[Z06] Root Cause for Excessive Guide Wear Found
11-28-2012, 01:49 PM
#721
Burning Brakes
Thread Starter
11-28-2012, 01:53 PM
#722
The Consigliere
Member Since: May 2006
Location: 2023 Z06 & 2010 ZR1
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I don't think pushrods or aspiration figure into it. I think it's the natural limit of two-valve engines with the compression and valve size employed here.
X amount of heat is generated in the combustion chamber; it's transferred to the various parts with which it comes into contact - the head, the piston, the exhaust system, and the valves.
Since the LS7 exhaust valve is bigger than any other LS engine, it's gonna absorb more heat. But it's got the same size stem and guide, so it's trying to pass more heat into the same size 'sink'.
I'll posit that, at similar conditions (throttle position, engine speed, etc.) the combustion temps in the LS7 are higher than the other LS engines, since the static CR is higher. I'm talking partial-throttle here, I presume the boosted engines get hotter at WOT. Note that there is some indication that exhaust valve guides might be an issue in the LS9, although not nearly to the degree seen here.
The LS7 also revs higher, potentially creating more combustion events per unit time - ergo, more heat to dissipate.
I'd feel a lot better if the LS7 had piston oil squirters and thermal barrier-coated exhaust valves.
X amount of heat is generated in the combustion chamber; it's transferred to the various parts with which it comes into contact - the head, the piston, the exhaust system, and the valves.
Since the LS7 exhaust valve is bigger than any other LS engine, it's gonna absorb more heat. But it's got the same size stem and guide, so it's trying to pass more heat into the same size 'sink'.
I'll posit that, at similar conditions (throttle position, engine speed, etc.) the combustion temps in the LS7 are higher than the other LS engines, since the static CR is higher. I'm talking partial-throttle here, I presume the boosted engines get hotter at WOT. Note that there is some indication that exhaust valve guides might be an issue in the LS9, although not nearly to the degree seen here.
The LS7 also revs higher, potentially creating more combustion events per unit time - ergo, more heat to dissipate.
I'd feel a lot better if the LS7 had piston oil squirters and thermal barrier-coated exhaust valves.
I agree, there is something going here that is having a bigger negative impact in the LS7 than the LS9. So far to date, only one ZR has been reported on the forum to have failed due to a dropped valve. One.
Mutliple people, tho, have reported valve guide wear on LS9 heads. Yet the LS9 doesn't seem to be grenading like the LS7. Now certainly there's only going to be about 4000 of them total on the road, and they didn't start making them until 2009, so perhaps it just hasn't caught up to it yet.
On the other hand, there are a whole bunch of LS9s getting worked - and worked hard. I know of one that has over 70 track days and 30k miles. Sure some are garage queens, but many of them are getting tracked and tracked hard.
I've wondered a lot about this. LS9 has a smaller valve, which as you point out aids in heat transfer to the head, different rocker ratio, lower compression, forged pistons with piston oil squirters, lower redline etc.
Don't know. Maybe it's simply premature to conclude anything. Yet it does seem to be manifesting differently between the two engines.
11-28-2012, 01:53 PM
#723
Burning Brakes
11-28-2012, 01:57 PM
#724
Melting Slicks
Yep, and yep.
Wipe test was perfect, but I still pulled the valve springs and set-up a dial indicator to confirm that valve to guide clearance wasn't excessive. Subsequently I still felt the need to go with full roller rockers and used the same dial indicator to get the appropriate lifter preload. Did this all years ago and it's part of why I've always rebelled at the idea that every LS7 is defective and eveybody needs to do what Ricky did to 'fix' his engine without first determining IF there's actually something to be fixed
My car is an '06 which has had at least 587 rwhp since it was 2,900 miles old and now has over 21,000 miles. It's also been beat within an inch of it's life on a RR course and snapped an axle on a drag strip.
Oh and I'm not sure where ramair got the idea that I didn't think SS valves could make a car go nines (h*ll that's what I use in my CTS-V and at 4200 lbs it'll run 10.40s), I just don't tend to fix what's not broke and have yet to be convinced the sodium valves in my car need to be 'fixed'.
Cheers, Paul.
Wipe test was perfect, but I still pulled the valve springs and set-up a dial indicator to confirm that valve to guide clearance wasn't excessive. Subsequently I still felt the need to go with full roller rockers and used the same dial indicator to get the appropriate lifter preload. Did this all years ago and it's part of why I've always rebelled at the idea that every LS7 is defective and eveybody needs to do what Ricky did to 'fix' his engine without first determining IF there's actually something to be fixed
My car is an '06 which has had at least 587 rwhp since it was 2,900 miles old and now has over 21,000 miles. It's also been beat within an inch of it's life on a RR course and snapped an axle on a drag strip.
