Cylinder Head gasket failures
02-22-2003, 03:34 AM
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Cylinder Head gasket failures
We have seen several Cylinder Heads lately and have had approximately 6 calls from people who have had the misfortune to lose the Head gasket on their 944 turbo engine. These failures prompted me to take a hard look at this problem and to see if we could come up with some reason why some engines lose these gaskets. In the last couple of years we have repaired many Blocks and rebuilt Heads and seen the same evidence of some problem. I have followed the posts on this forum reading many good reasons for possible causes. most I am sure are based upon lots of experience and knowledge. Some unfortunately from personal failures. The following is from our 944T engine experience, our experience of other aluminum engines and some theory.
Head studs.
These are an areas of concern, both stock and a common after market version. Both versions have the threads rolled, but the root diameter is the same diameter as the stud itself. The problem with this design of fastener is that when the stud is torqued and put in tensile tension, (stretch) all of the tensile is put into the threaded length not into the stud length. In all of the blocks we have seen, many have had the threads stretched in the block. All of the tensile has thus gone into the threaded area of the block and not into the stud, hence not holding down the head properly and evenly. The solution here is to design the stud properly so that the tensile is transferred to the stud and not to the threads. Guru Racing are coming out with a stud designed properly.
Headgasket design.
The stock gasket is ok, but most open decked engines in my opinion should be using a more stiffer type of gasket. Most of the gaskets we have seen that have failed have done so between the fire rings. This can be from simple deteriation,(time, water), Heads lifting,(studs) or Cylinder Head surfaces. On another note, these 944T engines which are using higher than stock boost should be running higher engine water pressures. Not system pressures, block and Head pressures. This can help with airpockets, hot spots etc. These factors contribute to engine detenation and possible gasket failures. Lean mixtures also have a serious effect on the gaskets, but I would expect to see this more on or around the fire ring. The MLS gasket Guru are offering is a superior design to the stock type and should be used.
Cylinder Head and Block surfaces.
This is an area we have concentrated on very heavily in the past few years. Especially with more and more aluminum engines now in service.
Smooth decks are are critical on aluminum engines like the 944T, as uneven expansion rates between block and head can shred head gaskets in contact with rough surfaces. We have found that the 944T engine is extremely sensitive to uneven combustion pressures between cylinders. A minute 2cc difference in combustion-chamber volume can cause detectable varations in peak cylinder pressure. The knock sensors could often misinterpret this as a misfiring cylinder and retard timing while also triggering other safety functions, check lights etc. To counter this problem, the deck surfaces are milled just enough to equalize chamber volumes after reworking individual chambers. Today we are required to buy expensive machinery which can do this work. The use of special CBN cutters which can produce very low RA numbers over stock allow us to lower these RA numbers. The stock heads typically have a RA number around 28-30, and now we usually get RA numbers down around 9.
All of the above can have a cause and effect on this Head Gasket problem. I have seem the gasket posted here by Guru, and would recommend this over the stock design. The MLS gasket, properly designed Fasteners and proper head machining should lower or hopefully eliminate this problem. Next time you need to have your head repaired and surfaced, ask your machine shop if they can control and get low RA numbers.
I hope this information can be of some help to those who have had failures, and to those who may have the misfortune to have a failure in the future.
Neil Harvey
Performance Developments
Head studs.
These are an areas of concern, both stock and a common after market version. Both versions have the threads rolled, but the root diameter is the same diameter as the stud itself. The problem with this design of fastener is that when the stud is torqued and put in tensile tension, (stretch) all of the tensile is put into the threaded length not into the stud length. In all of the blocks we have seen, many have had the threads stretched in the block. All of the tensile has thus gone into the threaded area of the block and not into the stud, hence not holding down the head properly and evenly. The solution here is to design the stud properly so that the tensile is transferred to the stud and not to the threads. Guru Racing are coming out with a stud designed properly.
Headgasket design.
The stock gasket is ok, but most open decked engines in my opinion should be using a more stiffer type of gasket. Most of the gaskets we have seen that have failed have done so between the fire rings. This can be from simple deteriation,(time, water), Heads lifting,(studs) or Cylinder Head surfaces. On another note, these 944T engines which are using higher than stock boost should be running higher engine water pressures. Not system pressures, block and Head pressures. This can help with airpockets, hot spots etc. These factors contribute to engine detenation and possible gasket failures. Lean mixtures also have a serious effect on the gaskets, but I would expect to see this more on or around the fire ring. The MLS gasket Guru are offering is a superior design to the stock type and should be used.
