928 Block Thoughts
02-19-2007, 02:58 AM
#1
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928 Block Thoughts
I've noticed that some people, who are building engines, are cutting out the existing cylinders and installing free standing cylinders. I just wondered if anyone seriously thought that this was going to work?
The 928 blocks are a silicon/aluminum mixture, which makes them very brittle. The lower cradle is actually aluminum, not this silicon mixture. They did this for strength.
The upper portion of the block has a huge tendency to crack the main journals towards the cylinders. This happens almost every time the thrust bearing spins in an automatic car. This happens on engines that are highly stressed, on a rather unpredicable time table.
When someone removes the existing cylinders, they seriously weaken the structure for the main "webs" in the existing block. Porsche did indeed do this for the 944 GTRs and supplied free standing cylinders for these race engines. They found that they needed to recast the existing block in aluminum (without the silicon), because the original silicon/aluminum blocks would "last only minutes" on the dyno before they cracked. Even when they switched the material, they had to re-inforce the areas around the main webs to make them strong enough to live. Why would a 928 engine be any stronger/different?
greg brown
The 928 blocks are a silicon/aluminum mixture, which makes them very brittle. The lower cradle is actually aluminum, not this silicon mixture. They did this for strength.
The upper portion of the block has a huge tendency to crack the main journals towards the cylinders. This happens almost every time the thrust bearing spins in an automatic car. This happens on engines that are highly stressed, on a rather unpredicable time table.
When someone removes the existing cylinders, they seriously weaken the structure for the main "webs" in the existing block. Porsche did indeed do this for the 944 GTRs and supplied free standing cylinders for these race engines. They found that they needed to recast the existing block in aluminum (without the silicon), because the original silicon/aluminum blocks would "last only minutes" on the dyno before they cracked. Even when they switched the material, they had to re-inforce the areas around the main webs to make them strong enough to live. Why would a 928 engine be any stronger/different?
greg brown
02-19-2007, 05:11 AM
#2
Nordschleife Master
Good point you raise. It seems the idea behind the aluminum aloy they used was mainly for cylinder wear and longevity on that part of the block specifically. So the top halves fo the bearings should be more suceptable to cracks and problems then the lower girdle. It is plainly obvious that the lower girdle is of a different composition. (have no idea what exactly) Its interesting that porsche used a steel girdle for M96 engines (996 and 986 boxster) They have the crank bolted to a steel "block" where the main bearings and the entire crank run on steel and only the cylinders are in the aluminum. I dont think aluminum is the best thing to run a crank through. Its pretty soft, but with current metalurgy who knows. I am sure there are some pretty awesome alloys that are both strong and lightweight. Not to mention somewhat forgiving. I guess when you have 7 main bearings like 911 engines you can get away an all aluminum case. With a V8 and only 5 mains, those parts better be pretty strong and quite resistant to fatique and cracking.
Are you now tearing apart someone else's 427ci 928 engine or something? I know you hate to rebuild what you feel is someone else's crap design. I dont blame you. Is Danglerb trying to get you to put steel liners in some engine? He dosent listen does he?
Are you now tearing apart someone else's 427ci 928 engine or something? I know you hate to rebuild what you feel is someone else's crap design. I dont blame you. Is Danglerb trying to get you to put steel liners in some engine? He dosent listen does he?
02-19-2007, 06:20 AM
#3
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While I don't doubt your experience and observations as an Engine Builder of some repute, as an engineer I can't see that the cylinder bores add very much stiffness to the lower half of the block.
Without knowing how exactly how the 944GTRs bores were added or in what material, wonder if the issue experienced in the 944GTRs was more heat transfer related or differential expansion related - think nice new strong steel? bore versus the removal of the relatively flimsy silicon alloy cylinder bores with very good heat transfer characeristics and similar expansion properties to the parent block.
From what I've seen of TBF'd blocks, unlocalised damage looked more like the effects of forces acting in an undesirable direction (Theusual root cause of TBF) combined with undesirable thermal expansion due to the heat created by the failing/overloaded/spinning thrust bearing.
