LS7 exhaust valves Problems
06-09-2010, 07:25 AM
#3
There have been quite a few reported cases of exhaust valve failure with the stockers. Most of the information out there is pointing to problems with the guides having poor clearances or also that the head of the valve is not seating in the pocket straight there for ripping the head off the two piece sodium filled valve. Also GM had a bad batch of rockers in 2007 where the bearings failed. I also have WCCH heads from Richard and the upgraded Harland Sharp mod and haven't had any issues yet "knock on wood!"
Last edited by ramairws6; 08-14-2010 at 05:39 AM.
06-10-2010, 10:29 AM
#7
Regardless if I find that this concern applied to L92 rockers I will post up about it. Thanks again
cam
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06-10-2010, 11:03 AM
#8
As for the stock exhaust valves, we have not had a failure on any here yet....that being said, when we port them or have the heads off for whatever reason, we go ahead and put in a solid stainless valve to replace them. Better safe than sorry I guess.
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06-10-2010, 11:29 AM
#10
Regarding the LS7 rocker arm failures iirc the LS7 uses a 1.8 ratio rocker but the L92 uses a 1.7 so perhaps its the ratio increase and the fact that the stock LS7 cam is pretty big and has a 7K RPM redline that caused the rocker issues.
Regardless I havent seen many threads at all with L92 stock rocker arm failures and this leads me to think they are good to go. Besides I'd like to think if one started to go you'd hear the valvetrain warning you so you could shut it down before catastrophic failure.
I did find this interesting link over at Corvetteforum which is an ib site so i dont think linking it will be a problem. This is certainly something to think about
http://forums.corvetteforum.com/c6-z...ngs-issue.html
Regardless I havent seen many threads at all with L92 stock rocker arm failures and this leads me to think they are good to go. Besides I'd like to think if one started to go you'd hear the valvetrain warning you so you could shut it down before catastrophic failure.
I did find this interesting link over at Corvetteforum which is an ib site so i dont think linking it will be a problem. This is certainly something to think about
http://forums.corvetteforum.com/c6-z...ngs-issue.html
06-10-2010, 01:10 PM
#11
Other than reading about the issues on the internet, I have no experience with LS7 rocker arms breaking. None of the LS7 cars we've had come through the shop have had any issues with them.
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06-10-2010, 03:30 PM
#12
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I picked Richard@WCCH's brain on this subject as it was a hot topic over on corvetteforum.com. Below are his comments:
And the thread that started it all http://forums.corvetteforum.com/c6-z...wn-begins.html
Originally Posted by Richard@WCCH
Hello Michael,
We have definitely seen excessive wear in some LS7 exhaust guides. Mostly coming from engines with over 30k on the clock. We have seen some heads show inconsistent wear requiring only one or two guides needing replacement. It appears that the sodium exhaut valves are transmitting an excessive amount of heat from the stem to the guide. In some cases the oil is BBQ'd in the guide and the lack of lubrication and cooling from the oil causes the excessive wear (mostly in the lower half of the exhaut guide). We've measured some guides as having over .004" of wear in the lower 1" of exhaust guide length. Porsche 911's suffer the same type of related guide wear on the exhaust side.
The next item to note regarding the sodium exhaust valves relates to the wall thickness of the sodium exhaust valve stem. It's only .040" thick and makes for a fragile exhaust valve. GM uses a good quality steel material but the design spec. renders the valves brittle at the neck. Hence the reason we choose the replace the factory units with stainless or inconel. The exhaust valves we use are manufactured with a .001" oversized stem size which allows us to hone the exhaust guides and remove most or all of the taper in the bore. In cases with excessive wear we replace the guides with bronze.
I can't comment on weather the excessive guide wear is causing the valves to rattle around in the guide on on the seat causing the head failures. It may be due to excessive heat buildup from the restricted heat flow path to the guide. As I mentioned before, there's only 1mm of wall thickness at the valve neck. Not a good situation in engines producing high specific horespower. The stainless exhaust valves don't show the rapid wear that the sodium valves have.
Hope this helps Michael.