Cylinder Head and Block surfaces.
This is an area we have concentrated on very heavily in the past few years. Especially with more and more aluminum engines now in service.
Smooth decks are are critical on aluminum engines like the 944T, as uneven expansion rates between block and head can shred head gaskets in contact with rough surfaces. We have found that the 944T engine is extremely sensitive to uneven combustion pressures between cylinders. A minute 2cc difference in combustion-chamber volume can cause detectable varations in peak cylinder pressure. The knock sensors could often misinterpret this as a misfiring cylinder and retard timing while also triggering other safety functions, check lights etc. To counter this problem, the deck surfaces are milled just enough to equalize chamber volumes after reworking individual chambers. Today we are required to buy expensive machinery which can do this work. The use of special CBN cutters which can produce very low RA numbers over stock allow us to lower these RA numbers. The stock heads typically have a RA number around 28-30, and now we usually get RA numbers down around 9.
All of the above can have a cause and effect on this Head Gasket problem. I have seem the gasket posted here by Guru, and would recommend this over the stock design. The MLS gasket, properly designed Fasteners and proper head machining should lower or hopefully eliminate this problem. Next time you need to have your head repaired and surfaced, ask your machine shop if they can control and get low RA numbers.
I hope this information can be of some help to those who have had failures, and to those who may have the misfortune to have a failure in the future.
Neil Harvey
Performance Developments
02-22-2003, 10:39 AM
#2
Rennlist Member
Interesting information Neil. I copied the following from MMS online for an explanation to those that may not understand surface texture:
"In North America, the most common parameter for surface texture is Average Roughness (Ra). Ra is calculated by an algorithm that measures the average length between the peaks and valleys and the deviation from the mean line on the entire surface within the sampling length. Ra averages all peaks and valleys of the roughness profile and then neutralizes the few outlying points so that the extreme points have no significant impact on the final results. It's a simple and effective method for monitoring surface texture and ensuring consistency in measurement of multiple surfaces."
More information can be found here:
<a href="http://www.mmsonline.com/articles/1102gage.html" target="_blank">http://www.mmsonline.com/articles/1102gage.html</a>
Alan C.
"In North America, the most common parameter for surface texture is Average Roughness (Ra). Ra is calculated by an algorithm that measures the average length between the peaks and valleys and the deviation from the mean line on the entire surface within the sampling length. Ra averages all peaks and valleys of the roughness profile and then neutralizes the few outlying points so that the extreme points have no significant impact on the final results. It's a simple and effective method for monitoring surface texture and ensuring consistency in measurement of multiple surfaces."
More information can be found here:
<a href="http://www.mmsonline.com/articles/1102gage.html" target="_blank">http://www.mmsonline.com/articles/1102gage.html</a>
Alan C.
02-22-2003, 04:34 PM
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well my car has the same gasket for over 12 years. i run as much as 1.7 bar pressure (25 PSI). I opened the engine since the head was lifting and the gas pass through the head gasket and the head. The gasket fire rings still were perfect. I am sur eif teh watsegate wouldn't stuck closed and I didn't run many times over 25 PSI the head gasket would go for more than 12 years.
the studs nuts the block and the head all was in perfect condition. the same with the cylinder walls. they still look like new. I think it has to do WHO make teh work and HOW GOOD he is.
My car has all the work made at Porsche Motorsport since it was a factory car. Thats why everythink lasted (and still last) for so long time.
I personaly do not think it is the head the gasket or the block that has the problems. the problem is on the engine builder. the car has been maintened by a Porsche factory race mechanic for the last 12 years. The only Problem I had was a clutch at 130000 km and at 215000 km. the car has now 230000 km and is still ok.
So be carefull who makes the engine work and how experienced he is with the 944 engine. That's all.
Konstantin
the studs nuts the block and the head all was in perfect condition. the same with the cylinder walls. they still look like new. I think it has to do WHO make teh work and HOW GOOD he is.
My car has all the work made at Porsche Motorsport since it was a factory car. Thats why everythink lasted (and still last) for so long time.
I personaly do not think it is the head the gasket or the block that has the problems. the problem is on the engine builder. the car has been maintened by a Porsche factory race mechanic for the last 12 years. The only Problem I had was a clutch at 130000 km and at 215000 km. the car has now 230000 km and is still ok.
So be carefull who makes the engine work and how experienced he is with the 944 engine. That's all.
