New valve guides- the only key to durability?
01-24-2007, 01:10 PM
#1
Drifting
Thread Starter
Join Date: Aug 2006
Location: South Bay, Los Angeles
Posts: 2,733
Likes: 0
Received 3 Likes
on
3 Posts
New valve guides- the only key to durability?
Why is it that 3.2 guys seem to get much longer service life than 3.6 guys?
Once you've replaced the valve guides in your 3.6 , is there any reason why you can't get the 300K miles like some of the 3.2 guys? Or is there still an inherent design flaw in the 3.6 that prevents this longevity?
If there is an inherent flaw in the 3.6, once you've got your 3.6 pulled for a top-end job, why not replace the whole thing with a "better" 3.2, or even a 3.2 Turbo, if you're worried about missing the HP?
Once you've replaced the valve guides in your 3.6 , is there any reason why you can't get the 300K miles like some of the 3.2 guys? Or is there still an inherent design flaw in the 3.6 that prevents this longevity?
If there is an inherent flaw in the 3.6, once you've got your 3.6 pulled for a top-end job, why not replace the whole thing with a "better" 3.2, or even a 3.2 Turbo, if you're worried about missing the HP?
01-24-2007, 02:02 PM
#3
Nordschleife Master
All Porsche valve guides are soft. It appears that on the 993 the tolerances are not as tight as the previous air cooled heads and when they wear, they leak oil though badly. So, yes, when you do a top end rebuild, use better valve guides, that is key in the longevity of the head. Obviously there are some other issue like cam pitting that seems more prevalant on the 964/993 engines, but that may be an oil formulation issue. There is the hydraulic lifter issue, and of course the SAI port issue. Personally, I think the 964 engine has some benefits in terms of less maintenance than the 993 engine.
One of the reasons the 3.0SC engine rus so long is that is is considered understressed. It has a better rod to stroke ratio and a smaller bore piston so it rocks less and therefore contacts the cylinder less, and there is less wear. Further, the smaller bore allows for larger cooling fin surface area and so it cools better.
The 3.6l engine is a good engine and will go 150k with only maintenance, although if you have the premature valve guide issue, then you'll need a top end rebuild at least.
One of the reasons the 3.0SC engine rus so long is that is is considered understressed. It has a better rod to stroke ratio and a smaller bore piston so it rocks less and therefore contacts the cylinder less, and there is less wear. Further, the smaller bore allows for larger cooling fin surface area and so it cools better.
The 3.6l engine is a good engine and will go 150k with only maintenance, although if you have the premature valve guide issue, then you'll need a top end rebuild at least.
01-24-2007, 02:09 PM
#4
RL Technical Advisor
Hi:
You've asked a big question,...
There are several reasons why Porsche guides don't last long and some of these are:
1) Excessive heat (3.6's do run hotter than 3.2's)
2) Poor OEM guide material (very soft)
3) Poor & insconsistent stem-to-guide fitment at the factory when the engines were built
This latter item helps explain why some motors begin using excessive oil at 40-50K and some go beyond 100K.
For example, a customer sent his 3.6 heads (with 3000 miles!) to us for service since he wanted to proactively deal with a potential SAI problem. Upon disassembly, we found almost .015 clearance between the exhaust valve stems and the guides which both allowed the valve to wobble all over the valve seat as well as permit excess oil down the guides. Its been a QC issue at the factory since this situation, while excessive, was not unusual.
IMHO, based on hundreds of sets of heads using our own guides and very careful fitment between valve stems and guides, these last well over 100K with proper maintenance and good oil temp control. We use factory Viton seals and these valves all now have hard chrome stems. If a PO keeps clean oil in the engine and prevents high oil temps, I think they can reach 200-300K before needing to be rebuilt.
You've asked a big question,...
There are several reasons why Porsche guides don't last long and some of these are:
1) Excessive heat (3.6's do run hotter than 3.2's)
2) Poor OEM guide material (very soft)
3) Poor & insconsistent stem-to-guide fitment at the factory when the engines were built
This latter item helps explain why some motors begin using excessive oil at 40-50K and some go beyond 100K.
For example, a customer sent his 3.6 heads (with 3000 miles!) to us for service since he wanted to proactively deal with a potential SAI problem. Upon disassembly, we found almost .015 clearance between the exhaust valve stems and the guides which both allowed the valve to wobble all over the valve seat as well as permit excess oil down the guides. Its been a QC issue at the factory since this situation, while excessive, was not unusual.
