996 What best year?
02-07-2013, 11:42 AM
#91
02-07-2013, 11:55 AM
#92
That 'something' isn't necessarily the product itself. Sometimes, it's a manufacturing process. These engines were hand assembled on a bench. The 996 was designed and built during a financially difficult time for Porsche, and they were getting production engineering advice from Toyota at the time. An 'improvement' could just as easily have been made to shave an hour off the build time as to make the product work better. We just don't know, and never will.
02-07-2013, 12:28 PM
#93
For those who want particular detail about the IMSB design, the following was cut from a discussion over on Renntech I was following. This bearing analysis was not done by Jake, but by a company that primarily makes replacement airplane components and sells an M96 IMSB replacement:
When you start a project that involves a ball bearing, the first steps are to figure out the loads and speed the bearing will operate under. The loads will be used to determine the rolling contact stress. Intuitively, you might think the peak contact stress to be directly at the point of ball contact, but actually it is just forward of the ball and below the surface. It is easy to visualize if you think of a heavily loaded wheel-barrow tire rolling over soft earth. The wheel pushes forward soil, so it is like the wheel is always trying to "climb out" of a depression. As long as the contact stress is below the material yield point, the bearing survives. From a practical standpoint, the higher the contact stress, the more distortion. The more distortion, the more heat generated. Heat makes the steel softer. So, when designing for big loads, you need bearings with either lots of *****…. or big ***** to keep heat down. Speed factors determine which approach is used.
When you see a ball race that is pitted or flaking, the allowable contact stress limt was exceeded. It could be dirt. lube, or overload that caused the problem.
Now we need to move back in time. Here is a guess as to how the bearing ended up submerged. The engine was designed wet sump, the first Porsche boxer engine with that approach since the 912/356. When they started testing it in the prototype 996, they found the oil pick-up was sucking air instead of oil (with the new suspension and great tires under high lateral "G’s"). When sump baffling changes did not fix it, they added a couple liters of oil to the sump. Cheap fix, new dipstick. But, the bearing (which was designed dry) was now wet when not running. Well,….. someone had to sign off the engineering change forms, including the impact of the change assessment. Now we have a paper trail to the guilty. When the bearing problems started, the cover up began.
So, the guy who messed up ….either found a way to get it blamed on the least liked guy in the group (and is now the area manager), or…..he has long since left Stuggart, and made a career making weinerschnitzel in the Black Forest. You might laugh, but this happens all the time in big organizations. The case everybody remembers ……..is the space shuttle o-ring disaster.
When we do the math on the grease lubed (but not immersed) dual row bearing, it looks really good . Lots of load margin, and the combination of small OD and small ***** keeps the ball speed and centrifugal loads down. No problem at all grease lubed. The problems start when the grease is gone.
The second generation single row bearing is simply the dual row cut in half. It is really that basic. Same ID and OD, half the ball count. So the contact stresses are doubled. We now have a bearing that is far more likely to overheat and die. Based on the failures seen , this is exactly what is happening. These engines should have the hybrid ceramic upgrade done now, don't wait for the clutch to go.
You can’t admit you made a mistake and go backward , so……..you make a third design! You fix the contact stress by putting this big single row bearing in place. The load capacity math on this bearing looks fine, but the limiting speed is very questionable. The increased diameter makes a big jump in ball speed (speed varies as the square of the radius) because the OD of this bearing spins. The ***** are large and heavy, making high centrifugal force that wipes lube off the race. If you are driving around town, and rarely redline the engine, it is OK. Endurance running this design at sustained high rpm is asking for trouble. We have not completed the analysis on the impact of just changing the ***** to ceramic on this bearing. With the ceramic ball’s weight reduction, it will help the centrifugal issue quite a bit. (We do have a ceramic ball version of this big bearing on the road, but it is far to early to tell if the solution is adequate. All indications are it is an improvement, and we can custom make one of these for you if you really want it.) Question is, will it be enough. Since you have to tear the engine down to service this bearing, we are considering making the shaft go back to the (field service capable) dual row. It will require a very precise, cylindrically ground spacer sleeve to make it happen. We would have to do the upgrade in-house to control the fit. More work is needed on this one. Since every engine has to be torn down to fix this bearing, we don’t expect very many sales for a period of years.
So, all this probably happened because ……….no one wanted to tell the boss a $300 bearing was needed to fix it, or the business people could not make a case for spending the money.
When you start a project that involves a ball bearing, the first steps are to figure out the loads and speed the bearing will operate under. The loads will be used to determine the rolling contact stress. Intuitively, you might think the peak contact stress to be directly at the point of ball contact, but actually it is just forward of the ball and below the surface. It is easy to visualize if you think of a heavily loaded wheel-barrow tire rolling over soft earth. The wheel pushes forward soil, so it is like the wheel is always trying to "climb out" of a depression. As long as the contact stress is below the material yield point, the bearing survives. From a practical standpoint, the higher the contact stress, the more distortion. The more distortion, the more heat generated. Heat makes the steel softer. So, when designing for big loads, you need bearings with either lots of *****…. or big ***** to keep heat down. Speed factors determine which approach is used.