Without knowing how exactly how the 944GTRs bores were added or in what material, wonder if the issue experienced in the 944GTRs was more heat transfer related or differential expansion related - think nice new strong steel? bore versus the removal of the relatively flimsy silicon alloy cylinder bores with very good heat transfer characeristics and similar expansion properties to the parent block.
From what I've seen of TBF'd blocks, unlocalised damage looked more like the effects of forces acting in an undesirable direction (Theusual root cause of TBF) combined with undesirable thermal expansion due to the heat created by the failing/overloaded/spinning thrust bearing.
02-19-2007, 06:40 AM
#4
Three Wheelin'
Greg, I must be one of the 2 people that you refer to using sleeves.
https://rennlist.com/forums/928-forum/326231-sleeved-engine-project-pics.html
The lower block strength was considered as very important for my design and dictated features, the original cylinders are cut no deeper than the point at which they meet the rest of the block. I went into the reasons for the machining methods in another thread and explained how I left material around the bottom area and designed the sleeves and sealing around the premise of removing no more material around the bottom than necesarry. Alot of planning, research and testing has gone into my project that you don't know about, nor do you understand my background and abilities. I could certainly go into detail about details that would bore most, I could talk about how the o-ring material choice came from discussions with engineers, making test grooved parts that were then sectioned and observed under a microscope for compression characteristics to adjust the groove geometry or the 3 month test were the o-ring was immersed in the planned 'lubricant' and then measured for effects to compressability. That's just the stupid o-ring. Now you come along and say "I just wondered if anyone seriously thought that this was going to work?". I find your tone to be insulting and resent your assumption that I haven't considered basic design criteria for a project like this.
https://rennlist.com/forums/928-forum/326231-sleeved-engine-project-pics.html
The lower block strength was considered as very important for my design and dictated features, the original cylinders are cut no deeper than the point at which they meet the rest of the block. I went into the reasons for the machining methods in another thread and explained how I left material around the bottom area and designed the sleeves and sealing around the premise of removing no more material around the bottom than necesarry. Alot of planning, research and testing has gone into my project that you don't know about, nor do you understand my background and abilities. I could certainly go into detail about details that would bore most, I could talk about how the o-ring material choice came from discussions with engineers, making test grooved parts that were then sectioned and observed under a microscope for compression characteristics to adjust the groove geometry or the 3 month test were the o-ring was immersed in the planned 'lubricant' and then measured for effects to compressability. That's just the stupid o-ring. Now you come along and say "I just wondered if anyone seriously thought that this was going to work?". I find your tone to be insulting and resent your assumption that I haven't considered basic design criteria for a project like this.
02-19-2007, 07:09 AM
#5
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Originally Posted by GregBBRD
When someone removes the existing cylinders, they seriously weaken the structure for the main "webs" in the existing block. Porsche did indeed do this for the 944 GTRs and supplied free standing cylinders for these race engines.
02-19-2007, 07:28 AM
#6
Three Wheelin'
Here's a post from my block thread when I got into the considerations of block strength and implications to sleeve configuration:
https://rennlist.com/forums/showpost...6&postcount=20
To sum it up, here are some ways I kept block strength:
1-Machine cylinders only to the depth that they are unsupported. The lower pic in the above post shows a machined area in between bores. Cutting downward stops at that point with only enough material cut to provide a register.
2-Machine the bore for the sleeve as small as possible. That makes the sleeve thin and prevents an o-ring from being used on the sleeve, the thin sleeve dictates the steel material.
I disagree that a 928 block is not well suited to having removable sleeves, in fact I believe it's ample cylinder spacing make it especially well suited because of the material available to act as 'webs'. A comparison with an inline 944 block could be helpful to point out the sometimes unconsidered stresses that the bottom part of an engine is subjected to but it's not helpful to say a 928 would be no different, with no hope of correcting issues. To consider keeping the original cylinders in place for the purpose of strengthening the bottom end is defeatist, it's missing the opportunity of gaining significant displacement by not trying to engineer a sleeved block WITH bottom end strength. You could be stuck with an original block but it's STILL going to break under high stresses anyway and you also have the material/scuffing issues complicating your life, you'd also have the unsupported cylinder top waving like a flagpole as high cylinder pressures slacken the force on the gasket allowing the brittle material to move, losing ring seal and power, or split.
https://rennlist.com/forums/showpost...6&postcount=20
To sum it up, here are some ways I kept block strength:
1-Machine cylinders only to the depth that they are unsupported. The lower pic in the above post shows a machined area in between bores. Cutting downward stops at that point with only enough material cut to provide a register.
