cam boys haven't read his book. I didn't key on this either first time around. I went back and found this info.
Strange that this info is not more universal.

Chris: On the second ring gap being larger than the first. I found that info as well on line.
My KB hyper pistons run a large first ring gap, .026" (.065" per inch of bore) for a regular street application. KB recommends a .016" (.004 per inch of bore) for the second ring gap. Would you change this second ring gap to something larger given the already large first ring gap and if so how big would you go on a street/strip application normally aspirated?

 

I did get the springs tested today. Took in 6. They all tested the same. So very consistent anyhow.
140# on the seat at installed height of 1.8".
318# @ 1.251 or .549" of lift.
So with a 370 lb/in spring reduced 10% for wear that gives me a 333 lb/in spring.
333 x .549 = 182.8#
182.8 + 140 = 322.8#

Mine read 318#. Sound about right? Springs still in good shape?

 

This is an important factor, but it is also important to try to keep the rocker tip centered on the top of the valve stem, otherwise the rocker will have less lateral support and risk the dreaded SBC failure of the rocker gradually wearing sideways and notching the rocker stud.

To have better centering of the narrow contact line on the valve stem, one needs to have the best installed height of the tip of the valve. Taller valves will move the contact line closer to the exhaust side, shorter valves will move the contact line closer to the intake side. Why?... The extended center line of the rocker stud and of the valve are not parallel and will intersect at some distance well above the head. Therefore, as the push rod is lengthened, raising the location of the trunnion point, the contact line will move closer to the exhaust side of the stem.

If one is concerned about lateral stability with taller valves, lash caps do a good job of providing a wider base of contact.

 

The rockers I'm using can move not only up and down and side to side but also in any axis in between. So I can't see them notching the rocker stud from lack of lateral support.
As long as the roller tip maintains contact with the valve stem by virtue of good spring control, wouldn't that provide the needed lateral support in the case of any type roller tip rocker without lash caps?

I mean wouldn't the roller tip first have to lose contact with the valve stem then, come back down onto a valve stem tip at some angle to start the process of rounding off the valve stem end and start riding at an angle to force the rocker to the side to eventually notch the stud as a result?

 

Cardo, this has got to be the sign of an unhappy valve train.
.015 valve shim under #4 intake spring.





That's right, worn completely through by the spring.

 

Time for new springs. Your down from 375 In/Lbs to under 330 In/Lbs. Make sure they are correct for your cam. Spring surge and harmonics will make the spring jump off the shim at the right resonance. This is what has been happening.

http://www.engr.colostate.edu/~dga/h...pm_1000fps.wmv

http://www.engr.colostate.edu/~dga/h...pm_1000fps.wmv

http://www.engr.colostate.edu/~dga/h...pm_3000fps.wmv

 

A guy by the name of Rick herrick made rocker arms for both smoky and a guy named Harvey crane. I worked for Rick for a couple of years making rockers.

One day the topic of offset rollers to center the roller and cover the entire diameter of the valve. Rick looked at me and said as long as sweep is no wider than .060" you have nothing to worry about.

I would open up 2nd ring by 20% more than top ring.

 

The cam calls for a 330 lb/in spring. The heads came with 370 lb/in springs. I've read that all springs lose 10% under normal use. That would make them a 333 lb/in spring after use.
Based on that would you still say springs are fatigued beyond use for this cam?
Correcting geometry will hopefully eliminate bad harmonics. More spring wouldn't control harmonics though would they? Just change the rpm at which it would occur if it still existed?
Spring seems adequate. Could it be shimmed a bit more as long as coil bind was not an issue to increase pressure in this scenario?

 

Good info. Thanks Chris.
Lots of ring gap for a KB piston then. Does blowby become a concern at some CR and ring gap?

 

Harmonics are strange. A stronger spring or weaker spring does not necessarily eliminate the problem. I built a solid flat tappet 440 BBC with a cam that had an operating range of 3500 to 7000 RPM. The cam had 2 recommended spring options. The heads came with springs that were in between the two on seat pressure, spring rate and open pressure. All tested consistent. I thought we were good to go. Got it on the dyno and power went flat at 5200. Timing was bouncing around. Changed distributors, same thing. Was scratching my head, couldn't figure it out. Changed to a third distributor, same problem. Figured out the springs might be the issue (thanks Gkull). Pulled a spring and found, after 5 or 6 dyno runs they were down 20% on pressure. Changed to the stronger recommended spring and it was still pulling at 6500 when we shut it down with rock solid timing stability. The characteristics of the original spring, whether it was the coil thickness, amount of coils, diameter, rate or a combination did not interact with the lobe profile well. The thing is the original springs that came with the heads were stronger than the weaker of the recommended springs on the seat, rate and open pressure. The spring surge/ harmonics knocked the pressure down 20% in a few under 6000 RPM dyno runs. See the video to see why. I would replace the springs with the exact spring recommended if it was mine because, looking at your shim your springs were doing the same thing that spring in the video I posted was doing. At 6000 RPM you valves are opening and closing 50 times a second. If those springs were slinkying enough to jump off the shim to the point it ate the shim up that bad they are not what you want. You should make noticeably more power without the bad harmonics.

 

You want to be careful not going too much over 300# with those rockers. I think somewhere in very small print Comp recommends that as a max open pressure not to go over 350# but I've seen them blued from less. (they used to recommend not exceeding 300#)

 

A spring's natural frequency needs to be extended out to the 13th harmonic and needs to be well ahead of (or behind) the operating frequency of the motion they control. At 6000rpm the fundamental spring motion has a frequency of 3000 cpm and the 13th harmonic would be 39000 cpm or 650 Hz.

 

Hopefully this thread will make some members check their stuff. Whether you accept this practice of selecting pushrod length or not, for you own sake a second look at what is happening could save you thousands of dollars if the valvetrain is failing.

 

I worked on boat in early 90's to pay for college. Mercury marine states not to worry about an engine up to 30% leak down.

Later in life one of my mentors was talking about leak figures and how useless they really are....the engine is not running....how many people have leaked an engine running at its operating range? I took this info and have stored it and shared it.

Don't worry about the leak down results....reduce the flutter.

 

ok. Does that translate to use the recommended spring for the application?

 

Depends on who''s recommending it and why.
One thinbg I like about Pac springs...they give frequencies in their catalog.

 

I believe that is correct 350# max for these rockers. So given your experience with these rockers it would not be advisable to shim the springs up or get a stronger spring as I am already over 300#.

http://www.compcams.com/Technical/Pr...gnumRoller.asp

Tucked away in my mind for the next build or if this one gets torn down in the future. Thanks!

 

Good info. Could be I have parts that don't play well together.
With correct pushrod length this problem may dissapear or it may not.
The Lunati springs spec'ed for the cam are these;

http://www.summitracing.com/parts/lun-73100-16

The springs on the head now are these, or at least close to these, Summit seems to have the wrong coil diameter listed, as the ones I have are 1.5" outer diameter coils;

http://www.summitracing.com/parts/dr...0111/overview/

So here is what I'm thinking, put in the pushrods (and replace shims while I'm at it) and run it.
If it's not ba-bam! better then give the Lunati springs a try. See if then it's better.
Sound like a decent plan?

 

Kinda depends on your driving. If you're going to have your foot in it a lot and drive it like you stole it, I'd just keep in mind the spring pressure. I had a set on a SBFord that had about 310 open pressure but I drove it like I stole it. Every rocker blued around the ball with Valvoline racing oil. If you;re more into cruising and an occasional romp on the throttle, then you're probably safer with a little more pressure.