Konstantin
02-23-2003, 01:10 AM
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My solutions:
streatching head studs (lifting head)
re-torque the studs, as the manufacture tells you to
head gasket design / high Ra numbers
copper coat the gasket - it'll cost you all of $5 for the aersol spray, and you'll have enough left over to copper coat your shoes or something too...
streatching head studs (lifting head)
re-torque the studs, as the manufacture tells you to
head gasket design / high Ra numbers
copper coat the gasket - it'll cost you all of $5 for the aersol spray, and you'll have enough left over to copper coat your shoes or something too...
02-23-2003, 01:46 AM
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One issue is the excessive heat that "burns" the head gasket. If one reduces the risk of one component failing, the heat burns the next weakest link, rings, pistons, valves. The solution is to eliminate the excessive heat that causes the failure. How do we do that, proper air to fuel ratio is the key.
02-23-2003, 02:15 PM
#6
As I was one of the people who recently lost my head gasket after approx 50K miles, I thought I should put in my 2c worth. I did ask Neil what may have caused this problem. I have been so busy I did not have the time to follow this thread. I have read what some of you have written. I read what he wrote completely different. Most of what he said I must admit I do not even understand. If we admit to ourselves, most of us don't. We should be thankful that people like Danno and Travis at Guru, Speedforce Tim, Huntely, Anderson and others are here to help us. If it wasn't for them we would all have spread our engines over the land long ago.I don't think he was advocating we all throw away our parts and buy some others, just that if we want to push the limits then we may have to look at other alternatives, and other areas that I have never seen any other tuners discuss. I run high boost like most of us do, and sometimes we need advice how we can make these engines live longer. Making HP is easy with Turbo's. Making them last is another story.
As for the comment about the copper spray. If you really think that engine development comes in a can, o boy! Your added comment about the re torquing of the studs, well IMHO, you proved you did not read and understand what he wrote. The comment about who builds these engines is right on! I for one have spent many $'s in the wrong place. There's a big difference between an assembler and a builder/engineer. My engine is an 88 and I'm sure development was over long ago. If we look and listen to what is been done on some later engines, maybe we can all benifit. Lets face it, 951 engines aren't cutting edge.
Danno, please jump in here and give your take and translate into language we can all understand.
As for the comment about the copper spray. If you really think that engine development comes in a can, o boy! Your added comment about the re torquing of the studs, well IMHO, you proved you did not read and understand what he wrote. The comment about who builds these engines is right on! I for one have spent many $'s in the wrong place. There's a big difference between an assembler and a builder/engineer. My engine is an 88 and I'm sure development was over long ago. If we look and listen to what is been done on some later engines, maybe we can all benifit. Lets face it, 951 engines aren't cutting edge.
Danno, please jump in here and give your take and translate into language we can all understand.
02-24-2003, 08:45 PM
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M42RACER wrote:
</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva"> As for the comment about the copper spray. If you really think that engine development comes in a can, o boy! Your added comment about the re torquing of the studs, well IMHO, you proved you did not read and understand what he wrote. </font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">I don't think rcldesign was saying it's engine development in a can. It's just an extra bit you can use. I put the copper spray on my last head gasket and I don't have any issues. Ran at 20+ psi all summer with 375 rwhp and it survived. I didn't re-torque the head. But I DO beleive I should have.
The re-torque will allow for the heat cycles and thermal expansion/contraction after initial set of the gasket. I'm not sure that you can cause a significant yield point movement in a Raceware or ARP stud without damaging the threads in the block.
I will agree that the design of the stud could be improved.
Alan C.
</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva"> As for the comment about the copper spray. If you really think that engine development comes in a can, o boy! Your added comment about the re torquing of the studs, well IMHO, you proved you did not read and understand what he wrote. </font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">I don't think rcldesign was saying it's engine development in a can. It's just an extra bit you can use. I put the copper spray on my last head gasket and I don't have any issues. Ran at 20+ psi all summer with 375 rwhp and it survived. I didn't re-torque the head. But I DO beleive I should have.
The re-torque will allow for the heat cycles and thermal expansion/contraction after initial set of the gasket. I'm not sure that you can cause a significant yield point movement in a Raceware or ARP stud without damaging the threads in the block.
I will agree that the design of the stud could be improved.
Alan C.