2-Machine the bore for the sleeve as small as possible. That makes the sleeve thin and prevents an o-ring from being used on the sleeve, the thin sleeve dictates the steel material.
I disagree that a 928 block is not well suited to having removable sleeves, in fact I believe it's ample cylinder spacing make it especially well suited because of the material available to act as 'webs'. A comparison with an inline 944 block could be helpful to point out the sometimes unconsidered stresses that the bottom part of an engine is subjected to but it's not helpful to say a 928 would be no different, with no hope of correcting issues. To consider keeping the original cylinders in place for the purpose of strengthening the bottom end is defeatist, it's missing the opportunity of gaining significant displacement by not trying to engineer a sleeved block WITH bottom end strength. You could be stuck with an original block but it's STILL going to break under high stresses anyway and you also have the material/scuffing issues complicating your life, you'd also have the unsupported cylinder top waving like a flagpole as high cylinder pressures slacken the force on the gasket allowing the brittle material to move, losing ring seal and power, or split.
Last edited by Mike Simard; 02-19-2007 at 08:10 AM.
02-19-2007, 08:55 AM
#7
Race Car
Firstly, what a GREAT topic! 
..even though I am one of those with a wet-sleeve engine which is nearly complete.
Because of Greg's reputation, I appreciate him voicing his concerns regarding the new design since he does have intimate knowledge of the 928 engine. Not only that, he provided an example with the 944GTR to support his concerns.
Likewise, 123 and Mike have good counter-points to consider.
Greg, how many time have you seen this on a non-automatic car? I know you know of at least one which occurred fairly recently, and whose cylinders are bored.
In the meantime though, only time will tell how the 'new era' wet-sleeve 928 block hold-up, but let the well-informed and thought-out speculations flow!
..even though I am one of those with a wet-sleeve engine which is nearly complete.
Because of Greg's reputation, I appreciate him voicing his concerns regarding the new design since he does have intimate knowledge of the 928 engine. Not only that, he provided an example with the 944GTR to support his concerns.
Likewise, 123 and Mike have good counter-points to consider.
Originally Posted by GregBBRD
The upper portion of the block has a huge tendency to crack the main journals towards the cylinders. ... This happens on engines that are highly stressed, on a rather unpredicable time table.
In the meantime though, only time will tell how the 'new era' wet-sleeve 928 block hold-up, but let the well-informed and thought-out speculations flow!
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02-19-2007, 09:16 AM
#8
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944 blocks are little different in a way that water area height is about 1" shallower that in 928 blocks. This obviously means support for wet towers will be higher up and supporting area is larger that in 928's case. This could have something to do why several 944 block based engines are using wet sleeves. I hope 928 can be made to work reliably as Alusil towers create annoying 105ish mm barrier which is impossible to cross with any other means.
02-19-2007, 12:46 PM
#9
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The 944 GTR as raced in TransAm was making 650 HP or so out of the 4 cylinder BUT as Greg mentioned virtually NOTHING in the engine was STOCK. Greg knows the GTR well , a brand new unraced one sits in a bubble in his shop. And I have seen bits and pieces of another such engine there as well.
02-19-2007, 02:33 PM
#10
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Oops, not my intention to make anyone angry or offend anyone. Sorry Mr. Simard.
I actually had no idea of who was doing this kind of work, or how many were doing it. I was just simply asking a question to a group of people that might have some input on this topic.
