96azhawk

I have been plagued with some high rpm valvetrain issues ever since putting this setup together. The car would hit almost what seemed to be a rev limiter at 6300 rpm. We eliminated all ignition components first to rule out ignition related even the opti. I ended up with a broken vavle spring. I would like to know if any of you have some good ideas as to why it broke and maybe some input on how to solve the problem. Thank you in advance for any help.

Here is the top end setup:
AFR LT4 195 ported heads, 2.02/1.60 valves, CC Pro Mag 1.6 rockers, 7/16" studs, Ti retainers, 10 degree locks, cam is 224/230 .607"/.603", 108 lsa

I adjusted the valves closer from 1/2 TPZL closer to 1/8 TPZL and it changed the miss from 6300 to about 6500. This was with the revised Comp 26918 springs installed at 1.750". I took a few off and had them checked and they spec's out around 145 lbs at 1.800" and about 375 lbs at 1.150"

I changed the valve springs out to PAC 1518's to try and help fix the problem. PAC 1518's, installed at 1.800" with about 1/8 turn past zero lash. It made my high rpm problem only a slight stutter around 6300 to 6500 rpm, but did not completely fix the problem.

So I was going to change the install height to 1.750" on the PAC 1518 springs and found that the #2 intake spring was broken. I am not even sure when it broke or noticed a big difference at any point to even think to check it.

MachinistOne

More info on the cam? full numbers including .200"

Looks like there's plenty of retainer to seal clearance...

96azhawk

Cam was a BRE spec'd cam. Here is all the info I have from the cam card:

Serial # L 9437-07 Spec Card# 12554307
Part # 07-000-8 LT1 CS LT1 ROLLER
Core C07-8B LT1 DUCT SEMI (H.L)
Grind # "B-SPEC F4"
ROCKER ARM RATIO
INTAKE 1.50
EXHAUST 1.50
DURATION @ .050 INTAKE 224 DURATION @ .050 EXHAUST 230
VALVE ADJ INTAKE .000
VALVE ADJ EXHAUST .000
LOBE LIFT INTAKE .3790
VALVE LIFT INTAKE .568
LOBE LIFT EXHAUST .3770
VALVE LIFT EXHAUST .565
LOBE SEPARATION 108.0
DURATION .006
INTAKE C/L 106.0
TAPPET LIFT INTAKE 275
CAM ADVANCE 2
TAPPET LIFT EXHAUST 286
HYDRAULIC Y
VALVE TIMING .050
OVERHEAD CAM N
VALVE OPEN INTAKE 6
JOURNAL DIAMETER STD 1.868
VALVE OPEN EXHAUST 45
RECOMMENDED VALVE SPRINGS
VALVE CLOSED INTAKE 38
DEPARTMENT R
VALVE CLOSED EXHAUST 5
SPC INSTR FOR CUSTOMER

MachinistOne

Contact Bret and ask him what he thinks is going on, I'd have to go digging through the Comp book to try and match those lift numbers up with a lobe that might not even be the right one.

I don't see anything obvious that would cause that other than a lobe that the spring can't control. I had one engine years ago(LT4) that broke 2 valve springs in a row(different sets) for no obvious reason and simply changing to a different manufacturer solved the problem. It was Manley Nexteks, and we changed to some Isky's IIRC, because there was nothing wrong with the setup...seemed like an odd harmonic or something because the guy didn't even race it, swore he didn't cold rev, etc...

96azhawk

I have sent an email to Bret asking him, but welcome any other input on the issue as well. Thank you.

96capricemgr

Might compare that to the 3192 lobe, I know it was a favorite.

MachinistOne

What's the piston to valve clearance?

96azhawk

This I do not know. We did not expect it to be an issue with this cam, so we did not verify.

This setup has actually been in and running for about 6 months and has about 2k total on the engine, the first 1500 with the Comp springs and the last 500 or less with the PAC's.

MachinistOne

What else is in the motor - do the pistons have valve reliefs? With that cam PtoV shouldn't be close if the pistons have a generic notch.

96azhawk

Ross Racing pistons part number 94455 with -4cc valve reliefs.

pgerst

Could be harmonics in the spring itself. odd that it does it with two different brands though both beehives.

http://www.ret-monitor.com/articles/...e-springs.pdf; also a couple posts on spring failures

PS: have you verified fuel? seems odd that you would get the same stutter with two valve springs, that others have used successfully. Possible that the failure was a stressor (imperfection, nicked during install) that is unrelated to the miss.

96azhawk

we did check and everything fuel and ignition is working perfectly. Maybe it is just a fluke with that one spring?

pgerst

It could be one bad spring. Can you inspect the spring under magnifcation and see if there is any evidence of a stressor such as a nick? Also looks like the failure may have occurred right beneath the spring end so possibly the end of the spring was malformed. This could cause an isolated incident of coil bind (the spring end hit the coil beneath it) or could be unequal seat pressure (the retainer is not evenly seated due to spring end malformity), causing the spring load to be applied as a cantilever. (An analogy would be using only one compressor on a coil over, you are adding an additional twisting/sideways load to the spring it was not designed for.) Third, which you would probably not see under visual magnification, is an inclusion or void in the spring material, usually caused by air or dust during the forming process.

PAC specs a frequency of 29277 cpm for the 1518 which works out to an 1/8 harmonic at 7319 rpm. Comp does not have that info on their web page but I suspect it will be similar. What are you running as a redline? In theory you should be safe below 7300 rpm but the 1/8 harmonic is a "rule of thumb." You may have geometry problems when you lose the narrow top of the beehive, but a dual spring is stable at much higher rpms as you basically add the natural harmonics of both springs together.

Did you get any feedback from either PAC or CompCams?

96azhawk

Thanks for all the input so far. I will be get more information and post up.

pgerst

Any update on the suspected cause?