Trending Topics
02-24-2003, 09:07 PM
#8
Race Director
Our new Head gasket will require no retorque at all and that alone makes it worth buying.Most of the head gasket failures ive seen were not caused buy improper tunning our high boost. The failures were caused buy water corroding threw the gasket itself around the water jackets leaving only the steel compression ring around the bore to take the grunt of the boost.Now with our new multi layer metal gasket this is no longer a problem and no retorqueing needed.
02-24-2003, 09:10 PM
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Race Director
That last post was by superjet.1 not Danno we share the same computer at our facility now.If it were Danno id have shown you some bitchen pics of the gasket.Thanks superjet.1 out!
02-25-2003, 02:46 AM
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</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Originally posted by Danno:
<strong>Our new Head gasket will require no retorque at all and that alone makes it worth buying.Most of the head gasket failures ive seen were not caused buy improper tunning our high boost. The failures were caused buy water corroding threw the gasket itself around the water jackets leaving only the steel compression ring around the bore to take the grunt of the boost.Now with our new multi layer metal gasket this is no longer a problem and no retorqueing needed.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Danno, here is a picture of just what you are referring to. It is off my 86 which lost a HG the first time since new this past fall. For everyones info the perforations in the water jacket gasket part are not supposed to be there. This let go while running only 14 psi and with 52 lb/hr injectors. There are no signs of detonation anywhere on the head or valves or pistons, however the fire ring does have a black spot but I think that occurred after the gasket just gave way.
<a href="http://boards.rennlist.com/upload/HG#4.jpg" target="_blank">http://boards.rennlist.com/upload/HG#4.jpg</a>
<strong>Our new Head gasket will require no retorque at all and that alone makes it worth buying.Most of the head gasket failures ive seen were not caused buy improper tunning our high boost. The failures were caused buy water corroding threw the gasket itself around the water jackets leaving only the steel compression ring around the bore to take the grunt of the boost.Now with our new multi layer metal gasket this is no longer a problem and no retorqueing needed.</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">Danno, here is a picture of just what you are referring to. It is off my 86 which lost a HG the first time since new this past fall. For everyones info the perforations in the water jacket gasket part are not supposed to be there. This let go while running only 14 psi and with 52 lb/hr injectors. There are no signs of detonation anywhere on the head or valves or pistons, however the fire ring does have a black spot but I think that occurred after the gasket just gave way.
<a href="http://boards.rennlist.com/upload/HG#4.jpg" target="_blank">http://boards.rennlist.com/upload/HG#4.jpg</a>
02-25-2003, 11:24 AM
#12
Rennlist Member
Danno,
So when Can I buy one the new gaskets? It's cold I'm bored and checking out the new gasket would take my mind off the weather.
Alan C.
So when Can I buy one the new gaskets? It's cold I'm bored and checking out the new gasket would take my mind off the weather.
Alan C.
02-25-2003, 03:54 PM
#13
Race Director
<a href="http://boards.rennlist.com/upload/HG%234.jpg" target="_blank">http://boards.rennlist.com/upload/HG%234.jpg</a>
I wish JohnD. would fix the non-alphabetic character conversions...
Uhh, that's not me posting up there, but an imposter! I know how to spell darn it!!! For a little background on headgasket failures for those who are just joining us, please review my <a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=18&t=002511" target="_blank">headgasket dissertation.</a>
"It is off my 86 which lost a HG the first time since new this past fall."
Well, 17 years for the stock headgasket's pretty darn good, eh? As having been alluded to in this disccusion, there's no 'one' cause of headgasket failures. There can be several contributing factors and the final-straw of a lean, detonating mixture will be the fatal blow. The factors may include some or all of the following:
- age and condition of fibre part of gasket
- head-stud tension
- flatness and smoothness of mating surfaces
- lean air-fuel ratio leading to detonation, high-pressure shockwaves and heat
Here's a gasket that was submitted to me for analysis a couple days ago:
<img src="http://www.gururacing.net/ImagesMisc/Headgasket030222.jpg" alt=" - " />
Other than the age factor, this gasket shows symptoms of all the other problems. As you can see from the photo, the surface of the cylinder tops looks very rough with lots of scratches. Each creates a small microscopic tunnel under the headgasket where combustion gases can escape.