IMHO, based on hundreds of sets of heads using our own guides and very careful fitment between valve stems and guides, these last well over 100K with proper maintenance and good oil temp control. We use factory Viton seals and these valves all now have hard chrome stems. If a PO keeps clean oil in the engine and prevents high oil temps, I think they can reach 200-300K before needing to be rebuilt.
01-24-2007, 02:19 PM
#5
steve--
what do you consider to be "high oil temps"...anything above 10 o'clock on the gauge?
what do you consider to be "high oil temps"...anything above 10 o'clock on the gauge?
01-24-2007, 02:32 PM
#6
Addict
Rennlist Member
Rennlist Member
Great topic, as I am considering doing a top-end on my own 97 C2 w/ 59k miles because it needs a clutch anyway and those SAI passages are clogged. I don't have a sense of oil consumption yet and it's difficult to drive the car because the clutch is slipping a little.
The mechanic I'm leaning toward using is convinced that a lot of the valvetrain wear can be associated with the use of Mobil One, which he says is outperformed by many other varieties; he swears by Amsoil. He builds race motors, FWIW.
He feels that with better valve guides the carbon problem will be reduced to the point where the SAI passages will not be a problem down the road. Any thoughts, Steve?
The mechanic I'm leaning toward using is convinced that a lot of the valvetrain wear can be associated with the use of Mobil One, which he says is outperformed by many other varieties; he swears by Amsoil. He builds race motors, FWIW.
He feels that with better valve guides the carbon problem will be reduced to the point where the SAI passages will not be a problem down the road. Any thoughts, Steve?
01-24-2007, 02:46 PM
#7
RL Technical Advisor
Originally Posted by FLYT993
steve--
what do you consider to be "high oil temps"...anything above 10 o'clock on the gauge?
what do you consider to be "high oil temps"...anything above 10 o'clock on the gauge?
Thats over 245 degrees F and while the motor will take it, its certainly NOT conducive to engine longevity.
Trending Topics
01-24-2007, 02:59 PM
#8
RL Technical Advisor
Originally Posted by vjd3
Great topic, as I am considering doing a top-end on my own 97 C2 w/ 59k miles because it needs a clutch anyway and those SAI passages are clogged. I don't have a sense of oil consumption yet and it's difficult to drive the car because the clutch is slipping a little.
The mechanic I'm leaning toward using is convinced that a lot of the valvetrain wear can be associated with the use of Mobil One, which he says is outperformed by many other varieties; he swears by Amsoil. He builds race motors, FWIW.
He feels that with better valve guides the carbon problem will be reduced to the point where the SAI passages will not be a problem down the road. Any thoughts, Steve?
The mechanic I'm leaning toward using is convinced that a lot of the valvetrain wear can be associated with the use of Mobil One, which he says is outperformed by many other varieties; he swears by Amsoil. He builds race motors, FWIW.
He feels that with better valve guides the carbon problem will be reduced to the point where the SAI passages will not be a problem down the road. Any thoughts, Steve?
With all due respect to your mechanic, I do not think Mobil 1 is the culprit as we've used that stuff since its inception along with many other lubricants. I've built 911/935 race motors since 1976 and guide wear was always an issue until we began using alternative materials for guides,....
With better guides and VERY careful fitment, the SAI issues will not be a problem. As an added measure, we do enlarge the SAI passages in both the heads and cam towers when we machine a set of heads here.
01-24-2007, 03:02 PM
#9
RL Technical Advisor
Originally Posted by Geoffrey
Obviously there are some other issue like cam pitting that seems more prevalant on the 964/993 engines, but that may be an oil formulation issue.
Do a search on Pelican on ZDDP and you'll be rewarded.
11-18-2007, 06:32 PM
#10
Three Wheelin'
Join Date: Dec 2002
Location: Tampa
Posts: 1,367
Likes: 0
Received 0 Likes
on
0 Posts
steve: thanks again for all your help in the past
Can you elaborate on your thoughts re heat and valve guide and/or engine life. I'm interested in your thoughts in general and also your thoughts on auxilary oil coolers. We debate this issue alot down here and I'm interested in your thoughts.
Thanks again
Jack
Can you elaborate on your thoughts re heat and valve guide and/or engine life. I'm interested in your thoughts in general and also your thoughts on auxilary oil coolers. We debate this issue alot down here and I'm interested in your thoughts.
Thanks again
Jack