When you see a ball race that is pitted or flaking, the allowable contact stress limt was exceeded. It could be dirt. lube, or overload that caused the problem.
Now we need to move back in time. Here is a guess as to how the bearing ended up submerged. The engine was designed wet sump, the first Porsche boxer engine with that approach since the 912/356. When they started testing it in the prototype 996, they found the oil pick-up was sucking air instead of oil (with the new suspension and great tires under high lateral "G’s"). When sump baffling changes did not fix it, they added a couple liters of oil to the sump. Cheap fix, new dipstick. But, the bearing (which was designed dry) was now wet when not running. Well,….. someone had to sign off the engineering change forms, including the impact of the change assessment. Now we have a paper trail to the guilty. When the bearing problems started, the cover up began.
So, the guy who messed up ….either found a way to get it blamed on the least liked guy in the group (and is now the area manager), or…..he has long since left Stuggart, and made a career making weinerschnitzel in the Black Forest. You might laugh, but this happens all the time in big organizations. The case everybody remembers ……..is the space shuttle o-ring disaster.
When we do the math on the grease lubed (but not immersed) dual row bearing, it looks really good . Lots of load margin, and the combination of small OD and small ***** keeps the ball speed and centrifugal loads down. No problem at all grease lubed. The problems start when the grease is gone.
The second generation single row bearing is simply the dual row cut in half. It is really that basic. Same ID and OD, half the ball count. So the contact stresses are doubled. We now have a bearing that is far more likely to overheat and die. Based on the failures seen , this is exactly what is happening. These engines should have the hybrid ceramic upgrade done now, don't wait for the clutch to go.
You can’t admit you made a mistake and go backward , so……..you make a third design! You fix the contact stress by putting this big single row bearing in place. The load capacity math on this bearing looks fine, but the limiting speed is very questionable. The increased diameter makes a big jump in ball speed (speed varies as the square of the radius) because the OD of this bearing spins. The ***** are large and heavy, making high centrifugal force that wipes lube off the race. If you are driving around town, and rarely redline the engine, it is OK. Endurance running this design at sustained high rpm is asking for trouble. We have not completed the analysis on the impact of just changing the ***** to ceramic on this bearing. With the ceramic ball’s weight reduction, it will help the centrifugal issue quite a bit. (We do have a ceramic ball version of this big bearing on the road, but it is far to early to tell if the solution is adequate. All indications are it is an improvement, and we can custom make one of these for you if you really want it.) Question is, will it be enough. Since you have to tear the engine down to service this bearing, we are considering making the shaft go back to the (field service capable) dual row. It will require a very precise, cylindrically ground spacer sleeve to make it happen. We would have to do the upgrade in-house to control the fit. More work is needed on this one. Since every engine has to be torn down to fix this bearing, we don’t expect very many sales for a period of years.
So, all this probably happened because ……….no one wanted to tell the boss a $300 bearing was needed to fix it, or the business people could not make a case for spending the money.
02-07-2013, 01:14 PM
#94
Track Day
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That 'something' isn't necessarily the product itself. Sometimes, it's a manufacturing process. These engines were hand assembled on a bench. The 996 was designed and built during a financially difficult time for Porsche, and they were getting production engineering advice from Toyota at the time. An 'improvement' could just as easily have been made to shave an hour off the build time as to make the product work better. We just don't know, and never will.
02-07-2013, 03:50 PM
#95
Race Director
What does mass (in the context of a captive bearing) have to do with speed of rotation? The increase in diameter is what you indicated caused the increased centrifugal force that purportedly causes the lubricant to be "wiped" from the race.
I don't understand this claim that the composition of the bearing alone - with no change in dimension - would affect its rotational speed.
Thanks in advance for clarifying.
I don't understand this claim that the composition of the bearing alone - with no change in dimension - would affect its rotational speed.
Thanks in advance for clarifying.
02-07-2013, 04:11 PM
#96
Race Director
Correct me if I'm wrong, but...
...the rotational velocity of a sphere has an inverse relationship with its diameter, not a direct relationship. Increasing the diameter of a ball means that the ball must rotate fewer times to travel the same distance. A soccer ball must make more revolutions than a beach ball to travel the same distance. If the soccer must travel the same distance as the beach ball in the same amount of time, it must spin more quickly.
Since rotational speeds should have decreased when the diameter of the ***** in the bearing was increased, the theory that increased centrifugal force causes lubricant to be thrown from the bearing seems counterintuitive.
Since rotational speeds should have decreased when the diameter of the ***** in the bearing was increased, the theory that increased centrifugal force causes lubricant to be thrown from the bearing seems counterintuitive.
02-07-2013, 05:36 PM
#97
I didn't write this. Like I noted it came from a competitor of LN/Jake. But the mass of a ceramic bearing is lower than a steel bearing. Thus the acceleration acting on the ceramic bearing results in a lower force than that of a steel bearing.