The fret marks also indicates lack of clamping tension. This may be due to the high combustion pressures that overcomes the clamping-force of the studs. It could be that there was inadequate torque on the nuts, or it could be that the studs were stressed beyond their yield-limit and are no longer elastic. There have been talk about these studs needing to be torqued to their yield-limits and that's not correct. Once the fastener is stressed beyond its yield point, it undergoes plastic deformation and provides zero tension. Best to have a stud that provides the minimum required tension at a torque that is well below its yield limit. This gives some overhead to deal with the added force of combustion pressures stretching the studs even further. For some background on bolts, studs and threaded fasteners, check this out: <a href="http://www.boltscience.com/pages/basics4.htm" target="_blank">Bolt Science</a>
Anyway, once the tension-force clamping the head down is relieved, the gasket and head can squirm around and rub on each other. Combined with the rough surface, you end up with the shiny fretting marks.
The lifting head also allows combustion to sneak past the gasket, as can be seen from the streaking marks moving perpendicular to the compression-ring to the outside.
And finally, the entire inner surface of the compression ring that faces the combustion chamber is melted showing lean mixtures. There are tiny pock marks that indicates detonantion. From the shape of the melted areas and the shape of the ledge at the top of the cylinders, I suspect that the compression-ring may have protruded into the combustion chamber a little bit.
"Superjet1, where are these gaskets listed on your web site?"
You can find it on our <a href="http://forums.gururacing.net" target="_blank">GURU Forums</a> under New Products.
"So when Can I buy one the new gaskets? It's cold I'm bored and checking out the new gasket would take my mind off the weather."
I'm cutting a check for the headgaskets today. Should be ready for shipping to customers by the end of the week! BTW, turn the heater on... <img border="0" title="" alt="[Wink]" src="wink.gif" />
I wish JohnD. would fix the non-alphabetic character conversions...
Uhh, that's not me posting up there, but an imposter! I know how to spell darn it!!! For a little background on headgasket failures for those who are just joining us, please review my <a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=18&t=002511" target="_blank">headgasket dissertation.</a>
"It is off my 86 which lost a HG the first time since new this past fall."
Well, 17 years for the stock headgasket's pretty darn good, eh? As having been alluded to in this disccusion, there's no 'one' cause of headgasket failures. There can be several contributing factors and the final-straw of a lean, detonating mixture will be the fatal blow. The factors may include some or all of the following:
- age and condition of fibre part of gasket
- head-stud tension
- flatness and smoothness of mating surfaces
- lean air-fuel ratio leading to detonation, high-pressure shockwaves and heat
Here's a gasket that was submitted to me for analysis a couple days ago:
<img src="http://www.gururacing.net/ImagesMisc/Headgasket030222.jpg" alt=" - " />
Other than the age factor, this gasket shows symptoms of all the other problems. As you can see from the photo, the surface of the cylinder tops looks very rough with lots of scratches. Each creates a small microscopic tunnel under the headgasket where combustion gases can escape.
The fret marks also indicates lack of clamping tension. This may be due to the high combustion pressures that overcomes the clamping-force of the studs. It could be that there was inadequate torque on the nuts, or it could be that the studs were stressed beyond their yield-limit and are no longer elastic. There have been talk about these studs needing to be torqued to their yield-limits and that's not correct. Once the fastener is stressed beyond its yield point, it undergoes plastic deformation and provides zero tension. Best to have a stud that provides the minimum required tension at a torque that is well below its yield limit. This gives some overhead to deal with the added force of combustion pressures stretching the studs even further. For some background on bolts, studs and threaded fasteners, check this out: <a href="http://www.boltscience.com/pages/basics4.htm" target="_blank">Bolt Science</a>
Anyway, once the tension-force clamping the head down is relieved, the gasket and head can squirm around and rub on each other. Combined with the rough surface, you end up with the shiny fretting marks.
The lifting head also allows combustion to sneak past the gasket, as can be seen from the streaking marks moving perpendicular to the compression-ring to the outside.
And finally, the entire inner surface of the compression ring that faces the combustion chamber is melted showing lean mixtures. There are tiny pock marks that indicates detonantion. From the shape of the melted areas and the shape of the ledge at the top of the cylinders, I suspect that the compression-ring may have protruded into the combustion chamber a little bit.
"Superjet1, where are these gaskets listed on your web site?"
You can find it on our <a href="http://forums.gururacing.net" target="_blank">GURU Forums</a> under New Products.
"So when Can I buy one the new gaskets? It's cold I'm bored and checking out the new gasket would take my mind off the weather."
I'm cutting a check for the headgaskets today. Should be ready for shipping to customers by the end of the week! BTW, turn the heater on... <img border="0" title="" alt="[Wink]" src="wink.gif" />