Oh and I'm not sure where ramair got the idea that I didn't think SS valves could make a car go nines (h*ll that's what I use in my CTS-V and at 4200 lbs it'll run 10.40s), I just don't tend to fix what's not broke and have yet to be convinced the sodium valves in my car need to be 'fixed'.
Cheers, Paul.
11-28-2012, 02:27 PM
#725
Race Director
I agree, there is something going here that is having a bigger negative impact in the LS7 than the LS9. So far to date, only one ZR has been reported on the forum to have failed due to a dropped valve. One.
Mutliple people, tho, have reported valve guide wear on LS9 heads. Yet the LS9 doesn't seem to be grenading like the LS7. Now certainly there's only going to be about 4000 of them total on the road, and they didn't start making them until 2009, so perhaps it just hasn't caught up to it yet.
On the other hand, there are a whole bunch of LS9s getting worked - and worked hard. I know of one that has over 70 track days and 30k miles. Sure some are garage queens, but many of them are getting tracked and tracked hard.
I've wondered a lot about this. LS9 has a smaller valve, which as you point out aids in heat transfer to the head, different rocker ratio, lower compression, forged pistons with piston oil squirters, lower redline etc.
Don't know. Maybe it's simply premature to conclude anything. Yet it does seem to be manifesting differently between the two engines.
Mutliple people, tho, have reported valve guide wear on LS9 heads. Yet the LS9 doesn't seem to be grenading like the LS7. Now certainly there's only going to be about 4000 of them total on the road, and they didn't start making them until 2009, so perhaps it just hasn't caught up to it yet.
On the other hand, there are a whole bunch of LS9s getting worked - and worked hard. I know of one that has over 70 track days and 30k miles. Sure some are garage queens, but many of them are getting tracked and tracked hard.
I've wondered a lot about this. LS9 has a smaller valve, which as you point out aids in heat transfer to the head, different rocker ratio, lower compression, forged pistons with piston oil squirters, lower redline etc.
Don't know. Maybe it's simply premature to conclude anything. Yet it does seem to be manifesting differently between the two engines.
those things you mention, I would be willing to bet that in the LS9 the issue does not even come close to having the problem of blown engines as the LS7.I mean, look at the number of 09 LS7's we are seeing go down, and other low mile LS7's ...which can then be more compared to LS9's being fairly "young" relative to how long the LS7 has been around
I think if we had not been seeing any 08+ newer LS7's go down, I would say yea it may take longer to rear its ugly head in the LS9...but the fact that we are seeing 09+ newer LS7's fail with little miles..and at least as far as the forum is considered we've only seen one LS9 go down.
11-28-2012, 02:53 PM
#726
Quick, I gotta ask ya...in my experience with a couple of previous LS motors (my son's C5s that we cammed) in regards to lifter preload has been the following:
Tighten the rocker bolt by hand just until you feel the lash taken out.
1/2 to 3/4 turns seat the rocker on a closed valve (important to know the valve is closed).
Here's my concern...everyone here that has reported on lifter preload has reported 1-5/8 to 1-3/4 turn to seat the rocker in its pedestal. 1-3/4 turn on a 1.25 pitch thread is 0.091". At the lifter it becomes 0.091x(1+1.8)/1.8=0.142. Seems a bunch of folks (including me with the oem setup) are either very close to or bottoming lifters.
Your thoughts?
JohnI've read that LS7 lifters (used as replacement for all LS engines now) can accommodate "more" preload. Never seen a value associated with how much more is. I've seen 060" as ideal for for LS1 and "more" for LS7. Not super helpful with the lack of specifics, I know, but it implies your preload might be okay.
-Chad
11-28-2012, 02:57 PM
#727
Drifting
Member Since: Feb 2006
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In another thread somewhere I seem to remember Katech/Jason saying the best preload for LS7 lifters was 80? I could be wrong but thats what I remember reading if that helps anyone?
11-28-2012, 02:59 PM
#728
Burning Brakes
Thread Starter
Hey John,
I've read that LS7 lifters (used as replacement for all LS engines now) can accommodate "more" preload. Never seen a value associated with how much more is. I've seen 060" as ideal for for LS1 and "more" for LS7. Not super helpful with the lack of specifics, I know, but it implies your preload might be okay.
-Chad
I've read that LS7 lifters (used as replacement for all LS engines now) can accommodate "more" preload. Never seen a value associated with how much more is. I've seen 060" as ideal for for LS1 and "more" for LS7. Not super helpful with the lack of specifics, I know, but it implies your preload might be okay.
-Chad
Another fella posted that there is more travel...more like 0.160. That's right at 2 turns at the rocker bolt. So 1-3/4 turns isn't bottomed, but it is damned close.The fact that everyone has reported 1-5/8 to 1-3/4 says to me that the design is biased towards bottoming a lifter...low pivot.