The 944 GTR program needed to have removable cylinders in order to make repairs to these engines practical. The earliest engines did not have removable cylinders. The time needed for repairs between races was impractical. They were forced to completely start over with a new block and block material to accomplish removable cylinders, as the alusil block simply would not work when the cylinder structure was removed. I have worked on both styles of engines. I have pieces from many different versions of these engines. I actually have the very first serial number race engine that Porsche put into the 944 GTRs, as well as the last "official" serial numbered engine. I'm somewhat familiar with the problems that the factory had with the 944 GTR, also.
I had noticed that several people were mentioning this method of putting larger pistons in the 928 block and I simply asked if this was a viable option. Perhaps I asked my question improperly. Again, sorry.
I'm certain you are a very smart person and a very qualified engineer and machinist. You should be very proud that you could fiqure out a way to make this work, while the extensive engineering staff at Porsche could not accomplish this. It would appear that Porsche had severe issues with durability of the block in the development of the 928 engine. They had to do more than a few changes before they re-inforced the block enough to make it live in a stock, fairly low horsepower, application. I just wondered if cutting away a huge amount of this structure would weaken the block and cause the cracking that Porsche experienced in the 928 development and in the 944 GTR program, as well as the cracking that I see on a fairly routine basis. It would seem that you have researched and solved this issue. Very nice.
greg brown
I actually had no idea of who was doing this kind of work, or how many were doing it. I was just simply asking a question to a group of people that might have some input on this topic.
The 944 GTR program needed to have removable cylinders in order to make repairs to these engines practical. The earliest engines did not have removable cylinders. The time needed for repairs between races was impractical. They were forced to completely start over with a new block and block material to accomplish removable cylinders, as the alusil block simply would not work when the cylinder structure was removed. I have worked on both styles of engines. I have pieces from many different versions of these engines. I actually have the very first serial number race engine that Porsche put into the 944 GTRs, as well as the last "official" serial numbered engine. I'm somewhat familiar with the problems that the factory had with the 944 GTR, also.
I had noticed that several people were mentioning this method of putting larger pistons in the 928 block and I simply asked if this was a viable option. Perhaps I asked my question improperly. Again, sorry.
I'm certain you are a very smart person and a very qualified engineer and machinist. You should be very proud that you could fiqure out a way to make this work, while the extensive engineering staff at Porsche could not accomplish this. It would appear that Porsche had severe issues with durability of the block in the development of the 928 engine. They had to do more than a few changes before they re-inforced the block enough to make it live in a stock, fairly low horsepower, application. I just wondered if cutting away a huge amount of this structure would weaken the block and cause the cracking that Porsche experienced in the 928 development and in the 944 GTR program, as well as the cracking that I see on a fairly routine basis. It would seem that you have researched and solved this issue. Very nice.
greg brown
02-19-2007, 03:00 PM
#11
Three Wheelin'
Thanks for the kind words Greg. Sorry if I was harsh, this project is the most effort I've ever invested in an engine and it's very dear to my heart (I sure hope my wife doesn't see this!). I'd love to hear more about the 944 GTR issues and if you have pictures that would be great too. You are right, the block structure is sometimes not considered and even many gearheads don't understand the movement going on in the bottom part of a block. How did Porsche go about sealing their sleeve?
Here's how I see the thought process for a sleeved project:
First the sealing method must be decided. Many would be tempted to Loctite it but IMHO that would be impractical because the bottom part of the block WILL be moving around. That leaves a good old fashined o-ring. Once that is decided there are 3 places to put it, if it goes on the sleeve it would be easy to cut on a lathe but a thicker sleeve would be needed and more material removed from the block. It can on the face of where the sleeve sits but that takes away register area and possibly cause the sleeve to compress the register and sink. The 3rd idea is to place the o-ring in the block, this is harder to machine but does allow a thin sleeve and the least amount of material to be removed and IMHO the groove itself doesn't sacrifice block strength. Well anyway, that's how I came up with mine, I'd love to hear your thoughts on the 944 GTR experience.