Best regards,
Richard
We have definitely seen excessive wear in some LS7 exhaust guides. Mostly coming from engines with over 30k on the clock. We have seen some heads show inconsistent wear requiring only one or two guides needing replacement. It appears that the sodium exhaut valves are transmitting an excessive amount of heat from the stem to the guide. In some cases the oil is BBQ'd in the guide and the lack of lubrication and cooling from the oil causes the excessive wear (mostly in the lower half of the exhaut guide). We've measured some guides as having over .004" of wear in the lower 1" of exhaust guide length. Porsche 911's suffer the same type of related guide wear on the exhaust side.
The next item to note regarding the sodium exhaust valves relates to the wall thickness of the sodium exhaust valve stem. It's only .040" thick and makes for a fragile exhaust valve. GM uses a good quality steel material but the design spec. renders the valves brittle at the neck. Hence the reason we choose the replace the factory units with stainless or inconel. The exhaust valves we use are manufactured with a .001" oversized stem size which allows us to hone the exhaust guides and remove most or all of the taper in the bore. In cases with excessive wear we replace the guides with bronze.
I can't comment on weather the excessive guide wear is causing the valves to rattle around in the guide on on the seat causing the head failures. It may be due to excessive heat buildup from the restricted heat flow path to the guide. As I mentioned before, there's only 1mm of wall thickness at the valve neck. Not a good situation in engines producing high specific horespower. The stainless exhaust valves don't show the rapid wear that the sodium valves have.
Hope this helps Michael.
Best regards,
Richard
Originally Posted by Richard@WCCH
Given the same conditions, the bronze guides will wear as well. The size of the small heat path must be increased. With a solid stem valve heat is allowed to travel further up the length of the stem. It provides a larger flow path for heat dissipation.
The primary heat path is through the valve seat. The secondary heat path is through the valve guide. Lean stoic. conditions at cruising speeds generate lots of heat on the exhaust valves with unleaded fuels. The heat bakes the small amounts of oil that makes it to the lower portion of the exhaust guide. Once the lubrication is gone there is metal to metal contact and on a molecular level electron sharing occurs. The steel molecues of the exhaust valve stem have a greater resistance to wear. The powdered metal guides have mixture of wear resisting alloys along with lubritious properties pressed into the matrix. You don't have to beat on the engine to experience valve guide wear.
The primarly reason we use bronze valve guides is because GM will not sell the guides separately. Rather they will sell you an entire replacement head. The aftermarket parts supply only furnishes bronze alloy valve guides. I personally like the powdered metal valve guides the factory uses but the supplier does not sell piece meal to the public. Additionally we have no long term wear issues with the bronze guides and solid stem stainless steel valves.
In the last decade GM has done a masterful job improving both metalurgy and rocker arm geometry to reduce overall valve guide wear. Rocker arm desings are also superbly executed. The issues seem to only be related to the sodium valves.
Hope this helps.
Richard
The primary heat path is through the valve seat. The secondary heat path is through the valve guide. Lean stoic. conditions at cruising speeds generate lots of heat on the exhaust valves with unleaded fuels. The heat bakes the small amounts of oil that makes it to the lower portion of the exhaust guide. Once the lubrication is gone there is metal to metal contact and on a molecular level electron sharing occurs. The steel molecues of the exhaust valve stem have a greater resistance to wear. The powdered metal guides have mixture of wear resisting alloys along with lubritious properties pressed into the matrix. You don't have to beat on the engine to experience valve guide wear.
The primarly reason we use bronze valve guides is because GM will not sell the guides separately. Rather they will sell you an entire replacement head. The aftermarket parts supply only furnishes bronze alloy valve guides. I personally like the powdered metal valve guides the factory uses but the supplier does not sell piece meal to the public. Additionally we have no long term wear issues with the bronze guides and solid stem stainless steel valves.
In the last decade GM has done a masterful job improving both metalurgy and rocker arm geometry to reduce overall valve guide wear. Rocker arm desings are also superbly executed. The issues seem to only be related to the sodium valves.
Hope this helps.
Richard