PM me.
John
11-28-2012, 03:02 PM
#729
Burning Brakes
Thread Starter
11-28-2012, 03:12 PM
#730
Melting Slicks
Cheers, Paul.
11-28-2012, 03:16 PM
#731
Burning Brakes
Thread Starter
I set mine using an adjustable PR at .090, then oredered the PRs to that length, when I got them and checked the preload it was closer to .120 which really pissed me off so I order another set of PRs which were .050 shorter, that got me closer to the .070-.080 I was looking for. IIRC the lifter bottoms out at closer to .200".
Cheers, Paul.
Cheers, Paul.
Thanks,
John
11-28-2012, 05:14 PM
#734
Burning Brakes
2007 Z06 9,700 MILES
Passenger side plugs.
Drivers side plugs.
Rocker.
Same rocker.
Exaust valve tip after removing the rocker and BEFORE applying ink.h
With sharpie.
I am not sure why my photos are posting like thumbnails, so I apologize. I will wait for the experts to make an educated analysis, but I do not like what I am seeing. I repeated the test a few different times feeling as though, depending on how I struck the top of the screwdriver, I could manipulate the witness mark. To me, it doesn't seem to roll completely off of the valve tip. It does however, leave VERY little, if any, blue to the outside. I am not sure what to think about the witness mark on the valve or the hour glass wipe mark on the rocker.
LMK Your thoughts.
Passenger side plugs.
Drivers side plugs.
Rocker.
Same rocker.
Exaust valve tip after removing the rocker and BEFORE applying ink.h
With sharpie.
I am not sure why my photos are posting like thumbnails, so I apologize. I will wait for the experts to make an educated analysis, but I do not like what I am seeing. I repeated the test a few different times feeling as though, depending on how I struck the top of the screwdriver, I could manipulate the witness mark. To me, it doesn't seem to roll completely off of the valve tip. It does however, leave VERY little, if any, blue to the outside. I am not sure what to think about the witness mark on the valve or the hour glass wipe mark on the rocker.
LMK Your thoughts.
Last edited by SSMOKE; 11-28-2012 at 06:21 PM. Reason: photobucket error
11-28-2012, 06:33 PM
#736
yes--- actually now im thinking that adjustable pushrods might also be an option..lol. i would be plenty happy with adjustable rocker arms!!! i guess we have to live with bandaids untill we can get these things to heal...lol lol.....why do we have to figure this out? this is bull$hit....
11-28-2012, 06:46 PM
#737
Burning Brakes
Thread Starter
BTW, your photos opened full size for me. ?
And the geometry score is:
Good: 7
Bad:7
I'm thinking more and more that two big pieces of this are **** poor QA and a nominal design that teeters on the edge of bad geometry.
John
Last edited by john_g_46; 11-28-2012 at 06:50 PM.
11-28-2012, 07:25 PM
#738
Burning Brakes
Thread Starter
I started taking a serious look at the possibility of shimming the oem rocker. I think it looks doable based upon measurements I've made on the oem rocker.
And speaking of measurements...
Remember I mentioned that the rockers do not seat down in the 1/2 round of the pedestal ?
Instead they have shoulders that sit on the flats. The shoulders are only 0.030" wide. The space between them on the rocker is 0.640.
I still have to measure up my pedestals this weekend to see if I really believe shimming is practical.
I'll keep y'all posted.
John
And speaking of measurements...
Remember I mentioned that the rockers do not seat down in the 1/2 round of the pedestal ?
Instead they have shoulders that sit on the flats. The shoulders are only 0.030" wide. The space between them on the rocker is 0.640.
I still have to measure up my pedestals this weekend to see if I really believe shimming is practical.
I'll keep y'all posted.
John
11-28-2012, 07:27 PM
#739
Drifting
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I have no idea what you just said john, i havent seen a rocker in person yet but i know you know your thing so rock on. I know every time you get an "ooh!" moment we all learn something.
11-28-2012, 07:37 PM
#740
Burning Brakes
Your witness mark is indeed very close to the edge...but still on. I'd have to say...geometry getting on the marginal side, but probably OK. I'm including you in my good list.
BTW, your photos opened full size for me. ?
And the geometry score is:
Good: 7
Bad:7
I'm thinking more and more that two big pieces of this are **** poor QA and a nominal design that teeters on the edge of bad geometry.
John
BTW, your photos opened full size for me. ?
And the geometry score is:
Good: 7
Bad:7
I'm thinking more and more that two big pieces of this are **** poor QA and a nominal design that teeters on the edge of bad geometry.
John
With only 9,700 miles on my car do you think that I am on my way to bad geometry and worn guides and/or is the geometry basically "fixed" and I should be good to go? I know that there are many variables.
I sent you a PM.
Thanks