You will note he says "because the outside of the bearing spins", i.e. The larger diameter bearing race spins faster faster than the smaller bearing race.
You will note he says "because the outside of the bearing spins", i.e. The larger diameter bearing race spins faster faster than the smaller bearing race.
02-07-2013, 08:38 PM
#100
Three Wheelin'
That's weird you say that because I was *just* thinking about boobies. huh.
Also - my 2002 has an M97 big bearing motor installed. When I replaced the clutch last summer and was both disappointed/pleased to see this.
It hasn't slowed me down at all. If the motor blows up I'll replace it. It'll suck - but what are you going to do?
Also - my 2002 has an M97 big bearing motor installed. When I replaced the clutch last summer and was both disappointed/pleased to see this.
It hasn't slowed me down at all. If the motor blows up I'll replace it. It'll suck - but what are you going to do?
02-09-2013, 10:47 AM
#102
996: The Essential Companion
The above subjected book recently became available for about $75 on Amazon.com. I would recommend both current and prospective 996 owners order it. Besides, having a copy handy will make you a real power on rennlist!
Back in my 993 days, I acquired the 993 version of this pub by the same author. It was encyclopedic in detail with good photo illustrations. I expect this new one to be every bit as useful.
relinuca
Back in my 993 days, I acquired the 993 version of this pub by the same author. It was encyclopedic in detail with good photo illustrations. I expect this new one to be every bit as useful.
relinuca
02-10-2013, 09:02 PM
#103
Then theres this:
And this:
And:
Note, not my words so dont blame me.
About 1 in 3000 996 intermediate shaft failures happened, but nearly all were in 1999-2002 vehicles with less than 20,000 miles on them and typically on cars not driven/used at least 40% of the time. Also an even higher incidence of 1999-2002 vehicles failed when not maintained per Porche requirements (oil) or driven often past the RPM limit.
1. 97% of all of these failures occur in 1999-2002 cars and 3% in 2003-2004 cars.
1. 96% of all of these failures occur with less than 20,000 miles.
1. 99% of all of these failures occur when it sits unused for more than 4 weeks often.
1. 92% of all of these failures occur in cars not maintained properly or driven abusively.
1. 96% of all of these failures occur with less than 20,000 miles.
1. 99% of all of these failures occur when it sits unused for more than 4 weeks often.
1. 92% of all of these failures occur in cars not maintained properly or driven abusively.
If you have a 2003-2004 3.4 or 3.6L boxer engine Porsche, you are at much less risk due to number of failures in these years, but a very small number of failures are still possible. However, if you have passed 30-000-40-000 miles, do not leave the car unused for months at a time, maintain the 0-40 Mobile 1 oil and the car per Porsche, and do not abuse the car past it's tach limit, you have almost no risk of this failure.
02-11-2013, 01:32 AM
#104
Hope this is all true, it makes sense to me, except i just don't understand how can not driving the car increase the risk of failure? What do you think the percentage of C2's that are parked for at least 3 months over the winter? My guess would be at least 75% of all sold. We in Canada should have these things blown up on the side of the road non stop as the percentage of cars that are parked over the winter is probably 90%. I am of the opinion that when the car is parked it's parked, when you get it out of storage, drive it often and don't baby it too much. But i didn't do the research, these people did, so i can't talk about it with too much confidence but without a little further explanation as to how this is bad i have hard time accepting it as a fact. Cheers!
02-11-2013, 06:27 PM
#105
Track Day
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Nedster,
The way I understand it if the bearing seal has been compromised then the only lubrication is from engine oil. If the car has been stored and not brought to temp over an extended period, the bearing would be dry and thus damaged with engine start.
I did this to try and assure less likely catastrophe with my Boxster:
1) Always warm the car to operating temp before driving or at least before getting "heavy footed."
2) Never lug down the engine. Keep the revs at 3K or above when possible.
3) Keep engine oil fresh. 3000 to 5000 max before change. (U can decide what weight best in ur mind.That's another can of worms to be opened)
4)Inspect oil filter after each change looking for metal fragments
5)Once warmed up and ready to go, drive it like u stole it!
During cooler weather in SW Missouri, when not driving it on a regular basis, I would start the car and let it run until it reached 180 degrees or so just to keep the oil "moving."
Maybe that will help with reasoning.
The way I understand it if the bearing seal has been compromised then the only lubrication is from engine oil. If the car has been stored and not brought to temp over an extended period, the bearing would be dry and thus damaged with engine start.
I did this to try and assure less likely catastrophe with my Boxster:
1) Always warm the car to operating temp before driving or at least before getting "heavy footed."
2) Never lug down the engine. Keep the revs at 3K or above when possible.
3) Keep engine oil fresh. 3000 to 5000 max before change. (U can decide what weight best in ur mind.That's another can of worms to be opened)
4)Inspect oil filter after each change looking for metal fragments
5)Once warmed up and ready to go, drive it like u stole it!
During cooler weather in SW Missouri, when not driving it on a regular basis, I would start the car and let it run until it reached 180 degrees or so just to keep the oil "moving."
Maybe that will help with reasoning.