Here's how I see the thought process for a sleeved project:
First the sealing method must be decided. Many would be tempted to Loctite it but IMHO that would be impractical because the bottom part of the block WILL be moving around. That leaves a good old fashined o-ring. Once that is decided there are 3 places to put it, if it goes on the sleeve it would be easy to cut on a lathe but a thicker sleeve would be needed and more material removed from the block. It can on the face of where the sleeve sits but that takes away register area and possibly cause the sleeve to compress the register and sink. The 3rd idea is to place the o-ring in the block, this is harder to machine but does allow a thin sleeve and the least amount of material to be removed and IMHO the groove itself doesn't sacrifice block strength. Well anyway, that's how I came up with mine, I'd love to hear your thoughts on the 944 GTR experience.
02-19-2007, 03:12 PM
#12
Race Director
No doubt Greg is an 928 engine building expert...& there are a few others on this forum.....Greg has made several strokers in the convential overbore-long stroke method that results in a high power, but long lasting engine.....I think he built both race engines for Mark Anderson and Joe Fan.....he knows his stuff.....
Only time will tell about the "liner" 928 engine & how it ends up....it seems 6.5L is the max for the 928 engine using stock bores......vs up to a 7 to 7.4L possible with liners? Damm a 451 cubic inch 928 engine....damm that would be another 50-75 whp more than the 6.5L......but how long will it last?
Only time will tell about the "liner" 928 engine & how it ends up....it seems 6.5L is the max for the 928 engine using stock bores......vs up to a 7 to 7.4L possible with liners? Damm a 451 cubic inch 928 engine....damm that would be another 50-75 whp more than the 6.5L......but how long will it last?
02-19-2007, 05:56 PM
#13
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Ryan:
You really need to give up the Danglerb thing, IMHO. You have certainly been the most vocal of the people that didn't like his postings. I really don't understand why you didn't just ignore his stuff, if you didn't have anything constructive to say. It seems that this forum should welcome new people, questions, and ideas. Sure, some of people's stuff is bound to be stupid and boring, but no one seems to have a gun to your head to make you read it. I actually found your attacks on him to be far more stupid and boring than his postings. I also found his "alter ego" that someone created to be some of the funniest things that I've ever read. That thing about installing the "juicer" had to be one funniest things ever written. I literally laughed for 15 minutes.
Forget him and get a life of your own. He's gone and you are still bitching on about him. Seems like enough.
greg brown
You really need to give up the Danglerb thing, IMHO. You have certainly been the most vocal of the people that didn't like his postings. I really don't understand why you didn't just ignore his stuff, if you didn't have anything constructive to say. It seems that this forum should welcome new people, questions, and ideas. Sure, some of people's stuff is bound to be stupid and boring, but no one seems to have a gun to your head to make you read it. I actually found your attacks on him to be far more stupid and boring than his postings. I also found his "alter ego" that someone created to be some of the funniest things that I've ever read. That thing about installing the "juicer" had to be one funniest things ever written. I literally laughed for 15 minutes.
Forget him and get a life of your own. He's gone and you are still bitching on about him. Seems like enough.
greg brown
02-19-2007, 06:14 PM
#14
Road Warrior
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https://rennlist.com/forums/928-forum/239222-110mm-bore-928-engine-project-plans-for-a-7-4-liter-in-the-works.html a 7.0L project.... seems they had some interesting ideas... iirc, mr ott is putting out around 586rwhp NA, and bob devore managed nearly 700hp NA w/just under 6.5L. http://www.928s4vr.com/devore.htm so why bother w/sleeves? either one will already trash the drive train if abused. heck, if we just got the same hp/liter as a GT3 (136) we'd be living large 
course, one could always just drop a 7L cheby crate motor in and be done w/it instead of trying to patch something together too, but I suppose either way this is probably safer than having multiple GF's in one's spare time...
course, one could always just drop a 7L cheby crate motor in and be done w/it instead of trying to patch something together too, but I suppose either way this is probably safer than having multiple GF's in one's spare time...
02-19-2007, 08:03 PM
#15
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Back to the block question. Has anyone run one of these engines with the free standing liners or is this all still experimental? If there is an engine that has been run, how long has it run? How hard has the engine been run? Has it been inspected for cracks in the main bearing areas? Believe me, I'm interested. The engineers at Porsche who couldn't make it work will be interested.
greg brown
greg brown